Patent Publication Number: US-9852149-B1

Title: Transferring and caching a cloud file in a distributed filesystem

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part application of pending U.S. patent application Ser. No. 13/725,738 filed on 21 Dec. 2012 by inventors John Richard Taylor, Randy Yen-pang Chou, and Andrew P. Davis, entitled “Distributing Data for a Distributed Filesystem Across Multiple Cloud Storage Systems”. U.S. patent application Ser. No. 13/725,738 is a continuation-in-part application of each of the following U.S. patent applications: (1) Ser. No. 12/772,927 filed on 3 May 2010 by inventors Randy Yen-pang Chou, Ravi Mulam, and Steve Jung, entitled, “Efficient Cloud Network Attached Storage”; (2) Ser. No. 13/225,194 filed on 2 Sep. 2011 by inventor Randy Yen-pang Chou, entitled “Forwarding Filesystem-Level Information to a Storage Management System”; and (3) Ser. No. 13/295,844 filed on 14 Nov. 2011 by inventors John Richard Taylor, Randy Yen-pang Chou, and Andrew P. Davis, entitled “Pre-Fetching Data for a Distributed Filesystem”. This application hereby claims priority under 35 U.S.C. §120 to the above-listed U.S. patent applications. The contents of U.S. patent application Ser. Nos. 13/725,738, 12/772,927, 13/225,194, and 13/295,844 are incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     This disclosure generally relates to techniques for providing flexible and extensible network storage systems. More specifically, this disclosure relates to techniques for storing and accessing data in a distributed filesystem. 
     RELATED ART 
     Enterprise data centers typically include large numbers of storage devices that can be accessed using high-speed networks. However, the management overhead for a large number of storage devices can become substantial. For instance, maintaining data consistency, redundancy, and storage system performance in the presence of hard drive failures can involve substantial time, effort, and expertise. 
     A number of “cloud-based storage” vendors attempt to simplify storage management by providing large-scale remote network storage solutions. Such vendors can leverage economies of scale to provide extensive data storage capacity that can be leased and accessed by clients. Clients can leverage such storage solutions to offload storage management overhead and to quickly and easily increase their data storage capacity on an as-needed basis. However, cloud-based storage involves another set of inherent risks and overheads. For instance, storing data remotely (“in the cloud”) often increases access latency, and multiple clients simultaneously accessing a shared data set in a cloud-based storage system may suffer from data consistency problems. Furthermore, network failures and/or outages in cloud-based storage systems can prevent clients from accessing their data for significant periods of time. 
     Hence, what is needed are techniques for providing network-based storage capabilities without the above-described problems of existing techniques. 
     SUMMARY 
     The disclosed embodiments disclose techniques for transferring and caching a cloud file in a cloud controller. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems; the cloud controllers cache and ensure data consistency for the stored data. During operation, a cloud controller receives a client request for a data block of a target file that is stored in the distributed filesystem but not currently cached in the cloud controller. The cloud controller initiates a request to a cloud storage system for a cloud file containing the requested data block. While receiving the cloud file from the cloud storage system, the cloud controller uses a set of block metadata in the portion of the cloud file that has already been received to determine the portions of the cloud file that should be downloaded to and cached in the cloud controller. 
     In some embodiments, the block metadata stored in a cloud file includes metadata for each data block that is stored in the cloud file. For instance, the block metadata may include a unique file identifier associated with each data block that is stored in the cloud file. 
     In some embodiments, block metadata is located at the beginning of cloud files, and received by a cloud controller prior to the data blocks stored in the cloud file. This arrangement allows cloud controllers to read and analyze the block metadata in parallel with downloading additional portions of the cloud file, and hence determine a set of actions to be taken for the data blocks in the cloud file. 
     In some embodiments, a cloud controller determines a set of file identifiers for files and directories that are logically proximate to the target file in the distributed filesystem. While downloading the cloud file containing the requested data block, the cloud controller compares the file identifiers for these logically proximate files and directories with the file identifiers associated with the data blocks stored in the cloud file, and determines a subset of the data blocks from the cloud file that will be opportunistically cached in the cloud controller. By caching logically proximate blocks from the cloud file, such opportunistic caching techniques seek to leverage spatial and temporal locality to improve access performance for the distributed filesystem. 
     In some embodiments, a cloud controller may determine that a portion of the cloud file comprises data blocks that are not likely to be needed, and does not download this unneeded portion. 
     In some embodiments, cloud files may be serially encrypted such that all data blocks in a cloud file that precede a requested data block need to be downloaded in order to decrypt the requested data block. In such scenarios, a cloud may determine that the end portion of the cloud file that follows a requested data block is not likely to be needed, and terminate the download of the end portion of the cloud file. 
     In some embodiments, an entry in the block metadata of a cloud file specifies one or more of the following: a unique filename identifier for an associated data block in the cloud file; a compression technique used to compress the associated data block; the logical size of the associated data block; the physical size of the associated data block; a checksum for the associated data block; the checksum technique used to calculate the checksum; and the type of the checksum. 
     In some embodiments, determining a subset of the data blocks from the cloud file that will be opportunistically cached in the cloud controller comprises using a locality policy that specifies a level of pre-fetching and opportunistic caching for at least one of the cloud controller and the portion of the distributed filesystem that includes the target file. 
     In some embodiments, the data for the distributed filesystem is stored in one or more cloud storage systems, and the collected metadata stored in each cloud controller is used to cache and ensure data consistency for this stored data. A cloud controller receiving new data from a client: (1) generates an incremental metadata snapshot for the new data that is propagated to the other cloud controllers of the distributed filesystem; and (2) stores an incremental data snapshot containing the new data to a cloud storage system. 
     In some embodiments, cloud files comprise logical storage volumes in the cloud storage system that store data and meta-data for the distributed filesystem. Cloud controllers manage the layout of data being written into cloud files to facilitate opportunistic caching for subsequent accesses. Note that the one or more cloud storage systems are unaware of the organization and structure of the distributed filesystem. 
     In some embodiments, data in the distributed filesystem is indexed using a global address space, and each cloud file is uniquely indexed in this global address space. In this context, a cloud controller receiving a request for a data block of a target file: (1) accesses the metadata for the distributed filesystem to determine a metadata entry that is associated with the target file; (2) uses the metadata entry to determine that the desired data block is not presently cached locally in the cloud controller; (3) uses a global address stored in the metadata entry to identify the cloud file that includes the desired data block; and (4) uses an offset stored in the metadata entry to determine the location of the data block in the cloud file. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a set of clients that are configured to access NAS devices. 
         FIG. 1B  illustrates a set of clients that are configured to access NAS devices via a load balancer. 
         FIG. 2  illustrates a network storage system that provides remote storage with a disk-level abstraction. 
         FIG. 3  illustrates an exemplary system in which a cloud controller manages and accesses data stored in a cloud storage system in accordance with an embodiment. 
         FIG. 4A  illustrates the process of generating a cloud file for a snapshot in the context of the exemplary system of  FIG. 3  in accordance with an embodiment. 
         FIG. 4B  illustrates a set of overlay metadata and a virtual cloud file in the exemplary system of  FIG. 3  in accordance with an embodiment. 
         FIG. 4C  illustrates a second cloud controller that responds to a snapshot sent by the first cloud controller of  FIGS. 3-4B  in accordance with an embodiment. 
         FIG. 4D  illustrates the process of accessing data from a cloud file in accordance with an embodiment. 
         FIG. 4E  illustrates an alternative exemplary cache configuration for cloud controllers in accordance with an embodiment. 
         FIG. 5  presents a flow chart that illustrates the process of pre-fetching data for a distributed filesystem in accordance with an embodiment. 
         FIG. 6A  illustrates a computing device that receives and forwards requests for filesystem operations in accordance with an embodiment. 
         FIG. 6B  illustrates a computing device that forwards requests for filesystem operations to a cloud controller in accordance with an embodiment. 
         FIG. 6C  illustrates a cloud controller in which a filesystem device driver in a guest operating system forwards requests for filesystem operations in accordance with an embodiment. 
         FIG. 7A  presents a flow chart that illustrates the process of forwarding filesystem-level information in accordance with an embodiment. 
         FIG. 7B  presents a flow chart that illustrates the process of using a guest operating system to forward filesystem-level information in accordance with an embodiment. 
         FIG. 8  illustrates a distributed system in which updates are mirrored to an additional mirror storage system in accordance with an embodiment. 
         FIG. 9  illustrates a computing environment in accordance with an embodiment. 
         FIG. 10  illustrates a computing device in accordance with an embodiment. 
         FIG. 11A  illustrates a distributed filesystem in which a backup cloud controller serves as a backup for set of cloud controllers that manage and access data stored in a cloud storage system in accordance with an embodiment. 
         FIG. 11B  illustrates the distributed filesystem of  FIG. 11A  after the backup cloud controller has taken over the load of a failed cloud controller in accordance with an embodiment. 
         FIG. 12  presents a flow chart that illustrates the process of providing disaster recovery for a distributed filesystem in accordance with an embodiment. 
         FIG. 13A  illustrates a distributed filesystem in which a set of cloud controllers manage and access data mirrored across multiple cloud storage systems in accordance with an embodiment. 
         FIG. 13B  illustrates a cloud virtual address (CVA) format that facilitates identifying and locating cloud files across different cloud storage configurations in accordance with an embodiment. 
         FIG. 14  illustrates the process of migrating a cloud file from a first cloud storage system to a second cloud storage system in accordance with an embodiment. 
         FIG. 15  presents a flow chart that illustrates the process of distributing data for a distributed filesystem across multiple cloud storage systems in accordance with an embodiment. 
         FIG. 16A  illustrates a set of snapshots that are taken for the distributed filesystem over time in accordance with an embodiment. 
         FIG. 16B  illustrates the process of migrating a cloud file from a first cloud storage system to an archival cloud storage system in accordance with an embodiment. 
         FIG. 17  presents a flow chart that illustrates the process of archiving data for a distributed filesystem in accordance with an embodiment. 
         FIG. 18  illustrates a distributed filesystem in which a cloud controller issues peer cache requests to one or more peer cloud controllers in accordance with an embodiment. 
         FIG. 19  presents a flow chart that illustrates the process of archiving data for a distributed filesystem in accordance with an embodiment. 
         FIG. 20  presents a flow chart that illustrates the process of using a set of overlay metadata in a cloud controller to generate incremental snapshots for a distributed filesystem in accordance with an embodiment. 
         FIG. 21A  illustrates metadata-based fragmentation in a storage drive of a cloud controller in accordance with an embodiment. 
         FIG. 21B  illustrates an exemplary partitioning of a disk platter of a storage drive into distinct metadata and data storage regions in accordance with an embodiment. 
         FIG. 22  presents a flow chart that illustrates the process of managing metadata and data storage for a cloud controller in a distributed filesystem in accordance with an embodiment. 
         FIG. 23  illustrates a cloud controller that performs exemplary opportunistic caching and transport termination operations in accordance with an embodiment. 
         FIG. 24  presents a flow chart that illustrates the process of transferring and caching a cloud file in a distributed filesystem in accordance with an embodiment. 
         FIG. 25  illustrates an exemplary distributed filesystem environment in which the characteristics of the distributed filesystem are guided and/or adjusted by a set of locality policies in accordance with an embodiment. 
         FIG. 26  presents a flow chart that illustrates the process of adjusting the characteristics of a distributed filesystem using a locality policy in accordance with an embodiment. 
         FIG. 27A  illustrates the state of a cloud controller prior to a delete request in accordance with an embodiment. 
         FIG. 27B  illustrates the initiation of a delete operation at a cloud controller in accordance with an embodiment. 
         FIG. 27C  illustrates the execution of the delete process initiated in  FIG. 27C  in accordance with an embodiment. 
         FIG. 28  presents a flow chart that illustrates the process of deleting a file from a distributed filesystem in accordance with an embodiment. 
         FIG. 29A  illustrates the process of writing new data blocks in an exemplary deduplication architecture in accordance with an embodiment. 
         FIG. 29B  illustrates an exemplary temporal deduplication architecture in accordance with an embodiment. 
         FIG. 29C  illustrates the process of using incremental metadata snapshots to update deduplication information in accordance with an embodiment. 
         FIG. 30  presents a flow chart that illustrates the process of performing deduplication in a distributed filesystem in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a non-transitory computer-readable storage medium, which may be any device or non-transitory medium that can store code and/or data for use by a computer system. The non-transitory computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing code and/or data now known or later developed. 
     The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a non-transitory computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the non-transitory computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the non-transitory computer-readable storage medium. 
     Furthermore, the methods and processes described below can be included in hardware modules. For example, the hardware modules can include, but are not limited to, application-specific integrated circuit (ASIC) chips, a full-custom implementation as part of an integrated circuit (or another type of hardware implementation on an integrated circuit), field-programmable gate arrays (FPGAs), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. 
     Evolution of Network-Based Storage Systems 
     The proliferation of the Internet and large data sets has motivated a range of specialized data storage techniques. For instance, network-attached storage (NAS) devices often serve as centralized storage devices that provide large amounts of storage space for a set of heterogeneous clients in an enterprise. Such devices are typically tuned to provide a desired level of performance, redundancy (e.g., using a redundant array of independent disks (RAID)), and high availability. For example, while typical filesystems may take a substantial amount of time to recover from a crash (as the system has to process logs and/or journals to correctly rebuild modified data that was queued or in the process of being written at the time of the crash), NAS devices often incorporate transactional copy-on-write filesystems, which sacrifice some read performance in exchange for faster crash recovery. In a transactional copy-on-write filesystem, a file is not modified in place; instead, the system uses a delta encoding to append modifications (“deltas”) to the previous file data. Such encodings increase the overhead associated with read operations, because the system incurs additional computation and access time to read and process deltas stored at the end of a file. However, this encoding also ensures that files are “data-consistent” (e.g., reliably incorruptible and consistent across crashes and reboots), thereby allowing NAS devices to recover quickly from failures. Such characteristics and capabilities have made NAS devices popular in enterprise environments. 
     Unfortunately, storage scalability can become problematic when data needs outscale the capabilities of a single NAS device; providing redundancy across multiple separate NAS devices (as illustrated in  FIG. 1A ) can involve substantial configuration expertise. For instance, consider the scenario of responding to a drive failure. Typically, a redundant storage system attempts to restore lost bits and re-establish redundancy as quickly as possible. However, in some scenarios, depending on the application and load, the storage system may need to place higher priority on continuing to serve client requests with a specified level of performance, and hence may need to delay restoration efforts. Storage systems typically need to be architected very carefully based on expected client needs, application needs, and load characteristics. 
       FIG. 1A  illustrates a set of clients ( 100 - 102 ) that are configured to access NAS devices ( 110 - 114 ). Note that management overhead typically increases in proportion with the amount of storage available. For instance, as the number of supported applications and storage space increase, a storage system may need to grow to include a load balancer  120  between the clients ( 100 - 102 ) and the NAS devices ( 110 - 114 ), as illustrated in  FIG. 1B . Load balancer  120  can explicitly partition applications and clients to a given NAS device, and then route requests accordingly. While initial NAS vendors primarily focused on speed and reliability, as storage needs have continued to grow NAS vendors have also begun to compete by including sophisticated system management solutions that facilitate adapting to different storage, performance, and failure scenarios. 
       FIG. 2  illustrates another network storage system that provides remote storage, but with a disk-level abstraction. In such an architecture, a computing device  200  manages metadata for a filesystem  202  locally, and then sends block-level read/write requests to a remote block storage device  204  via a storage area network (SAN) (e.g., by using the Internet Small Computer System Interface (ISCSI) or a Fibre Channel protocol). More specifically, block storage device  204  provides only a block storage interface, and is unaware of any filesystem structure associations (e.g., file names and/or structures) for the stored blocks. Such storage systems typically do not use transactional copy-on-write filesystems, and hence are not data-consistent. 
     Note that there is a significant distinction between filesystem-level operations and block-level (e.g., disk-level) operations. A filesystem typically serves as an intermediary between an operating system and one or more block-level devices. More specifically, a filesystem typically attempts to efficiently manage one or more block-level devices to provide more sophisticated storage services to an operating system. For instance, filesystems often manage disk blocks and metadata to provide structure (e.g., files and directories) and some notion of access rights and data consistency (e.g., via file lock operations) for an underlying block storage mechanism. Hence, filesystem-level operations provide a higher level of abstraction (e.g., a filename and an ordering associated with an underlying set of disk blocks) for the block storage mechanism. 
     Typically, a filesystem and an associated block storage device both operate in the context of the same computing device, and the block storage device has been specially initialized (e.g., formatted) to support the filesystem. Upon receiving a request for a filesystem operation (e.g., from an operating system and/or application), the filesystem determines and initiates a set of block-level operations needed to service the request. Hence, there is a notion of “filesystem-level information” (e.g., the level of information managed by the filesystem and received in a request for a file operation) and a separate notion of “block-level information” that is used when the filesystem operates upon the underlying block storage device. In the example of  FIG. 2 , the functionality of the filesystem and the underlying block storage are split across two different devices (computing device  200  and block storage device  204 ). As mentioned above, block storage device  204  provides only a block storage interface, and is unaware of any filesystem structure associations for the stored blocks. Block storage device  204  may store filesystem metadata on behalf of filesystem  202 , but it is filesystem  202  that provides the higher level of abstraction to the operating system of computing device  200 . 
     A number of “cloud-based storage” vendors (also sometimes referred to as “cloud storage providers”) attempt to simplify storage management by providing large-scale network storage solutions. Such vendors can leverage economies of scale to provide data centers with extensive data storage capacity that can then be rented and accessed by clients, thereby allowing clients to offload storage management overhead and easily increase their data storage on an as-needed basis. However, cloud-based storage also includes another set of inherent risks and overheads. Storing data remotely (“in the cloud”) often increases access latency, and network failures and/or outages in cloud-based storage systems can prevent clients from accessing their data for substantial time intervals. Furthermore, multiple clients simultaneously accessing a shared data set in a cloud-based storage system may suffer from data consistency issues. 
     Consider a scenario where one remote client attempts to write a set of data to a cloud storage system, and a second remote client attempts to read the data that is being written. In some systems, a reader may not be able to see the existence of newly written file data until the entire write operation has completed (e.g., after the first remote client has closed the file). In other (non-data-consistent) arrangements, the reader may see and access the file, but because writes are stateless and potentially out-of-order (e.g., as in the Network File System (NFS) protocol), does not know which file sections have already been written, and hence may access a mix of valid data and garbage. 
     Embodiments of the present invention combine aspects of NAS capabilities and cloud-based storage capabilities to provide a high-capacity, high-reliability storage system that ensures that data can be accessed in a data-consistent manner. 
     Providing Data Consistency in a Cloud Storage System 
     In some embodiments, a set of caching storage devices (referred to as “cloud controllers”) collectively cache, manage, and ensure data consistency for a set of data that is stored in a network storage system (e.g., a cloud-based storage system, which is also referred to as a cloud storage system). More specifically, one or more cloud controllers work together (e.g., as a federation) to manage a distributed filesystem with a global address space. Each cloud controller maintains (e.g., stores and updates) metadata that describes the file and directory layout of the distributed filesystem and the location of the data blocks in the cloud storage system. Each cloud controller can also cache a subset of the data that is stored in the cloud storage system. A cloud controller that writes (or modifies) data ensures that: (1) data changes are reflected in the cloud storage system; and (2) other cloud controllers in the system are informed of file and metadata changes. 
     Note that while the cloud storage system stores the data for the distributed filesystem, the cloud storage capabilities may be provided by an external vendor. An enterprise storing sensitive data in the distributed filesystem may not want this vendor to be able to access such data, and hence, the cloud storage system may be configured to store the distributed filesystem&#39;s data in the form of encrypted storage volumes (referred to as “cloud files” or “drive files”). This configuration enhances data security, but also prevents the cloud storage system from actively assisting in ensuring data consistency and performing other operations that require knowledge of the data and data layout. More specifically, in some embodiments the cloud controllers fully manage the filesystem and manage data consistency, with the cloud storage system providing purely storage capabilities. 
       FIG. 3  illustrates an exemplary system in which a cloud controller  300  (e.g., a caching storage device) manages and accesses data stored in a cloud storage system  302 . A request server  304  in cloud controller  300  may receive file requests from either local processes or via a network from a client  306 . These requests are presented to a storage management system that includes a transactional filesystem  308  that manages a set of filesystem metadata  310  and a local storage system  312 . In  FIG. 3 , the filesystem structure defined by metadata  310  is illustrated as a tree of pointers that define one or more levels of directories and files residing in directories. Each file is described using a set of ordered metadata structures that indicate the set of disk blocks that contain the file&#39;s data. A set of block records  314  in metadata  310  include pointer fields that indicate the location of the file data in a disk block  316  in local storage  312  (if the given block is currently being cached in the storage  312  of cloud controller  300 ), as well as the location of the file data in a cloud file  318 . Note that disk blocks  316  and cloud files  318  may have substantially different sizes. For instance, cloud files might be much larger than disk blocks, and hence the data contained in a disk block  316  may occupy only a portion of a cloud file  320 . Hence, one pointer field in block record  314  may consist of a block pointer (labeled “BLOCK PTR” in  FIG. 3 ) that points to a specific disk block, while another field (labeled “CVA&amp;OFFSET”) may include both a pointer to a cloud file (also referred to as a “cloud virtual address,” or CVA) and an offset into the cloud file. 
     Note that using a transactional filesystem in each cloud controller does involve some additional overhead. As described above, the transactional filesystem tracks modifications using delta encoding (instead of the more typical read/copy/modify operations used in many non-data-consistent filesystems). For instance, consider a 1 KB modification to an existing 3 KB file in a filesystem that supports 4 KB blocks. Using a traditional approach, the filesystem might read out the original 4 KB block, modify the block to reflect the updates, and then write the modified file back to the same block. In contrast, in a transactional filesystem, the original block is left unchanged, and the filesystem writes out the modifications and additional data to another empty 4 KB block. The metadata for the transactional filesystem is extended to support the notion of partial blocks and deltas (e.g., including one pointer that points to 3 KB of data in one block and another pointer that points to another block that contains 1 KB of additional data and a set of changes that should be applied to the initial 3 KB of data). 
     In some embodiments, using a transactional filesystem (e.g., transactional filesystem  308  in  FIG. 3 ) in a cloud controller facilitates providing ongoing incremental snapshots of changes to a cloud storage system and other cloud controllers. More specifically, the transactional nature (e.g., the delta encoding of changes) can be extended to include a set of additional metadata structures that track recently changed data in the cloud controller. These additional metadata structures can then be used to quickly and efficiently construct compact snapshots that identify file metadata and file data that has changed due to recent write operations. Note that these snapshots do not involve copying a full set of metadata and/or every byte that was previously written for a file; instead, such snapshots compactly convey only the set of changes for the data set. Sending only a compact set of changes facilitates maintaining data consistency while minimizing the amount of data (and metadata) that needs to be transferred and processed. Sending frequent snapshots ensures that changes are quickly propagated to other cloud controllers and the cloud storage system. 
     In some embodiments, cloud controllers generate separate metadata snapshots and file data snapshots. Metadata is typically much smaller than file data, and is needed to access file data. Furthermore, each cloud controller is typically configured to maintain (and update) the full set of metadata, but only caches file data that is needed by local clients. Hence, uploading (or sending) a metadata snapshot separately means that the updated metadata will be more quickly available to other peer cloud controllers. Each of these peer cloud controllers can then determine (e.g., based on client data usage and needs) whether to access the related file data associated with the updated metadata. Note that a cloud controller may still upload both metadata updates and file data updates to the cloud storage system, but may split them into different sets of cloud files (or both include the metadata with the file data as well as generate another separate, duplicative update that includes only metadata) so that other cloud controllers can access the two separately. In such an organization, a cloud controller might then send a message to other cloud controllers specifying the location of the stored metadata snapshot. Alternatively, cloud controllers may also be configured to send metadata snapshots directly to a set of peer cloud controllers. 
     Consider an example of a cloud controller receiving a request from a client to store a 10 GB file, in an environment where the network link between the cloud controller and a cloud storage system supports a transfer speed of 1 GB/minute and the cloud controller is configured to send a metadata snapshot every minute. Upon determining the scope of the file operation, the cloud controller can already allocate a set of corresponding disk blocks and cloud files, and generate a set of corresponding metadata that indicates the respective disk addresses and CVAs for the file&#39;s data blocks. The cloud controller then uploads the file data to the cloud storage system over a time interval (e.g., roughly ten minutes), and sends out metadata snapshots that indicate the existence and location of the data blocks. The cloud controller may convey a range of information about the data being uploaded to other cloud controllers depending on the level of transparency and availability desired for modified data. For instance, in some embodiments, the file remains accessible by clients via the originating cloud controller throughout the upload process. However, other cloud controllers that have received the corresponding metadata and seek to access modified data that has not yet been received by the cloud storage system may receive an indication that the data is not yet available, and that their access attempts should be re-tried at a later time (or after a specified time interval). Alternatively, in some instances, when a set of data has not yet been uploaded to the cloud storage system, a client (and/or cloud controller) that hence cannot yet access this data via the cloud storage system may be configured to gain access to the desired data by directly interacting with the cloud controller hosting the desired data. Such alternative access techniques may depend on the capabilities and topography of the network connecting the cloud controllers and cloud storage system. 
     In some embodiments, the originating cloud controller may propagate additional intermediate metadata that informs other cloud controllers as portions of the modified data become available in the cloud storage system. For instance, metadata snapshots may indicate files that are in the process of being uploaded, and include a field that indicates whether a given data block has been successfully stored in the cloud storage system. The cloud controller updates (and propagates) this metadata as it receives acknowledgments of receipt from the cloud storage system, thereby indicating that some of the data being uploaded is now already available in the cloud storage system. For example, immediately after first storing the 10 GB file locally, the cloud controller may have already reserved 10 GB of space in cloud files at a given set of CVA addresses (e.g., in the cloud storage system), but have not yet transferred any file data. A snapshot sent at this point includes metadata that indicates the existence of the file, but also indicates that none of the data is available in the cloud storage system yet. After one minute, the cloud controller sends out another snapshot containing metadata that reflects the set of data that has already been transferred to (and been acknowledged as received by) the cloud storage system. 
     In some embodiments, each cloud controller maintains a set of structures that track snapshots and changes in metadata, and updates its local metadata to reflect updates from the rest of the distributed system. For instance, a cloud controller receiving the first snapshot from the above example may note the creation of a 10 GB file (as described in the above example), but then also determine that none of the associated data blocks is available yet. After receiving and processing the second snapshot, the receiving cloud controller determines the presence and location of the first GB of stored data that is now available. At this point, the receiving cloud controller may, if desired, use the received metadata to already download and access the available file data from the cloud storage system on behalf of a client. If, however, a client requests additional parts of the file that have not yet been stored in the cloud storage system (as indicated by the metadata in the most recent snapshot), the cloud controller can signal that the desired data is not yet available, and delay the access. More of the file data becomes available over time, as indicated by the subsequent snapshots. 
     Note that cloud controllers can use the detailed information received in snapshots to provide a range of data access and data consistency capabilities. More specifically, each cloud controller receives ongoing updates that identify valid data, and indicate how to find and access such data. If data is written to the cloud storage system out-of-order, this is reflected in the received snapshot(s), and the cloud controller (and/or a requesting client) can use such received snapshot information to determine how to proceed. 
     In some embodiments, cloud controllers may use stored snapshot data to provide access to different versions of a file. For instance, in the preceding example, a cloud controller may allow a client to already access the uploaded (and acknowledged) portions of a new file before the file has been completely uploaded to the cloud storage system. Similarly, the cloud controller may allow the client to access modified file data as it becomes available in the cloud storage system. Alternatively, in other scenarios, when an existing file is being modified, a cloud controller may be configured to present a previous version of the file to clients until the complete set of data for the modified version is available in the cloud storage system. In some embodiments, cloud controllers may maintain records of past snapshots to allow file accesses to be rolled back across multiple different versions, thereby allowing clients to view historical versions of files and/or the changes made to files over time. 
     In general, the disclosed techniques leverage transactional filesystem techniques and snapshots to ensure that only valid data can be accessed. While these techniques involve some additional complexity, they also provide an assurance of data consistency for a distributed filesystem that leverages cloud storage. The following sections describe additional aspects of storing and accessing data in the disclosed distributed filesystem. 
     Generating Snapshots and Cloud Files 
     A number of factors affect the performance of accessing data from a cloud storage system. In a typical computer data is stored locally on a disk, and a number of hardware and operating system mechanisms attempt to minimize the latency of reads and writes. For instance, processors and operating systems strive to load frequently used data into memory and multiple levels of hardware caches, thereby reducing the latency associated with reading data from disk. Accessing data stored on a cloud storage system involves an additional set of latencies. For instance, in addition to normal disk latency, accessing a cloud storage system may involve additional latency due to network latency, network protocol handshaking, network transfer times, and delays associated with encryption or decryption. One of the challenges of a distributed filesystem is minimizing such latencies as much as possible. 
     One factor that can significantly affect the latency of data access in the described distributed filesystem is cloud file size. Overly small cloud files can result in higher network negotiation and transfer overhead. Conversely, overly large cloud files can result in large transfer delays; for instance, a cloud controller that needs only a small piece of data from a large cloud file that is serially encrypted may need to wait for the entire file to be downloaded and decrypted before it can access the desired data. Determining a reasonable cloud file size that maximizes throughput to and from the cloud storage system may depend on factors such as network link size and latency (e.g., transfer speeds), local filesystem block sizes (e.g., making the cloud file size a multiple of a local block size), and CVA pointer sizes or boundaries. Another trade-off involves determining whether to use fixed-sized or variable-sized cloud files. Variable-sized cloud files allow some level of customization to match network and application characteristics, but also involve additional complexity to manage the different sizes. Hence, in some embodiments the system reduces management overhead by using a single fixed cloud file size (e.g., 32 MB) throughout the cloud controllers and cloud storage system. Note, however, that the contents of each cloud file may vary based on the set of data currently being generated or modified. For instance, data blocks being stored for large files (e.g., larger than 32 MB in the case of 32 MB cloud files) may be split across two or more cloud files. Alternatively, if the current load involves storing data for multiple small files or making a large number of small file modifications, a corresponding cloud file may contain multiple user files and deltas. Note also that, in some embodiments, data and meta-data are always separated into different cloud files. In general, cloud controllers may use a range of techniques to stripe chunks of data across cloud files in an attempt to optimize subsequent accesses from such cloud files. 
       FIG. 4A  illustrates the process of generating a cloud file for a snapshot in the context of the exemplary system of  FIG. 3 . As indicated by filesystem metadata  310 , a file (“file X”) includes a set of metadata  400  and a set of disk blocks (the highlighted blocks among disk blocks  316 ) that have been modified since a previous snapshot. During the snapshot process, cloud controller  300  freezes the pointers in the blocks, and determines the set of metadata and data that should be written out to cloud storage system  302 . The modified data is then packaged into units that match the granularity of cloud files (e.g., into 32 MB segments), optionally encrypted, and then uploaded to cloud storage system  302 . 
     Note that cloud files are also written to in an incremental, transactional fashion, to preserve data consistency. More specifically, new and modified file data is written to a separate cloud file, as in a transactional filesystem, to ensure that the consistency of previous file versions is preserved. Thus, an initial set of data for a given file is written to one cloud file, and later additions or modifications to the file detected by a subsequent snapshot are written to a new, different cloud file. 
     The filesystem metadata for each disk block includes information that specifically identifies the location and enables the lookup of the disk block in a cloud file. For instance, the metadata may include one or more of the following: a CVA (cloud virtual address) that uniquely addresses the cloud file; the offset of the disk block in the cloud file; a physical and logical size for the disk block; the portions of the disk block that are valid; compression information; a checksum hash value or other checksum information; and information that indicates whether the disk block has already been successfully uploaded to the cloud storage system. 
     To ensure data consistency, cloud controllers need to ensure that each cloud controller assigns unique CVAs that create non-overlapping cloud files. More specifically, the cloud controllers need to collectively manage the global address space for the distributed filesystem. In some embodiments, each cloud controller is assigned a unique identifier, the collective set of cloud controllers are associated with a total amount of cloud storage space, and each cloud controller is pre-allocated a portion of the global address space. In such embodiments, a cloud controller can already allocate a cloud file in this pre-allocated address range at the time that it writes a new disk block, and store the CVA of the cloud file in the block&#39;s metadata. This organization ensures that there are no collisions in allocating cloud file addresses, and also ensures that even the first metadata snapshot for a new disk block already includes an accurate (future) location of the disk block in the cloud storage system. Note that the allocations of the global address space can be adjusted as needed over time if more data is created on a subset of the cloud controllers. 
     While most updates and file changes can propagate through the distributed filesystem via incremental snapshots, some data consistency issues can arise if multiple clients accessing two different cloud controllers attempt to simultaneously write the same file. Downloading and importing snapshot data may involve some latency, and thus such conflicting operations may lead to race conditions and errors. Hence, in some embodiments, each file is associated with a cloud controller that “owns” (e.g., actively manages) the file. For instance, the cloud controller from which a file was first written may by default be registered (in the file block metadata) as the owner (e.g., the owning cloud controller) of the file. A cloud controller attempting to write a file owned by another cloud controller first contacts the owner with a request to lock the file. The owner can determine whether to grant or deny the lock request. In some embodiments, even if this request is granted, all write operations may be required to go through the cloud controller that owns the file (e.g., new data is written to the local filesystem of the owning cloud controller). Note that while every cloud controller actively manages a set of files, a given cloud controller may not need to continue to cache every disk block of files that it owns; once such blocks have been written to the cloud storage system, they may subsequently be cleared from the cloud controller to make space for other needed data. However, the metadata for all of the files in the distributed system is typically maintained in every cloud controller. In some embodiments, the system may also include mechanisms for transferring ownership of files between cloud controllers (e.g., migrating file ownership to cloud controllers that are the primary modifiers of the file to reduce network latency). 
     Optimizing the Creation of Cloud Files 
     Note that a cloud controller may use a range of techniques to generate cloud files. For instance, one such technique may involve: (1) allocating one or more cloud-file-size memory buffers; (2) copying the file and metadata for the cloud file into a memory buffer; (3) encrypting the contents of the memory buffer; and (4) uploading the encrypted contents of the memory buffer to a cloud storage system as a cloud file. Note, however, that this technique involves allocating and using additional memory buffers, and potentially performing a large number of data copy operations. 
     In some embodiments, a cloud controller generates an additional set of filesystem overlay metadata that allows existing file data and metadata to be virtually linked together into a cloud file view. For instance, the system can construct such overlay metadata when writing new blocks (and corresponding metadata). Alternatively, the cloud controller may instead generate such overlay metadata while traversing the filesystem to find changed data to include in the next incremental snapshot. Either way, unlike the above memory-buffer technique, this overlay metadata facilitates minimizing the use of additional resources by creating cloud files “in place” (e.g., without allocating additional memory buffers or additional copy operations); instead, a set of pointers point to the original blocks in the transactional filesystem that contain the modified data and metadata. Note that while such additional overlay metadata may involve some additional space and computational complexity, these additional needs are typically small compared to the space and copy overhead associated with other approaches. 
     When creating a snapshot, a cloud controller can access the overlay metadata to read, encrypt, and upload the cloud file to the cloud storage system. For instance, the overlay metadata may facilitate accessing the virtual cloud file via a special filesystem directory that presents a view of the disparate data blocks as a single, consolidated cloud file that can be read and transferred. In many scenarios the cloud controller primarily maintains overlay metadata for data that has not yet been written out to a cloud file; in some embodiments, once the data has been uploaded to the cloud storage system, the cloud controller clears the overlay metadata and begins generating new overlay metadata to track changes destined for a subsequent set of cloud files. In other embodiments, a cloud controller may maintain snapshot information and overlay cloud files locally for a longer time interval (e.g., until space constraints on the cloud controller prompt the removal of infrequently used data). 
       FIG. 4B  illustrates a set of overlay metadata  410  and a virtual cloud file  412  in the exemplary system of  FIG. 3 . During operation, cloud controller  300  uses overlay metadata  410  to track the metadata  400  and data disk blocks (the highlighted blocks among disk blocks  316 ) that have been modified since a previous snapshot. During the snapshot process, cloud controller  300  reads and uploads the virtual cloud file  412  presented in the overlay metadata  410  into a cloud file in cloud storage system  302 . Note that the transactional nature of cloud files can lead to substantial distribution of file data in a cloud storage system over time. For instance, files that are modified multiple times across multiple snapshots will be stored in different cloud files. Thus, a cloud controller that has flushed the file data (as described in more detail in the following sections) may need to download and access all of the relevant cloud files to reconstruct the file at a later time, which may involve considerable network bandwidth and time. Unfortunately, the initial cloud file for a given file is generated at the time that the first set of file data is written; at this time, little is known about the likely future access patterns for the file. 
     In some embodiments, a cloud controller attempts to optimize the placement of data into cloud files to reduce future access overhead. For instance, the cloud controller may strive to, when possible, store all blocks for a file in the same cloud file (e.g., assuming the size of the file and/or file modifications are smaller than the size of a cloud file). Toward this end, the cloud controller may place data into multiple cloud files in parallel, and avoid storing data for multiple files in the same cloud file unless the complete set of data for some or all of the files will fit. A cloud controller may also perform additional file grouping based on user configuration and/or automatic analysis of file access trends. For example, users may be provided with a way to configure a policy that reflects anticipated file access patterns, groupings, and/or priorities (e.g., a user policy that indicates files with a certain extension are likely to be accessed together, and thus should be grouped together). 
     Note that some cloud files may be partially empty. For instance, a cloud controller that is generating a snapshot based on a time interval or a cloud controller that is placing data into multiple cloud files in parallel to optimize future read operations may not have enough data to fill a complete cloud file. In such scenarios, the cloud controller may simply write out the available data, and leave the wasted space to be reclaimed using a future reclamation operation. For example, in some embodiments a cloud controller may be configured to: (1) download file data spread across two or more cloud files; (2) reassemble the desired data into a new cloud file; (3) upload the new cloud file to the cloud storage system; and (4) distribute a metadata snapshot that updates the access information for the affected files. In some scenarios, such optimizations may be performed by the cloud controller that owns the files, and involve locking the files during the cloud file defragmentation process. In scenarios where locking files for a long interval is undesirable, such operations may involve: (1) maintaining the previous cloud files the final metadata snapshot has been generated; (2) determining that the file data has not changed during the download, reassembly, and upload operations; and (3) briefly locking the original cloud files and metadata sections until the system can confirm that the updated metadata has been successfully propagated to all of the cloud controllers in the system. If the file data being compacted has changed during the interval, the changes can be discarded and compaction can be re-tried at a later time. In some embodiments, compaction operations may be limited to older (e.g., less likely to be modified) cloud files, to reduce the likelihood of collisions. Alternatively, such operations may involve determining timeframes in which particular cloud files are less likely to be used, and performing compaction operations at those times. 
       FIG. 20  presents a flow chart that illustrates the process of using a set of overlay metadata in a cloud controller to generate incremental snapshots for a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems. More specifically, the cloud controllers cache and ensure data consistency for the data stored in the cloud storage systems, with each cloud controller maintaining a metadata hierarchy that reflects the current state of the distributed filesystem (operation  2000 ). During operation, a cloud controller receiving new data from a client: (1) stores the new data in the cloud controller (operation  2010 ); (2) creates a metadata entry for the new data in the locally maintained metadata hierarchy (operation  2020 ); (3) updates the overlay metadata to point to the metadata entry and the new data stored in the cloud controller (operation  2030 ); and (4) then uses the overlay metadata to generate an incremental snapshot for the new data (operation  2040 ). 
     Accessing Cloud Files and Managing Local Disk Layout 
     The previous sections disclose techniques for generating snapshots and uploading data to cloud files. Using such techniques, cloud controllers can treat the cloud storage system as an object store. Other cloud controllers receiving metadata updates can then access data from cloud files as needed. Furthermore, a cloud controller that has uploaded data can, if needed, flush data that has been uploaded from its local filesystem (e.g., “clear its cache”) to make space for other data that is more likely to be needed immediately. Note, however, that a cloud controller flushing data still keeps the accompanying metadata, so that the flushed data can be found and reloaded from the cloud storage system if needed again. 
       FIG. 4C  illustrates a second cloud controller  420  that responds to a snapshot sent by the cloud controller  300  of  FIGS. 3-4A . As described previously, cloud controller  300  generates a cloud file during a snapshot, and uploads the cloud file and the (incremental) metadata snapshot to cloud storage system  302  (as indicated by ( 1 ) in  FIG. 4C ). Upon receiving confirmation of the successful upload, cloud controller  300  then sends a notification to other peer cloud controllers (including cloud controller  420 ) that informs them of the availability of the new snapshot (as indicated by ( 2 ) in  FIG. 4C ). Cloud controller  420  then downloads the metadata snapshot from cloud storage system  302  (as indicated by ( 3 ) in  FIG. 4C ), and updates its local metadata accordingly (as indicated by ( 4 ) in  FIG. 4D ). After updating the metadata, cloud controller  420  can proceed to download any desired data from the corresponding cloud files. Note that in some scenarios cloud controller  300  may also be configured to directly send the metadata snapshot as part of the notification ( 2 ) to cloud controller  420 . 
       FIG. 4D  illustrates the process of accessing data from a cloud file. At some point after receiving updated metadata from a snapshot (as described for  FIG. 4C ), cloud controller  420  receives a request from a client  421 . The storage system on cloud controller  420  inspects its updated filesystem metadata  424 , and determines that the request requires data that is not currently cached in local storage  426 . The system then uses the lookup information in the block records of the metadata (e.g., the CVA and offset values) to determine the appropriate cloud file(s) to download. Cloud controller  420  then downloads (and decrypts, if necessary) the indicated cloud files, and uses the offset information in the metadata to unpack the desired contents of the downloaded cloud file(s). 
     In some embodiments, a cloud controller downloads a cloud file into a dedicated memory buffer, and operates directly upon this memory buffer to access the desired data. The cloud file format is easy to operate upon in memory, and the downloaded data can be accessed very quickly from memory. However, storing such blocks in memory also constrains memory use, and (depending on the application) client data demands may involve reloading and caching more data than can be stored in memory. Furthermore, operating upon downloaded cloud files purely in memory may require a cloud file to be re-downloaded if the cloud controller is power cycled. Hence, in alternative embodiments, the cloud file is unpacked and re-integrated into the local transactional filesystem of a downloading cloud controller (e.g., into the highlighted subset of disk blocks  428  in  FIG. 4D ). Integrating downloaded cloud file data into the local filesystem allows all filesystem accesses to operate in the same manner (e.g., as opposed to special memory-buffer accesses for downloaded cloud file data), and facilitates keeping the downloaded data persistent across power cycles. 
     Note that a cloud controller can choose the target disk blocks that will receive the downloaded data blocks; for instance, in the context of  FIG. 4D , cloud controller  420  is not constrained to use the same set of disk blocks used by cloud controller  300  to store the same data. In some instances, this selection process may involve determining disk blocks containing data that has not been accessed recently (e.g., via a least-recently-used (LRU) policy), and flushing such data. Each cloud controller tracks the usage of its cached data blocks, and strives to cache data blocks that minimize that latency experienced by clients. 
       FIG. 4E  illustrates an alternative exemplary cache configuration for cloud controllers in which the local storage capabilities of cloud controller  420  serve as a persistent resident cache (PRC)  430  for a subset of the data blocks  432  stored in cloud storage system  302 . In the exemplary configuration of  FIG. 4E , a lookup structure  434  (e.g., a hash table comprising one or more levels) is used to determine whether a desired data block is currently being cached by cloud controller  420 . Upon receiving a client request (not shown), cloud controller  420  traverses filesystem metadata  424  to find the block record  436  for a requested file data block. The CVA and offset from block record  436  are then used to perform a lookup operation (operation  1 ) in lookup structure  434  to determine whether the requested file data block is cached, and, if so, retrieve it (operation  2 ). 
     In some implementations, lookup structure  434  is implemented as a single-level hash table that is indexed using a hash value derived from the CVA and cloud file offset associated with a requested file data block. In such implementations, the hash lookup determines whether the requested file data block is cached, and if so, returns a pointer to the data block&#39;s location in PRC  430 . If the block is not cached, the cloud file referenced by the CVA is downloaded from cloud storage system  302 , the requested data block is loaded into PRC  430 , and lookup structure  434  is updated to point to the now-cached data block. 
     Unfortunately, a single-level hash table implementation may sometimes become unwieldy and fragmented; for instance, hashing on a per-block basis (e.g., basing hash values on a combination of the CVA and offset) can lead to the hash table becoming sufficiently large that part of the hash table may be paged out of memory (e.g., written to disk), thus leading to a large number of disk reads which may negatively affect the performance of data accesses. Hence, in some implementations, lookup structure  434  is structured as a multi-level hash table and/or lookup structure (as illustrated in  FIG. 4E ). For example, a CVA may be used to perform a lookup on a level one (L1) hash table to return a second-level structure for the associated cloud file. The offset for the data block could then be used to perform a level two (L2) lookup upon this second-level structure to determine whether the requested data block is cached and, if so, determine the address of the data block in PRC  430 . Note that lookup structure  434  and the contents of PRC  430  are stored in persistent storage to ensure that the cached data is not lost (e.g., does not have to be re-downloaded from cloud storage system  302 ) if cloud controller  420  crashes and/or reboots. 
       FIG. 4E  can also be used to describe the process by which new data blocks become referenced by lookup structure  434  and cached in PRC  430 . For instance, consider a scenario in which block record  436  is being created for a new data block that was just received from a client (not shown). Cloud controller  420  can store the new data block in an available space in PRC  430 , and then set the block pointer field in block record  436  to temporarily directly point to that location in PRC  430  (e.g., for an initial timeframe in which the new data block has not yet been written to cloud storage system  302  as part of a new cloud file). After the data block has been written to a cloud file, cloud controller  420  can then update lookup structure  434  appropriately (e.g., with the needed hash table entries and structures, and a pointer to the cached block in PRC  430 ), update the CVA and offset field in block record  436 , and then clear the block pointer in block record  436 . 
     Note that the use of a separate lookup structure  434  adds a level of indirection that can facilitate updating the system when disk blocks are flushed from PRC  430 . More specifically, when a disk block is being flushed from PRC  430 , cloud controller  420  can use the disk block&#39;s CVA and offset to look up and remove the pointer to the data block from the lookup structure  434 . In an alternative embodiment that did not use a lookup structure (e.g., instead having the block pointer in a block record point directly to a cached data block), cloud controller  420  might need to traverse filesystem metadata  424  to find and update a target block record or maintain a reverse mapping that facilitates looking up the block records for data blocks  432  stored in PRC  430 . 
     While  FIGS. 4A-4E  illustrate filesystem metadata and cached data blocks as being logically separate, in practice both metadata and data are interleaved in cloud controllers&#39; local storage systems. Some fragmentation issues can arise on cloud controllers over time, as cached data is flushed and new (and/or old) data is loaded from cloud files. Recall that cloud controllers typically maintain the full set of metadata, but flush actual data as needed. In general, as new data is created and written into files, target disk blocks are found to store the resulting data and metadata. To optimize reads and writes, a filesystem normally places metadata in close proximity to its referenced data. However, such organization can lead to fragmentation issues for a system in which data can be flushed (and reloaded), but metadata remains persistent. More specifically, if such data is stored interleaved and relatively continuously (e.g., to improve read access), the subsequent holes left by flushed data may be refilled with new metadata and data. Over time, these operations lead to substantial fragmentation, with small pieces of metadata strewn across the disk and a rarity of larger contiguous disk areas that can be used to store new file data. In some cases disk defragmentation techniques may be used to alleviate some of these issues, but defragmentation is typically both time- and disk-intensive, and client data access performance may be degraded during the defragmentation process. 
       FIG. 21A  illustrates metadata-based fragmentation in a storage drive  2102  of a cloud controller  2100 . Over time, opportunistic writes result in metadata blocks  2104  being randomly spread across the disk platters of storage drive  2102 . As a result, it can be difficult to find large, contiguous areas in which to write related file data. Splitting related file data across multiple small, separated disk areas involves additional seek and rotational latency, thereby increasing the total latency for both the initial write as well as subsequent reads. 
     In some embodiments, the block allocation policy used in a cloud controller&#39;s transactional filesystem is altered to prioritize a selected set of disk sectors toward either data or metadata. More specifically, by dynamically weighting some disk regions toward metadata, the filesystem can create dedicated, metadata areas on the disk that are distinct from their respective data blocks, and no longer interleaved on a per-file basis. While distinct, these metadata areas can still be allocated in close-enough proximity to the data blocks that they reference that both can be read without substantially degrading performance. When data is subsequently flushed, all of the disk blocks holding data are cleared, and new data and metadata can be written into the disk region; new metadata is written into the disk blocks weighted toward metadata, while the new data blocks can be stored into the nearby (flushed) disk regions. Because metadata is typically much smaller than the actual file data (e.g., in many scenarios metadata is on the order of 0.1% of the size of the file data that it manages), this arrangement facilitates avoiding fragmentation across a large number of write/flush cycles. 
     Note that the amount of metadata in a distributed filesystem may, depending on the client and application load, grow significantly over time. Hence, cloud controllers may need to periodically adjust the number and weighting of disk regions containing metadata (and/or a number of allocated metadata regions), so that more blocks that store metadata are allocated in proximity to a set of blocks used to store file data. Note also that in some embodiments the filesystem may attempt to optimize read accesses by trying to store (where possible) disk blocks that have been loaded from cloud files in proximity to their metadata. 
       FIG. 21B  illustrates an exemplary partitioning of a disk platter  2110  of a storage drive into distinct metadata  2112  and data  2114  storage regions. Note that while metadata and data are written to distinct regions ( 2112  and  2114 , respectively), a cloud controller may still attempt to locate metadata and data as close together as possible (e.g., by choosing target destinations that are in neighboring regions, and as close together as possible). Note also that  FIG. 21B  illustrates only a single example; allocation strategies may vary, resulting in layouts that are not as regular as illustrated and/or are adjusted over time. For example, metadata is typically accessed more frequently than data, so one layout policy may involve biasing toward storing metadata on “faster” sectors (e.g., on the outer edge of a disk platter, where spatial density is higher) and data on “slower” sectors. The actual layout of metadata and data storage regions may also be determined based on an expected (or tracked) load for the cloud controller and/or its clients. 
       FIG. 22  presents a flow chart that illustrates the process of managing metadata and data storage for a cloud controller in a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems. More specifically, the cloud controllers cache and ensure data consistency for the data stored in the cloud storage systems, with each cloud controller maintaining (e.g., storing) in a local storage device: (1) one or more metadata regions containing a metadata hierarchy that reflects the current state of the distributed filesystem; and (2) cached data for the distributed filesystem (operation  2200 ). During operation, the cloud controller receives an incremental metadata snapshot that references new data written to the distributed filesystem (operation  2210 ). The cloud controller stores updated metadata from this incremental metadata snapshot in one of the metadata regions on the local storage device (operation  2220 ). 
     In some embodiments, metadata and data may be divided across multiple storage volumes (e.g., multiple storage devices or redundant arrays of independent disks (RAIDs)). Storing metadata and its associated cached data on different storage volumes facilitates reducing seek latencies by allowing metadata and cached data to be accessed in parallel from their respective storage volumes. Note that such organizations may still involve splitting (and balancing) metadata and cached data across the different volumes; for instance, the metadata and cached data may be split such that each volume stores a subset of the metadata and a subset of the cached data, but no individual volume stores cached data referenced by its stored metadata. Such an arrangement allows metadata to be accessed in parallel from all of the volumes when high throughput is needed, while also still allowing associated sets of metadata and cached data to be accessed in parallel. Note also that each individual module may still be partitioned into metadata and data regions, to avoid the same fragmentation issues described above. 
     In some embodiments, a cloud controller may arrange data cached in local storage to optimize performance. For instance, the level of indirection described in  FIG. 4E  (e.g., by the lookup structure  434  used to track cached data blocks in PRC  430 ) facilitates rearranging cached data that was previously written to a storage device. For example, some applications and/or file formats continually append new data at the end of a file, thereby introducing potential fragmentation in a cloud controller that caches the data blocks for such a file. More specifically, while a cloud controller may be configured to try to place new data for such a file in proximity to the file&#39;s other cached data as new parts of the file are created, downloaded, and cached, in some situations this may not be possible (e.g., if there are currently no free data blocks in the vicinity of the other data blocks previously cached for the file). To alleviate such situations, a cloud controller may be configured to track file accesses to determine access trends and performance, and periodically perform a “defragmentation” operation for cached data to selectively relocate the cached data blocks for one or more files into a contiguous section of local storage, thereby improving read performance. Such defragmentation operations may involve copying a set of cached data blocks to a new location, updating lookup structures accordingly, and then freeing the original locations of the cached blocks. 
     In some embodiments, a cloud controller may be configured to perform defragmentation operations for cached data blocks based on a range of factors. For instance, a cloud controller may track usage statistics for file data blocks on an ongoing basis, and then use such tracking information to ensure that cached data blocks that are frequently accessed together are grouped together in the local cache. However, because tracking (and defragmentation in general) can involve fairly high overhead, a cloud controller may alternatively be configured to track and configure specific file types that are known to exhibit fragmentation, or to emphasize optimizing frequently accessed files whose tracking data indicates a certain common access pattern (e.g., a contiguous access pattern). In some embodiments, a locality policy may be used to specify: (1) specific file types to be considered and/or emphasized for defragmentation; (2) specific access patterns to detect and optimize for; and (3) a frequency and/or time interval for performing fragmentation checks and/or operations. 
     Pre-Fetching Cloud Files 
     As mentioned previously, cloud files are also written to in an incremental, transactional fashion. For instance, files that are written and/or modified across multiple snapshots may have data stored in different cloud files. Unfortunately, accessing data split across multiple cloud files can increase access latency substantially. Consider a client that sends a request to a cloud controller to access a data block in a file. The cloud controller inspects its current set of metadata, determines that the data block is not currently cached, downloads the corresponding cloud file containing the data block, and presents the data block to the client. The client may then request additional data blocks from the same file. In some scenarios, all of these blocks will have been stored in the same cloud file, and are now available in the cloud controller. If, however, some of these file blocks are in another cloud file, the client will have to endure additional network, download, and processing latencies as one or more additional cloud files are accessed. While an initial delay may be acceptable, recurring access delays may cause a noticeable delay and substantial user frustration. In some embodiments, pre-fetching techniques facilitate reducing the access latency experienced by a user accessing files from a distributed filesystem. 
     In some embodiments, a cloud controller predictively pre-fetches additional cloud files in an attempt to reduce access latency. For instance, upon receiving a request to access a given data block for a file, a cloud controller may analyze the metadata for the file and then predictively pre-fetch other cloud files that contain other nearby data blocks (or even all other data blocks for the file, depending on the file size). Alternatively (and/or additionally), the cloud controller may also pre-fetch data for other associated files that are likely to be accessed in conjunction with the original file. For instance, when a user browses a given file directory, the cloud controller may pre-fetch the first data block for every file in that directory and the contents of each subdirectory to anticipate the user&#39;s next actions. Alternatively, the cloud controller may pre-fetch files that are known to be associated with a requested file (e.g., an icon file that contains an icon representation for a selected file, along with any other support files for the requested file). In such situations, the cloud controller can traverse its stored set of metadata to look up the physical locations (e.g., the CVAs and offsets) for cloud files that should be pre-fetched from the cloud storage system. 
       FIG. 5  presents a flow chart that illustrates the process of pre-fetching data for a distributed filesystem. During operation, a cloud controller that maintains a set of metadata for the distributed filesystem receives a request to access a data block for a file (operation  500 ). The cloud controller traverses the metadata to identify a metadata entry that is associated with the data block (operation  510 ), and then uses this metadata entry to download a cloud file containing the data block from a cloud storage system (operation  520 ). While performing these operations, the cloud controller additionally determines that an additional cloud file in the cloud storage system includes data that is likely to be accessed in conjunction with the data block (operation  530 ), and proceeds to pre-fetch this additional cloud file from the cloud storage system (operation  540 ). 
     Note that NAS workloads are very user-based, and hence can be quite random as a large number of users access a fairly random set of files; the amount and range of pre-fetching performed may vary based on factors such as the available network bandwidth, cloud controller load, and types of accessing clients or applications. For instance, a basic optimization may involve pre-fetching data for other files in the same directory as the current file being accessed. More sophisticated pre-fetching techniques may involve receiving application (or user) feedback that specifies file associations and/or files that are likely to be needed soon. Cloud controllers may also be configured to track access patterns over time to determine files and file sections that have temporal locality. For example, consider a large (e.g., terabyte-sized) set of seismic data that is typically accessed sequentially. Cloud controller tracking may: (1) detect the sequential nature of the data; (2) make note of this behavior in the file&#39;s metadata; and then (3) perform selective pre-fetching upon successive accesses to ensure that each subsequent cloud file is pre-fetched, thereby providing latency-free file access for a client. Note that some of these techniques may also facilitate initially grouping commonly accessed files and/or data blocks into the same cloud file so that they can be accessed more efficiently at a later time. Pre-fetching configurations may be specified and customized for a range of different granularities (e.g., globally, per-site, or for individual cloud controllers) using locality policies (as described in a following section). 
     While the previous paragraphs describe techniques for identifying and pre-fetching additional cloud files, pre-fetching and caching decisions may also occur at the granularity of an individual cloud file. As described previously, a cloud file may contain data blocks associated with a large number of files. However, not all of these data blocks may be related to and/or useful for a given user request; furthermore, caching all of the data blocks for every cloud file accessed by a cloud controller can quickly fill the cloud controller&#39;s cache with potentially un-useful data. Hence, in some embodiments, cloud controllers may be configured to analyze local filesystem metadata and metadata from a cloud file being downloaded to determine the portions of the cloud file that should be downloaded and/or cached. 
     In some embodiments, a cloud controller performs “opportunistic caching” by using block metadata stored in a cloud file to determine other data blocks in the cloud file that should be downloaded and cached. For instance, the beginning of each cloud file may include a set of metadata for each data block that is stored in the cloud file. This metadata block specifies, but is not limited to, one or more of the following: a unique filename and/or number that identifies the file in the distributed filesystem that its corresponding data block in the cloud file is associated with; a compression algorithm used to compress the data block; the logical size of the data block; the physical size of the data block; a checksum for the data block; the checksum algorithm used to calculate the checksum; and the type of checksum. The cloud controller can analyze distributed filesystem metadata and metadata for the cloud file to determine which data blocks may be associated with the current block request, and hence should be cached. 
     Upon receiving the metadata from the cloud file, the cloud controller can determine the unique filenames and/or file identifiers that are stored in the metadata. However, the cloud controller may not be able to perform a reverse mapping to determine the location (e.g., path) of this file data in the distributed filesystem; for instance, distributed filesystem metadata may be stored in a manner that optimizes using file data structures to determine the CVA for a given file but does not include a reverse lookup mechanism that maps file identifiers back to a location in the distributed filesystem. If no such reverse mapping is available, scanning the metadata of the distributed filesystem to determine the location of a given file&#39;s data block may involve substantial computation and delay. However, the cloud controller does know the location of the requested file data block in the distributed filesystem, and thus can determine the file identifiers for files and directories in proximity to the requested file without substantial additional effort. The cloud controller can compare the set of file identifiers for the data blocks in the cloud file with the file identifiers for these proximate files and directories to determine which data blocks from the cloud file should be opportunistically cached; unrelated data blocks are not cached. As previously noted, related files have temporal and special locality, and thus are likely to be stored in the same cloud file and accessed again in a substantially similar timeframe; hence, such opportunistic caching techniques are likely to cache blocks that will be used in the near future, thereby improving access performance. 
     In some embodiments, a cloud controller downloading a cloud file may determine from the metadata in the cloud file that a large number of data blocks at the end of the cloud file are not likely to be needed, and request “transport termination” for the remaining unneeded portion of the cloud file, thereby saving network bandwidth. In some implementations where the cloud file is not encrypted, the cloud controller may be able to analyze the metadata from the cloud file to determine sections of the cloud file that are not needed, and instruct the cloud storage system to only transfer needed portions of the cloud file. In alternative implementations where cloud files are encrypted and compressed from beginning to end, additional portions of the cloud file may need to be transferred. More specifically, the metadata for the blocks of the cloud file can be stored at the beginning of the file data, and are analyzed upon receipt and decryption. Because of the serial encryption and compression, all data up to and including a given target data block will need to be downloaded, decrypted, and decompressed to allow the target data block to be accessed. If, however, data after the location of the target data block (or the location of some other beneficial data block later in the cloud file) is not determined to be of likely benefit, the cloud controller can instruct the cloud storage system to stop sending any data after that specified location. In some embodiments, the cloud controller can make this determination after receiving and analyzing the cloud file metadata, thereby ensuring that no extraneous data is sent from the cloud storage system. Such techniques can be particularly beneficial in situations where cloud files contain data blocks for a large number of unrelated files (especially if the target data is near the beginning of the cloud file) and in situations where metadata and/or application hints can indicate that only a limited set of data blocks from the cloud file will be accessed. 
       FIG. 23  illustrates a cloud controller  2300  that performs exemplary opportunistic caching and transport termination operations. During operation, cloud controller  2300  receives a request from a client  2302  for a specific file data block. Cloud controller  2300  determines the cloud file  2304  containing the target data block  2306 , and requests cloud file  2304  from cloud storage system  302 . Cloud storage system  302  begins sending cloud file  2304  to cloud controller  2300 ; the first set of data received by cloud controller  2300  is the block metadata  2308  for cloud file  2304 . Cloud controller  2300  already has the CVA for cloud file  2304  and the offset of the target data block  2306  in cloud file  2304 , but can use the additional information included in the (locally maintained) distributed filesystem metadata and the block metadata  2308  to determine a set of related data blocks  2310  that should be opportunistically cached in cloud controller  2300 &#39;s local cache. Cloud controller  2300  can also determine from block metadata  2308  the set of data blocks in cloud file  2304  that are not likely to be needed, and inform cloud storage system  302  of a transport termination point  2312  in cloud file  2304 ; cloud storage system  302  in response does not send any of the data blocks in cloud file  2304  that follow transport termination point  2312 . 
       FIG. 24  presents a flow chart that illustrates the process of transferring and caching a cloud file in a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems (operation  2400 ); the cloud controllers cache and ensure data consistency for the stored data. During operation, a cloud controller receives a request from a client for a data block of a target file that is stored in the distributed filesystem but not currently cached in the cloud controller (operation  2410 ). The cloud controller initiates a request to a cloud storage system for a cloud file containing the requested data block (operation  2420 ). As the cloud controller begins receiving the cloud file from the cloud storage system, it uses a set of block metadata in the portion of the cloud file that has already been received to determine the portions of the cloud file that should be downloaded to and cached in the cloud controller (operation  2430 ). 
     Receiving and Servicing Client Requests 
     Client systems typically use network protocols (such as the Network File System (NFS) and the Common Internet File System (CIFS) protocols) to access network-based storage systems. CIFS (also sometimes referred to as Server Message Block (SMB)) is a complex application-layer network protocol that includes many application-specific capabilities that blur the typical separation between filesystems and applications. When a user accesses a file on a client system using software (e.g., Microsoft Office) that supports CIFS functionality, the client system may send CIFS requests to the network storage system to inform that the file has been opened and to store specific data. The CIFS server receiving the request for the file operation recognizes the file type, and can provide application-specific support. For instance, the CIFS server may maintain a network connection, and after determining that only that client is accessing the file, instruct the client to continue to cache file data indefinitely. Later, upon detecting that another client is attempting to open the file, the CIFS server can instruct the first client to flush the cached file data, and provide collaboration capabilities to both clients. Such functionality can enhance the user experience, but can complicate the interface between clients and storage systems; crafting a high-performance implementation of the CIFS protocol can involve substantial effort and expense. 
     In addition to complexity, CIFS also suffers from a range of other drawbacks and/or limitations. CIFS does not allow requests to be proxied; all requests need to be addressed directly to the hosting server, which locks the underlying data. CIFS is also a “chatty” protocol (e.g., CIFS generates substantial bi-directional status and update traffic). In a distributed environment, these characteristics can result in substantial load and latency issues. 
     Embodiments of the present invention combine cloud controllers with NAS capabilities and cloud-based storage to provide a high-capacity, high-reliability storage system that can be accessed from multiple front-ends via an application-layer network protocol (e.g., CIFS). 
     In some embodiments, a customized filesystem device driver in an operating system decouples filesystem functionality from an underlying block storage mechanism, thereby allowing filesystem-level information to be forwarded to another filesystem and/or data management mechanism. For instance, in some embodiments a customized filesystem device driver in an operating system may forward such filesystem-level request information to a range of network storage devices and/or distributed architectures that can provide enhanced data storage capabilities to client computing devices. 
     Operating systems often support several standard filesystems (e.g., the DOS filesystem, the New Technology File System (NTFS), and CDROM filesystems), but sometimes also include an open interface that facilitates accessing special devices and providing third-party interoperability (e.g., to support USB flash drives and filesystems associated with other operating systems, and to allow the development and support of future filesystems). In some embodiments, such interfaces can be used to create a filesystem device driver that emulates a local filesystem and storage device to the local operating system, but actually instead forwards filesystem-level request information to (and receives responses from) a non-block-level storage management system and/or filesystem. 
     In some embodiments, the disclosed forwarding techniques facilitate leveraging an existing implementation of a request server to reduce implementation overhead and complexity. For instance, as described above, implementing CIFS involves substantial effort and expense. Hence, some embodiments may provide CIFS capabilities by executing an operating system (e.g., Microsoft Windows Server 2008 Core, which includes embedded, high-performance CIFS server functionality with low memory usage) that can manage CIFS requests, but then use a customized filesystem device driver to extract and forward filesystem-level information that was received in CIFS requests. Note that this is distinct from techniques that direct file operation requests to a filesystem (such as NTFS) that then directly performs corresponding block-level operations upon a block storage device; forwarding filesystem-level information facilitates adding additional layers of capabilities and services prior to (or in place of) performing lower-level storage operations. 
       FIG. 6A  illustrates a computing device  600  that receives and forwards requests for filesystem operations. Computing device  600  executes a request server  608  that receives requests for file operations from clients ( 610 - 612 ) in its computing environment  614 . Request server  608  sends instructions to a filesystem device driver  616  to perform the requested file operations. However, instead of managing a disk drive and disk operations, filesystem device driver  616  can be configured to forward filesystem-level information associated with the request to a range of other devices and/or mechanisms. For instance, filesystem device driver  616  may be configured to forward filesystem-level request information to one or more of the following: a cloud storage system  302  that is outside local computing environment  614 ; a storage management system  632  on another computing device  630 ; and/or an NAS device  640 . Note that NAS device  640  may comprise a range of capabilities and architectures. For instance, NAS device  640  may comprise a compute server that uses an NAS filesystem  642  (e.g., a transactional copy-on-write filesystem) and a range of local storage capacities  644  to handle network file requests. 
     In some embodiments, an NAS device serves as a cloud controller for the cloud storage system. In this role, the NAS device presents a standard request interface (e.g., CIFS) to clients, and uses local storage capabilities to cache the working data set that is being accessed in its local computing environment. The high-capacity cloud storage system stores the full data set for an enterprise, and serves as a backing store for the NAS device. This architecture combines the performance and interface of a local NAS device with the capacity of a cloud storage system, while reducing management complexity. 
       FIG. 6B  illustrates a scenario in which storage management system  632 , NAS filesystem  642 , and storage  644  are co-located on an NAS device, cloud controller  601 . For instance, filesystem device driver  616  may forward filesystem-level information from requests to storage management system  632 , which can then use this information to determine whether file data should be stored (or accessed) in NAS filesystem  642  and storage  644  and/or cloud storage system  302 . For instance, storage management system  632  may determine how to distribute and/or duplicate file information associated with the request between storage  644  and cloud storage system  302 . The local working data set for an organization is usually relatively small (compared to the full enterprise data set), and hence can typically fit into a reasonably provisioned local storage  644  mechanism. From the client perspective, data access remains substantially similar to the simplest NAS device scenarios described above; computing device  600  serves as a single point of contact, no load balancer is needed to map applications of clients to specific NAS devices, and clients  610 - 612  are unaware of the interaction between storage management system  632  and cloud storage system  302 . Note also that while request server  608  is not limited to receiving requests from local computing environment  614 , request server  608  may also be configured to service requests for other clients outside of local computing environment  614 . Similarly, in some scenarios one or more front-end computing devices  600  may be co-located with cloud storage system  302 . 
     In some embodiments, filesystem-level information can be forwarded by executing an operating system with a desired server capability (e.g., Microsoft Windows Server 2008 Core) in a virtual machine. Note that, as above, this “guest” operating system does not execute in a stand-alone configuration (e.g., with an underlying NTFS filesystem and disk drive), but instead can be configured to forward filesystem-level information (e.g., CIFS requests) to (and receive responses from) an underlying storage management system that provides data-consistent capabilities. For instance, a customized filesystem device driver in the guest operating system can forward request information to (and receive responses from) a storage management system in the host operating system. Note that such forwarding behavior in the guest operating system is distinct from typical storage operations for a guest operating system. Typically, upon receiving a request for a file operation, a guest operating system accesses an associated (virtual) filesystem and outputs a block-level storage request that is received and handled by the host operating system; hence, no filesystem-level information is sent to the host operating system. In contrast, in the described embodiments, the guest operating system forwards filesystem-level information, not block-level information, to the host operating system. 
       FIG. 6C  illustrates a cloud controller  602  that includes all of the above-described capabilities. More specifically, cloud controller  602  receives requests directly from clients, and also serves as a front-end to remote cloud storage system  302 . Cloud controller  602  includes a host operating system  604  that executes a guest operating system  606  in a virtual machine. Guest operating system  606  includes a filesystem device driver  616  that forwards requests for filesystem operations. A request server  608  (e.g., a CIFS server) in guest operating system  606  receives requests for file operations from clients ( 610 - 612 ) in its local computing environment  614 . Request server  608  sends instructions to filesystem device driver  616  to perform the requested file operations. However, instead of managing a disk drive and disk operations (and/or using a virtual filesystem to send block-level requests to host operating system  604 ), filesystem device driver  616  is configured to forward filesystem-level information associated with the request to a storage management system  618  in host operating system  604  (e.g., via a virtual device interface in host operating system  604 ). As in  FIG. 6B , storage management system  618  then determines how to distribute and/or duplicate file information associated with the request between local storage  620  and cloud storage system  302 . 
       FIG. 7A  presents a flow chart that illustrates the process of forwarding filesystem-level information. During operation, a computing device receives a request for a file operation from a client computing device (operation  700 ). In response, the operating system of the computing device issues one or more filesystem operations for the request (operation  710 ). A filesystem device driver receives and extracts filesystem-level information from these filesystem operations (operation  720 ), and then forwards the extracted filesystem-level information to a storage management system (operation  730 ). 
       FIG. 7B  presents a flow chart that illustrates the process of using a guest operating system to forward filesystem-level information. During operation, a cloud controller that serves as a front-end for a cloud storage system executes a guest operating system on a virtual machine (operation  740 ). When this guest operating system receives a request for a file operation from a client computing device (operation  750 ), a filesystem device driver for the guest operating system forwards filesystem-level information associated with the request to the host operating system of the cloud controller (operation  760 ). The host operating system then uses this forwarded information to manage a file associated with the request in the cloud controller and in a cloud storage system (operation  770 ). 
     In some embodiments, the customized filesystem device driver extracts, tracks, and forwards client file interactions on a per-file and a per-directory basis. More specifically, semantic filesystem-level information included in the application-layer network protocol (e.g., CIFS) is forwarded by the filesystem device driver to a storage management system. This semantic information can include, but is not limited to: a file name; a file type; a requested file operation (e.g., a read, write, or update operation); a set of application information associated with the file; one or more users accessing the file; and security information for the file. Cloud controllers can use this information to determine whether a file and its associated information should be cached locally and/or forwarded to the cloud storage system (or other devices accessing the cloud storage system, as described below). For instance, the storage management system may know that certain files will be duplicated and/or shared shortly after being modified, and hence may ensure that such files are both cached locally and forwarded to the cloud storage system to facilitate the expected duplication operation. 
     In some embodiments, decoupling a filesystem from underlying block storage devices facilitates transparently changing (e.g., either increasing or decreasing) the amount of storage space accessible by clients. Operating systems typically assume that filesystem device drivers always manage fixed-size volumes; storage devices normally have a fixed size, so this usually is not an issue. However, one of the benefits of using cloud-based storage is the ability to easily increase data capacity on demand. For instance, for the above-described scenario where a cloud controller caches data for a cloud storage system, the amount of space available to clients can be increased by leasing additional space in the cloud (network) storage system and communicating the change to clients as needed (e.g., upon request) through the filesystem device driver. Hence, in such embodiments the customized filesystem device driver and the disclosed caching architecture substantially simplify adjusting data storage capabilities. In contrast, expanding traditional storage systems typically may involve shutting down a storage device, physically adding additional storage devices, and then reconfiguring the entire storage system to accommodate the added storage space. 
     In some embodiments, volume size can be changed transparently regardless of any pending client operations. In alternative embodiments, the presence of some active filesystem connections and/or operations may require some or all connected clients to be disconnected during volume size changes (e.g., preventing some clients from actively accessing files), which may be infeasible or inconvenient. Hence, in some embodiments, the filesystem device driver may be configured to claim an initial fixed size that substantially overstates the expected amount of storage, to prevent future resizing logistics. The allocated portion of the cloud storage system may initially provide only a small subset of this claimed storage size, but then subsequently be dynamically expanded as needed. In some embodiments, such size configurations may be accompanied by user quotas to prevent storage space wastage. 
     Note that a filesystem device driver provides a higher level of abstraction than techniques that attempt to analyze disk-block-level traffic (e.g., in the disk-level remote storage system illustrated in  FIG. 2 ). More specifically, such techniques attempt to recreate filesystem information by reverse-engineering block-level storage transactions. However, because these storage transactions typically do not include higher-level (e.g., file- and directory-level) information, such attempts typically involve substantial effort, and do not provide the same level of tracking capabilities. In contrast, a customized filesystem-level device driver facilitates forwarding reliable high-level semantic information without additional processing overhead and without modifying the guest operating system. 
     In some embodiments, the described techniques provide filesystem-level proxy functionality. In many situations, proxies are implemented at the abstraction level of networking protocols, but this becomes more difficult as network protocols become more complicated (e.g., by adding application-specific information, as in CIFS). Hence, instead of re-implementing a complicated network protocol, some of the disclosed embodiments create a proxy at another, simpler layer by using a customized filesystem device driver that extracts and “tunnels” (e.g., forwards) filesystem-level information to another storage management system. Note that a filesystem-level proxy can also provide additional benefits. For instance, by emulating a local filesystem and storage device, the disclosed techniques can also overcome restrictions imposed by certain resource-intensive applications (e.g., certain databases, email server products, and/or data protection managers) to only use local storage devices. 
     Using Multiple Cloud Controllers to Access Cloud-Based Storage Via CIFS 
     The previous section described some possible architectures for cloud controllers. As described previously, two or more cloud controllers may work together to collectively manage and access a shared set of files that are stored in a cloud storage system. 
       FIG. 8  illustrates multiple cloud controllers  800 - 802  (as individually illustrated in  FIGS. 3-6C  and described above) that collectively manage data in cloud storage system  302 . Both cloud controllers  800 - 802  support application-layer network requests (e.g., CIFS requests) from their respective clients, and then collectively ensure data coherency and access performance for the shared data. As described previously, storage management systems in cloud controllers  800 - 802  incorporate aspects of a transactional copy-on-write filesystem, thereby ensuring that file operations are data-consistent and that the system can quickly recover from crashes. 
     Note that the described architecture can overcome limitations in an application-layer network protocol. As described above, CIFS does not allow requests to be proxied, and requires that a single hosting server manage the underlying filesystem. However, the disclosed filesystem forwarding mechanisms provide a level of abstraction where each guest operating system assumes it is the sole manager of the shared data. The underlying storage management systems in the host operating systems can pass application-level request information to one another to ensure that consistency is maintained. Thus, the disclosed techniques leverage existing CIFS implementations (via the guest operating system), but also extend CIFS capabilities to allow multiple CIFS servers to share access to underlying data. Note that the storage management systems may not need to understand all of the application-level information being received via CIFS requests; in some instances, the cloud controllers may simply forward such information to each other. However, for some data access scenarios (e.g., multiple clients accessing a shared file via different gateways), the storage management systems in each cloud controller may need to track and actively manage file operations to ensure that data consistency is maintained for the underlying data. Note also that receiving and filtering CIFS requests at the cloud controllers can reduce the amount of protocol chatter that travels over wide-area network links. For instance, a storage management system in a local cloud controller may be able to resolve some subset of CIFS requests, thereby eliminating the need to forward associated request information to the other components of the distributed storage system and reducing request latency. 
     In some embodiments, an additional network storage system may be used to provide another level of redundancy (and ensure high availability). For instance, in  FIG. 8 , either the storage management systems in cloud controllers  800 - 802  and/or cloud storage system  302  may be configured to mirror updates (e.g., also send metadata and data snapshots) to a mirror storage system  804  which is primarily used in failure situations. For instance, if cloud storage system  302  were to crash or become unavailable due to a network partition, cloud controllers  800 - 802  could be configured to temporarily use mirror storage system  804  as their backing store. As an emergency backup, mirror storage system  804  may include fewer resources (e.g., a smaller network link and/or less storage capacity) than cloud storage system  302 , but still ensure that availability and performance guarantees can be met. Note also that cloud controllers may be deployed as mirrored pairs, to ensure high availability across cloud controller failures. The following sections discuss redundancy and recovery options in more detail. 
     Disaster Recovery and Business Continuity 
     Some embodiments of the disclosed system architecture support the rapid recovery and/or replacement of cloud controllers. For instance, a new (or recovering) cloud controller immediately begins reading and processing the available metadata snapshots (e.g., in the case of a recovering cloud controller, the set of incremental metadata snapshots written to the cloud storage system since the cloud controller went offline). Because metadata is relatively small in comparison to the actual file data, this process can be performed more quickly than transferring a full data set. A cloud controller with relatively up-to-date metadata can immediately begin processing client requests; while the cloud controller may not yet have the requested data (or any data at all) cached, once it has retrieved a sufficient set of metadata it can already identify and access the cloud files containing requested file data. 
     In some embodiments, one or more additional cloud controllers serve as backups for an existing set of cloud controllers. These additional backup cloud controllers can take over the load of damaged and/or failing cloud controllers to provide disaster recovery and business continuity. 
       FIG. 11A  illustrates a set of cloud controllers  1100 - 1112  that manage and access data stored in a cloud storage system  302 . Backup cloud controller  1100  serves as a “hot backup” for cloud controllers  1102 - 1112 . During operation, cloud controllers  1102 - 1112  write incremental snapshots containing new metadata and data to cloud storage system  302 . Cloud controllers  1102 - 1112  continuously receive incremental metadata snapshot updates (e.g., either from cloud storage system  302 , as shown, or directly from the other cloud controllers), and update their local metadata with these updates to maintain a current view of the data stored in the distributed filesystem. Cloud controllers  1102 - 1112  also download data (e.g., stored in cloud files) from cloud storage system  302  in response to data requests from a set of clients  1114 - 1130 . 
     Note that in  FIG. 11A  backup cloud controller  1100  is not associated with any clients, but is instead configured to take over the load of another failing cloud controller. In some embodiments, backup cloud controller  1100  does not (in its backup role) serve data to any clients; instead, backup cloud controller  1100  primarily receives and synchronizes the metadata updates sent by cloud controllers  1102 - 1112  to maintain a current view of the data stored in the distributed filesystem. In some alternative embodiments, backup cloud controller  1100  may also be configured to assist with serving client data requests during times of exceptional system load. 
     In some embodiments, a backup cloud controller may also be configured to predictively download some of the data created and/or cached by the other cloud controllers of the system, in an attempt to download data that is in the working set of other cloud controllers. While a backup cloud controller that is provisioned similarly to the other cloud controllers can only store a partial subset of the full caches of the combined set of other cloud controllers, caching the full set of metadata and at least some of the current data actively being operated on in the system can be of benefit if the backup cloud controller needs to take over the role of a failed cloud controller. 
     For instance, a backup cloud controller that is configured to serve as a “hot backup” may continuously load the (most-recently-created) data that was most recently uploaded to the cloud storage system. Due to temporal locality, recently created data is very likely to be accessed again, so in some scenarios caching such data in a backup cloud controller may be an optimal caching choice. For example, consider a scenario in which a set of globally distributed cloud controllers collectively manage data stored in a cloud storage system. Due to geographic constraints, only a subset of the cloud controllers may be active (e.g., actively servicing client requests) at a time, thereby allowing a backup cloud controller using the above-described technique to cache a larger portion of each active cloud controller&#39;s active data set. In the most extreme scenario, if only one cloud controller is active, the backup cloud controller may mirror a substantial percentage of the working set of that one active cloud controller. If an active cloud controller fails, the backup cloud controller&#39;s cache is already loaded with much of the recent data. Alternatively, if an inactive cloud controller were to fail, the backup cloud controller has more time and leeway (due to the failing cloud controller&#39;s current inactivity) to transparently load a data set that more closely matches the working set of the failed cloud controller. 
     Alternatively, if constantly downloading the most-recently-uploaded data to the backup cloud controller incurs too high a bandwidth cost, the backup cloud controller may instead be configured to periodically update the local cache (e.g., hourly). Other options include tracking the most-commonly-accessed set of files in each cloud controller (and sharing this tracking information with the backup cloud controller, which then caches that specific data) and/or allowing system users and/or administrators to select the set of data that is cached on the backup cloud controller (e.g., using locality policies, as discussed in a following section). 
       FIG. 11B  illustrates the system of  FIG. 11A  after the failure of a cloud controller,  1102 . Backup cloud controller  1100 , which was previously primarily synchronizing metadata (and perhaps predictively downloading some of the data cached by cloud controllers  1102 - 1112 , as described above), takes over the load of failed cloud controller  1102 , and begins servicing requests from clients  1114 - 1116 . More specifically, because of its ongoing metadata synchronization efforts, backup cloud controller  1100  can already provide clients  1114 - 1116  instant access to any needed metadata, and can either serve locally cached data or access the cloud storage system as needed in response to client data requests. Note that a backup cloud controller will generally be configured to stop performing general caching efforts after taking over the role of a failed cloud controller; at this point, the backup cloud controller typically manages its cache in a manner that optimizes the performance of the data requests received from its clients. 
     In some embodiments, a backup cloud controller may be located at a location that is distinct from any other cloud controller, to ensure that a local disaster or issue does not also affect the backup cloud controller. However, in alternative embodiments, and in embodiments that include multiple backup cloud controllers (which might also serve in a load-balancing capacity when needed), some or all of the one or more backup cloud controllers may be co-located with active (non-backup) cloud controllers. Note that in scenarios that include multiple backup cloud controllers, individual backup cloud controllers may be associated with (and cache data for) a distinct subset of the active (non-backup) cloud controllers. 
     In some embodiments, backup cloud controllers (and/or other cloud controllers or other system entities) may be configured to automatically detect the failure of a cloud controller and perform a set of network and/or client reconfiguration options that automatically reroute requests from orphaned clients to a backup cloud controller. In scenarios where multiple backup cloud controllers are available, backup cloud controllers may also be configured to periodically track the network latency and bandwidth between each backup cloud controller and non-backup cloud controller; such information can assist in determining which backup cloud controller can provide better service to the orphaned clients. In alternative embodiments, rerouting requests from orphaned clients to a backup cloud controller may involve manual intervention by a system administrator (e.g., explicitly changing a distributed filesystem mapping in a local network to route client requests to the backup cloud controller). 
     Note that transitioning from a failed cloud controller to a backup cloud controller may not be completely transparent from the perspective of orphaned clients of the failed cloud controller. For example, a client may have been transmitting data to be written to the failing cloud controllers, or the failing cloud controller may have been preparing (or even already been transmitting) data and/or metadata updates to the cloud storage system. Data in transit to or from a failed cloud controller should typically be considered lost; a primary consideration for the distributed storage system is that no data corruption occurs at the remaining cloud controllers or the cloud storage system. 
     In some embodiments, depending on the desired failure characteristics, the distributed storage system may have different modes of operation. For instance, in some modes of operation cloud controllers may perform strict ordering of updates; e.g., always upload an incremental data snapshot before its associated incremental metadata snapshot, to ensure that the global system metadata isn&#39;t updated until the uploading cloud controller has confirmed that both the data and metadata have been successfully received by the cloud storage system. For example, the cloud controller may calculate checksums (e.g., MD5 checksums) prior to uploading each snapshot, and confirm that these checksums match checksums generated by the cloud storage system upon receiving and successfully storing the snapshots. In the worst case, such strict ordering may result in the loss of some storage space in the cloud storage system (e.g., if the cloud controller fails after uploading snapshots but prior to sending notification of the metadata and/or data updates to other cloud controllers); this storage space can be recovered during cloud-controller recovery or via administrator actions. 
     In alternative embodiments, incremental metadata snapshots are uploaded first (to propagate the awareness of new data throughout the system as quickly as possible), with data snapshots following. In such embodiments, a cloud-controller failure that occurs between the upload of the metadata and data snapshots may result in metadata references to unavailable cloud files. In such situations, data corruption issues may need to be resolved by administrator actions or by inspecting log data to revert the metadata uploaded by the failing cloud controller. 
       FIG. 12  presents a flow chart that illustrates the process of providing disaster recovery for a distributed filesystem. During operation, two or more cloud controllers collectively manage the data of a distributed filesystem (operation  1200 ). More specifically, whenever each cloud controller receives new data from a client, it sends incremental metadata snapshots for the new data to the other cloud controllers, and sends an incremental data snapshot containing the new data to a cloud storage system. A backup cloud controller associated with the distributed filesystem is also configured to receive each (incremental) metadata snapshot (operation  1210 ), such that, upon determining the failure of a cloud controller, the backup cloud controller can immediately begin receiving data requests from clients associated with the failed cloud controller (operation  1220 ). 
     Multiple Cloud Storage Providers 
     Cloud storage providers may be differentiated (and selected) based on a range of characteristics including, but not limited to:
         capacity;   cost per unit of storage, network transfer, access, and/or processing activity;   request response/performance (e.g., internal latency when servicing a request);   frequency and/or severity of outages (with less reliable cloud storage providers typically offering cheaper pricing to make up for the potential unreliability);   location (which may involve political concerns) and network latency; and   level of replication/redundancy and number of points of presence (POPs) (e.g., some higher-cost cloud storage providers may immediately internally replicate newly stored data to multiple POPs at different geographic locations, thereby ensuring very high availability and low-latency access from anywhere in the world).
 
Choosing an appropriate cloud storage provider for a distributed filesystem may also depend on determining an anticipated data set and access patterns.
       

     In some embodiments, multiple cloud storage providers may also be used to provide high availability and disaster recovery. For example, data may be mirrored across multiple cloud storage providers to ensure that data is still available to clients even if there is an outage in one of the cloud storage providers. Alternatively, multiple different cloud storage providers that provide different tiers of performance (and have different cost structures) may also be combined to balance performance and overall system cost. 
       FIG. 13A  illustrates a set of cloud controllers  1300 - 1302  that manage and access data mirrored across multiple cloud storage systems (e.g., multiple cloud storage systems hosted by multiple distinct cloud storage providers). During operation, cloud controllers  1300 - 1302  write incremental data and metadata snapshots to both cloud storage systems  1304  and  1306 . Load (e.g., cloud controller read requests) may be balanced between the two cloud storage systems depending on a number of desired criteria. For instance, if cloud storage systems  1304  and  1306  typically provide comparable performance, but cloud storage system  1304  has a lower per-access cost because it has occasional outages, the cloud controllers may be biased to primarily read from cloud storage system  1304 , and only access cloud storage system  1306  when  1304  is suffering from an outage. In another example, cloud storage system  1304  may be more reliable and cheaper than  1306 , but have worse performance due to fewer POPs (and, as a result, higher network latencies); in such a scenario, the cloud controllers might be biased to optimize performance by primarily reading data from cloud storage system  1306 . Note that such choices are mostly transparent to the clients of cloud controllers  1300 - 1302 ; clients continue to access cloud controllers as previously, but may notice different levels of file access performance depending on the underlying cloud storage system choices (e.g., the cloud controllers&#39; caches may hide some, but not always all, of the performance and latency aspects of an underlying cloud storage system). 
     In some embodiments of a distributed filesystem with data mirrored across multiple cloud storage systems, a cloud controller may be configured to immediately write a cloud file to a first cloud storage provider (thereby allowing the data to be propagated to other cloud controllers), but then delay the transfer of the cloud file to the mirror to a time when network bandwidth is cheaper. In such embodiments, the cloud controller may be specially configured to ensure that the cached local copy of the data in the cloud file is not flushed until after it has been mirrored to the second cloud storage provider. 
       FIG. 13B  illustrates an exemplary CVA format  1310  that facilitates identifying and locating cloud files across different cloud storage configurations. As described previously, accessing a block from a cloud file involves using a CVA to download the needed cloud file and then using the offset into the cloud file to access a specific target block. Fields in exemplary CVA format  1310  include, but are not limited to: a cloud controller ID (CCID) that uniquely identifies the originating cloud controller for the respective cloud file (e.g., the cloud controller that uploaded the cloud file); a filesystem ID (FSID) that identifies one or more cloud storage providers and/or other storage options; a snapshot ID (SSID) that uniquely identifies the updates of each cloud controller (e.g., each CCID+SSID combination in the system is unique); and a file ID (FILEID) that identifies the cloud files within each snapshot (e.g., each FILEID for a given SSID is unique). Note that ensuring the uniqueness of CVAs can be quite important for avoiding data corruption, because cloud storage providers typically serve as “black boxes” to the cloud controllers. For instance, a cloud controller may not know anything about the organization or layout of data in a cloud storage system, but may simply use a CVA to generate a unique file name and/or identifier for a cloud file to be stored to the cloud storage system. For example, in the context of the exemplary CVA format  1310  of  FIG. 13B , a CVA may be converted to an object (file) name such as:
         “data-&lt;CCID&gt;-&lt;FSID&gt;-&lt;SSID&gt;-&lt;FILEID&gt;.snp”
 
By using such naming conventions, a CVA can easily be used to determine the identifier for the cloud file; during a subsequent access, the same (or another) cloud controller can use the same CVA to generate and present the same file name to the cloud storage system and receive in return the cloud file that was stored in association with that CVA. Hence, the fields of CVA format  1310  need to be sufficiently provisioned to ensure that two cloud files are never written to the same cloud storage system for the same identifier; for example, CVA format  1310  may include a 64-bit SSID field to ensure that there will never be duplicate snapshot numbers for a cloud controller.
       

     In some embodiments, an FSID field can be used to support multiple cloud storage providers. For instance, part of the FSID field may form a bit vector that indicates one or more cloud storage providers that are currently storing an associated cloud file. In another example, a cloud controller may use a value stored in a CVA&#39;s FSID field to perform a lookup in a table of cloud service provider credentials. This table may include a list of cloud storage providers that are currently storing the cloud file, as well as “cloud account” information (e.g., information identifying a specific user account at a cloud storage provider and credentials that are needed to access that user account). Note that in addition to accessing different cloud storage providers, a cloud controller may also be configured to access different cloud accounts at the same cloud storage provider (e.g., different user accounts with different configurations and/or levels of service at the same cloud storage provider). FSIDs may also be used to support other filesystem-level features, such as storage quotas. 
     In some embodiments, data in the distributed filesystem may be split across multiple different cloud storage providers based on factors such as access frequency, age, and cost. For instance, new data may initially be written to a higher-cost cloud storage provider that instantly replicates the stored data across multiple POPs; this wide initial distribution allows other cloud controllers requesting the new data (and metadata) to download it quickly. At some subsequent time, data that is no longer frequently accessed may be migrated into a cheaper lower-tier cloud storage provider (e.g., a cloud storage provider with higher latency and lower cost) and deleted from the first cloud storage provider. Such moves may be performed asynchronously and as background operations to ensure that users accessing cloud controllers and data are not adversely affected. For example, data may be migrated to the lower-tier cloud storage provider at a time of day when the load of the distributed filesystem and the cost of network bandwidth are both lower. 
       FIG. 14  illustrates the process of migrating a cloud file  1400  from a first cloud storage system  1402  to a second cloud storage system  1404 . Upon determining that all of the contents of cloud file  1400  are no longer being frequently accessed, cloud controller  1406  downloads cloud file  1400  from cloud storage system  1402  and then writes cloud file  1400  to cloud storage system  1404  (operation  1  in  FIG. 14 ). Alternatively, if the cloud storage providers support direct transfers, cloud controller  1406  may instead be able to instruct cloud storage system  1402  to copy cloud file  1400  directly to cloud storage system  1404  (operation  2  in  FIG. 14 ), thereby saving the overhead (and additional network bandwidth) involved in having cloud controller  1406  be an intermediary for the transfer. 
     In some embodiments, multiple factors are considered prior to migrating data between cloud storage providers. For instance, in some scenarios deciding whether to migrate a given cloud file may involve considering: the cost of storage at both the source and target cloud storage providers; a variable network bandwidth cost and/or the network bandwidth cost for the transfer; the access frequency and/or history for the contents of the cloud file; the potential performance impact of moving the cloud file to a lower tier; and the load of one or more cloud controllers. In some scenarios, cloud controllers actively monitor the cloud files and/or data files that they “own” (e.g., created) to determine how frequently they are accessed, and then use this information to determine candidates for migration. For example, a cloud controller may track the most recent access (e.g., the last read time) for individual blocks in its local persistent read cache (and/or in a persistent read cache that is distributed across multiple cloud controllers). After the last block for a cloud file is evicted from the read cache (e.g., due to not being used recently), the cloud controller may initiate a counter; if no blocks from the cloud file are used before the counter reaches zero, the cloud file becomes a candidate to be moved to a lower tier. Alternatively, the cloud storage system may be configured to track how often each given cloud file is accessed; the cloud controller that created a drive file may also check this access log to determine data that is no longer frequently used. Note that the above scenarios keep a cloud file in the higher tier cloud storage system if any of its blocks are still being actively used. In other scenarios, such decisions may be more complex (e.g., migration choices may also be affected by user-defined locality policies and/or cost-performance trade-offs). 
     In some embodiments, migrating a cloud file to a different cloud storage provider and deleting the copy from the previous cloud storage provider involves some additional logistical operations and/or policies to ensure that cloud controllers can still access the cloud file as needed. For instance, in one system metadata may not be updated to reflect migrations, and cloud controllers are configured to incrementally check cloud storage providers in priority order (using the same CVA as an identifier) until the target cloud file is found. In other scenarios, the metadata for files may be updated to indicate that the cloud file has moved. Such indications may range from changing a bit field that is associated with a CVA (but not included in the actual CVA itself) to actually creating a new CVA that correctly references the cloud file on the new cloud storage provider. In the latter scenario, updating the metadata may be a multi-step operation. For example, a cloud controller may: (1) determine that a given cloud file should be migrated; (2) determine a new CVA for the cloud file at the new cloud storage provider; (3) upload the cloud file to the new cloud storage provider using the new CVA as the identifier; (4) upon receiving confirmation of receipt from the new cloud storage provider, update the metadata for all of the file blocks in the migrated cloud file to point to the new CVA (ideally without otherwise modifying the access history or other characteristics for the file blocks); (5) distribute an incremental metadata snapshot to the other cloud controllers; and (6) after enough time has passed to ensure that the other cloud controllers have updated their metadata, delete the cloud file from the old cloud storage provider. 
       FIG. 15  presents a flow chart that illustrates the process of distributing data for a distributed filesystem across multiple cloud storage systems. During operation, two or more cloud controllers collectively manage the distributed filesystem data that is stored in one or more cloud storage systems (operation  1500 ); the cloud controllers cache and ensure data consistency for the stored data. Whenever each cloud controller receives new data from a client, it sends an incremental metadata snapshot for the new data to the other cloud controllers and an incremental data snapshot containing the new data to a cloud storage system. Data stored in the distributed filesystem can be distributed across two or more cloud storage systems to optimize performance and/or cost for the distributed filesystem (operation  1510 ). 
     Archival Cloud Storage Providers 
     In some embodiments, a distributed filesystem can also leverage an “archival” cloud storage system. Archival cloud storage systems typically provide storage for a very low cost, but are equipped with a limited set of storage capabilities that are geared toward storing data that is infrequently accessed. For instance, archival cloud storage systems may be architected such that new data can be received and written immediately, but read requests for stored data sometimes involve potentially substantial delays (e.g., sometimes on the order of multiple hours, or longer). Such relaxed service constraints allow the cloud storage provider to optimize its data center to substantially reduce costs (e.g., by receiving data in a rotating set of active servers and powering down all other servers and storage devices completely, thereby dramatically reducing power and cooling costs); storage costs for archival cloud storage systems may be one or more orders of magnitude cheaper than non-archival storage systems. While such access constraints may not be appropriate for data that is being actively accessed by users, such storage capabilities can provide low-cost storage options for certain sets of archival data in a distributed filesystem. 
     In some embodiments, data writes can be mirrored to an archival storage system for disaster recovery. In such embodiments, writes can be mirrored as described previously (for multiple tiers), but the archival storage system would typically only be read if a primary (non-archival) cloud storage system were to be irretrievably lost (e.g., if the primary cloud storage provider were to go out of business or suffer from a substantial national disaster). In such arrangements, data might be sequentially read out of the archival storage system (e.g., in order of priority) and then immediately written into another non-archival cloud storage system. 
     In some embodiments, the distributed filesystem collects and writes a set of archival data that is being retired from active use to an archival cloud storage system. This archived data will typically no longer be directly accessible by cloud controllers, but instead would need to be recovered by an administrator of the distributed filesystem. 
     As described earlier, cloud controllers notify each other whenever new data is created in the distributed filesystem. More specifically, cloud controllers periodically generate snapshots that reference every block, including both data and metadata, that is valid and stored in the cloud storage system at the time that the snapshot is taken. These snapshots are stored using data structures that allow each cloud controller to determine the set of new data and metadata that have been created in a recent time interval. Each cloud controller compares its two most recent snapshots (e.g., snapshot A and a later snapshot B) to determine the incremental difference between the two snapshots. For instance, data blocks that were created after snapshot A but before snapshot B (and are still referenced by snapshot B) are considered part of this incremental difference. Furthermore, any data blocks that were freed (e.g., deleted) between the snapshots are also considered part of this incremental difference. This incremental difference can be logically separated into incremental differences in metadata (e.g., new metadata created to reference newly created file data blocks) and incremental differences in data (e.g., the actual newly created file data blocks). In this disclosure, incremental differences in metadata are referred to as incremental metadata snapshots, and incremental differences in data are referred to as incremental data snapshots. 
     Each cloud controller stores both sets of incremental differences (e.g., the incremental metadata snapshot and the incremental data snapshot) in a cloud storage system. After uploading an incremental metadata snapshot to the cloud storage system, each cloud controller notifies the other cloud controllers. Each cloud controller then downloads these incremental metadata snapshots from the cloud storage system, updates its local metadata based on the downloaded metadata, and then deletes the downloaded incremental metadata snapshot. Other cloud controllers can then use the new metadata to access the new data stored in the incremental data snapshot (in the cloud storage system). In some scenarios, a cloud controller may also receive metadata (and data) directly from other cloud controllers. As mentioned previously, owning cloud controllers maintain data consistency by arbitrating multiple attempts to simultaneously write a given file. Cloud controllers can request updates directly from other cloud controllers if the data in the cloud storage system (or in a requesting cloud controller) is not up-to-date. In this scenario, the cloud controller storing the most recent copy of a file will determine the incremental difference between what the requesting controller has and the up-to-date version, and send the incremental difference directly to the requesting cloud controller. 
     Stored snapshots can be used for a range of features, including, but not limited to: “rolling-back” the filesystem to an earlier date; recovering accidentally deleted files; and archiving data from the distributed filesystem. The data structures used to store snapshots facilitate storing periodic snapshots compactly. For instance, a cloud controller may store hourly snapshots that capture the full state of the system at each hour interval; instead of storing each snapshot as a separate entity, however, these data structures may store incremental differences in a manner that is more compact but still allows each hourly snapshot to be accessed as needed. For example, in this scenario the incremental difference would reference the full set of changed data and metadata for all cloud controllers over the specified time interval (as opposed to the incremental metadata and incremental data snapshots, which only specify the set of changed data and metadata for one cloud controller). When using such an organization, accessing a desired snapshot for a previous time may involve creating a copy of an even earlier snapshot, and then applying one or more incremental differences for snapshots subsequent to the earlier snapshot to generate the desired snapshot. 
     A cloud controller may also periodically create “full” snapshots (e.g., complete, distinct copies of the metadata for the system at a given time that can be accessed without having to apply incremental differences). In summary, in this disclosure:
         1. the term “snapshot” refers to a complete set of references for the distributed filesystem at a given time;   2. a subsequent snapshot may be accessed by starting with an earlier snapshot and applying a set of incremental differences;   3. the term “incremental metadata snapshot” refers to a set of incremental differences that identify a set of metadata that has changed on a specific cloud controller since its previous snapshot;   4. the term “incremental data snapshot” refers to a set of incremental differences that include a set of data that has changed on a specific cloud controller since its previous snapshot; and   5. the term “full snapshot” refers to a specific type of snapshot that can be accessed without having to apply any incremental differences between snapshots.       

       FIGS. 16A-16B  illustrate an exemplary process for archiving data in a distributed filesystem.  FIG. 16A  illustrates a set of snapshots that are taken for the distributed filesystem over time. For instance, a designated cloud controller may be configured to write a snapshot on an hourly basis. Note that storing a large number of snapshots may consume substantial storage space (even when using delta encoding to store incremental differences); hence the system may be configured to retire some of the snapshots for older time intervals. For example, the system may (as illustrated in  FIG. 16A ) store hourly snapshots for a recent set of days, but then only maintain a select set of daily snapshots for recent weeks, a select set of weekly snapshots for recent months, and a select set of monthly snapshots for past years. 
     During an archival operation, a cloud controller analyzes file metadata changes for a time interval between two snapshots (e.g., snapshots  1600  and  1602 ) to determine files that existed and were deleted in the time interval  1604  but are still actively stored in cloud files in a cloud storage system. For example, cloud controller  1606  may be able to determine from snapshots  1600  and  1602  (and/or the intervening and surrounding snapshots) that a given file that existed (and/or was created) in snapshot  1600  and deleted in snapshot  1602  is still stored in a cloud file in a non-archival cloud storage system. If this deletion occurred sufficiently far in the past (e.g., more than 30 days ago), the cloud controller determines that the active lifespan of this file is over, and that any data for the file that is stored in cloud files no longer need to be kept in active storage. 
       FIG. 16B  illustrates how cloud controller  1606 , after determining from past snapshots that all of the blocks in cloud file  1608  were previously deleted, moves cloud file  1608  from cloud storage system  1610  to archival cloud storage system  1612 . As in  FIG. 14 , this transfer may involve either cloud controller  1606  as an intermediary (operation  1  in  FIG. 16B ) or a direct transfer from cloud storage system  1610  to archival cloud storage system  1612  (operation  2  in  FIG. 16B ). 
     The archival operation illustrated in  FIGS. 16A-16B  describes using data gleaned from previous snapshots to determine data that can be archived. In some embodiments, cloud controllers can also actively track deletes to determine archival candidates. For instance, each cloud controller can keep track of data blocks in the cloud files it previously uploaded to the cloud storage system (e.g., by maintaining a bitmap for each cloud file), and then mark deleted blocks whenever a file is deleted. When all of the blocks in a cloud file are marked as being deleted, the owning cloud controller can move the cloud file to the archival cloud storage system. Alternatively, the cloud controller may instead add the cloud file to a special delete list for a specified interval (e.g., 30 days) to ensure that the data still remains temporarily available and quickly recoverable for that interval in case of an accidental delete or other similar issue; if no client or cloud controller accesses the cloud file during this interval, the cloud file is then moved to the archival cloud storage system. One benefit of techniques that track deletes is that they can detect and archive files that were created and deleted between snapshots (e.g., depending on the time granularity between snapshots). In some embodiments, snapshot-comparison and delete-tracking approaches can be combined into a hybrid technique that more accurately archives unneeded data. 
     Note that in some embodiments a cloud file typically is only archived if all of the blocks of the cloud file have been deleted; if any of the blocks have not been deleted, the cloud file remains in non-archival cloud storage. In alternative embodiments, individual blocks in a cloud file may be archived, and cloud files can be shrunk to conserve cloud storage space. For instance, if a subset of the blocks in a cloud file are moved to an archival cloud storage system, the portion of the cloud file remaining in the primary cloud storage system may need to be reconstructed to preserve the remaining blocks. For example, filling the archived blocks of a cloud file with zeros and then compressing the cloud file effectively reduces the amount of storage space needed for the empty blocks without changing block indexing and accesses for the cloud file. In a specific (small) example, for an exemplary cloud file containing blocks “ABC”, a cloud controller archiving block B could generate a modified cloud file “A0C” for the primary cloud storage system and “0B0” for the archival cloud storage system (where ‘0’ represents a block filled with zeros). If block ‘C’ were later archived, the blocks could be updated to become “A00” and “0BC”, respectively. 
     In some embodiments, a storage administrator manages and/or initiates archival operations. Alternatively, such operations may be scheduled at regular time intervals. Note that some archival techniques (e.g., the snapshot-comparison technique) can be initiated on a single cloud controller that has access to the collected snapshot data. Furthermore, archival operations may be initiated as low-priority background jobs (in comparison with client accesses) and/or on a cloud controller during a timeframe that the cloud controller typically has low load. Alternatively, such operations may be initiated on a backup cloud controller if available, especially if backup cloud controllers are configured to not receive client requests. Archival transfers can also be timed to occur when network bandwidth is cheaper. 
     In some embodiments, a distributed filesystem may include additional tables (or other structures) that facilitate accessing archived data; note that reading data from an archival cloud storage system may involve special access techniques and/or administrator assistance. After ensuring that a cloud file has successfully been copied to an archival storage system, a cloud controller can delete that cloud file from the originating (non-archival) cloud storage system. At this point, the cloud controller may generate an incremental metadata snapshot to indicate that the metadata stored in previous snapshots can no longer be used to access the deleted data that was previously stored in the (now deleted) cloud files. However, a different set of tracking structures will need to be updated to reflect that the newly archived data in the archival cloud storage system can still be accessed if needed. For instance, archived data may be tracked in a separate set of metadata, and indexed by date such that an administrator can selectively recover and access archived data within a desired timeframe (e.g., data that is approximately one year old, data that existed during a certain timeframe, or data that was deleted between two specified dates) as needed. 
     Note that recovering archived data may also involve recovering archived snapshots. Because only a subset of snapshots are kept over time, a cloud controller performing an archival operation may also write an old snapshot of the distributed filesystem to the archival cloud storage provider; these old snapshots can be re-populated into the cloud controllers at a later point if needed to access cloud file data that is restored from archives. If the archived snapshot is written together with the deleted data that it references, a recovery operation may recover both simultaneously. For example, the index of archived data may indicate the deletion timeframe, original lifespan, data size, format, and identifier for each archival record, thereby facilitating archival recovery. 
     Note also that, in some embodiments, archival operations can also be opportunities to defragment and/or reorganize data. As described previously, cloud file size is often chosen to balance download throughput and latency; for data being archived, these are no longer issues, and hence cloud file constraints may be eliminated. For instance, file data distributed across multiple cloud files may be re-written into contiguous files (that can be recovered more easily in a single access) regardless of size during archiving. In such implementations, an index of archived files may simply store filenames, relevant metadata (e.g., creation and deletion dates, originating cloud controller, size, etc.), and a reference identifier to access the data from the archival cloud storage system. 
     In some embodiments, cloud controllers can also serve as intermediaries to an archival cloud storage provider for backup purposes. For instance, cloud controllers may implement a virtual tape library (VTL) interface that allows clients to perform backups as if the cloud controller were a tape backup device, with the data instead being written to the archival cloud storage provider. These techniques allow existing client backup infrastructure to be used transparently while adding additional capabilities (e.g., offsite storage in the archival cloud storage provider, redundant simultaneous writes to multiple archival cloud storage providers, etc.). Note that the high read latency of archival cloud storage providers is comparable to that of typical tape backup systems, and hence not a drawback in such usage scenarios. 
       FIG. 17  presents a flow chart that illustrates the process of archiving data for a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems (operation  1700 ); the cloud controllers cache and ensure data consistency for the stored data. Whenever each cloud controller receives new data from a client, it creates an incremental metadata snapshot for the new data that is propagated to the other cloud controllers and an incremental data snapshot containing the new data that is sent to a cloud storage system. During operation, a cloud controller determines that a cloud file in a previously stored data snapshot is no longer being actively referenced in the distributed filesystem (operation  1710 ). For instance, a cloud file from a previous data snapshot may no longer be referenced if all of the blocks it contains have been deleted some time ago. The cloud controller transfers this cloud file from the (first) cloud storage system to an archival cloud storage system (operation  1720 ), thereby reducing storage costs while preserving the data in the cloud file in case it is ever needed again. 
     Accessing Cloud Controllers as Peer Caches 
     Accessing cloud files involves some overhead—a cloud controller seeking to download a target file that is not locally cached needs to: (1) contact a cloud storage system hosting one or more cloud files that contain the target file&#39;s data blocks; (2) download some or all of the target cloud files; and (3) if the cloud files are encrypted, decrypt the portions of the target cloud files that precede and include the target data blocks. Delaying factors may include the distance and network latency between the requesting cloud controller and the cloud storage system, the load and delay in the cloud storage system, the size of cloud files, and the location of the target block in the cloud file. Furthermore, accessing the cloud storage system may be associated with an access cost and/or network bandwidth cost. Hence, while a requesting cloud controller will typically eventually get the file from the cloud storage system, accessing the data blocks for the target file may involve considerable delay. If another (peer) cloud controller in the distributed filesystem is already caching desired data blocks, requesting and transferring cached data blocks from a peer cloud controller potentially facilitates accessing the desired file data more quickly (and cheaply). 
     In some embodiments, cloud controllers can query one or more of the other cloud controllers of the distributed filesystem to locate and access a needed data block that is already being cached by a peer cloud controller. Each individual cloud controller strives to determine the working set of its clients and locally cache a set of data that reduces the average client access latency. However, cloud controllers have limited storage capacity, and hence can only cache a limited amount of data. Inter-cloud-controller cache-sharing techniques can effectively increase the size of a cached working set, thereby potentially reducing average file access latencies as well as the number of requests that are sent to the cloud storage system. Furthermore, such techniques transfer only the needed data blocks; in contrast, accessing cloud files from the cloud storage system to access the same set of data blocks potentially transfers a larger amount of data (due to the size and organization of cloud files). 
     However, attempts to leverage the caches of other cloud controllers can also introduce additional overhead and trade-offs. As described previously, a cloud controller writes new data received from a client to a cloud storage system. The cloud controller will typically continue to cache this data locally while it is actively used by its clients; data that is no longer actively used is eventually pushed out of the local cache by other frequently and/or recently accessed data, but can be re-loaded if needed from the cloud storage provider. Thus, while it is possible that other cloud controllers may cache data that a requesting cloud controller needs, determining which cloud controllers are likely to be currently caching a needed file block is difficult. One option is to query all of the cloud controllers in the distributed filesystem. Unfortunately, while broadcasting requests to all cloud controllers increases the probability of finding a cached block, processing and/or responding to such requests may substantially increase each cloud controller&#39;s load. As a result, broadcast techniques become increasingly inefficient as the number of cloud controllers in the distributed filesystem grows. Furthermore, cloud controllers that are distributed across a large internal network or wide-area network are typically not be in the same broadcast domain; hence, broadcasts will require a large number of point-to-point messages. Such messages can increase network load, potentially to the point of slowing down accesses to the cloud storage system. Directory-based techniques that track the location and status of cached data blocks provide an alternative to broadcast-based techniques, but also require storage space and substantial message update traffic to keep the cache directories up-to-date. 
     In some embodiments, cloud controllers are configured to query a limited subset of peer cloud controllers in the distributed system for cached data blocks. For instance, a cloud controller may be configured to only send such “peer cache requests” to: one or more co-located cloud controllers; the cloud controller that owns (e.g., created) the cloud file containing a needed data block; a backup cloud controller; one or more cloud controllers that are determined to be in close network proximity to the requesting cloud controller; and/or one or more cloud controllers that are specified using a locality policy. 
     In some embodiments, a cloud controller is configured to query the caches of one or more peer cloud controllers for a data block in parallel with sending a request to the cloud storage system for the cloud file containing the data block. If a target peer cloud controller is indeed caching the data block, it can send the cached data block to the requesting cloud controller, which can then cancel and/or interrupt the transfer of the cloud file from the cloud storage system (e.g., depending on whether the cloud storage system is already transferring the cloud file or is still processing the request). If the peer cloud controller indicates that it is not caching the data block (or does not respond at all), the requesting cloud controller still receives the cloud file from the cloud storage system, and caches the requested (and now received) data block. Performing these two operations simultaneously: (1) ensures that peer cache requests do not increase the worst-case latency for a data block access; (2) adds only a small number of additional network messages and cloud controller look-ups (that are proportional to the number of peer cloud controllers that are queried); and (3) in the best case facilitates transferring needed data blocks to a requesting cloud controller more quickly than would be possible from the cloud storage system, thereby allowing the requesting cloud controller to quickly present the requested data blocks to a requesting client and cancel the parallel request to the cloud storage system before part (or sometimes even any) of the cloud file has been transferred. 
       FIG. 18  illustrates a cloud controller  1800  that issues peer cache requests to one or more peer cloud controllers. In one scenario, upon receiving a request for a data block of a file from client  1802 , cloud controller  1800  uses the stored metadata for the distributed filesystem to determine that cloud controller  1804  is the owner of the cloud file containing the data block, and then sends a peer cache request to cloud controller  1804 . More specifically, cloud controller  1800 : (1) uses the stored metadata to determine the CVA for the data block; (2) determines that the data block is not currently locally cached in cloud controller  1800 ; (3) identifies cloud controller  1804  as the originating cloud controller for the data block based on the unique CCID in the CVA; and (4) sends a peer cache request for the data block to cloud controller  1804  (operation  2  in  FIG. 18 ). Cloud controller  1804  is targeted for the peer cache request based on temporal and spatial locality; as the originator and a previous cacher of the data block, cloud controller  1804  is more likely to be currently caching the data block, and thus is a good candidate for the request. As described above, cloud controller  1800  can send the peer cache request to cloud controller  1804  (operation  2 ) in parallel with sending a request for the cloud file referred to by the CVA to cloud storage system  302  (operation  1  in  FIG. 18 ). 
     In a second scenario for  FIG. 18 , the distributed filesystem also includes a backup cloud controller  1806 , and cloud controller  1800  instead sends a peer cache request for the data block to backup cloud controller  1806  (operation  3  in  FIG. 18 ). As described above, backup cloud controllers may be configured to store the most frequently accessed and/or recently created data in the distributed filesystem; in this role, backup cloud controllers are likely to be caching the most useful and most-frequently-requested data blocks in the system, and are therefore logical targets for peer cache requests. Furthermore, a backup cloud controller that is not currently associated with any active clients is likely to be lightly loaded, and hence able to handle and quickly respond to peer cache requests. As in the first scenario, the peer cache request to backup cloud controller  1806  (operation  3 ) may be issued in parallel with the request to the cloud storage system  302  (operation  1 ). Alternatively, operations  1  and  2  may both be performed in parallel to operation  3 . 
     In a third scenario for  FIG. 18 , cloud controllers  1800  and  1804  may be co-located at a single geographic location and collaborate to split the working set of clients at that location. For instance, two or more cloud controllers may be in very close network proximity and configured to query each other using peer cache requests before sending requests for cloud files to a cloud storage system. In such a collaborative environment, a set of tightly bound cloud controllers may operate (and segment a cache space) more efficiently if they are configured to track what their peer cloud controllers are caching (e.g., constantly update one another with the specific set of data blocks that each is caching). 
     In some embodiments, cloud controllers may determine a target cloud controller for a peer cache request based on a range of criteria. The preceding exemplary scenarios determined targets for peer cache requests based on ownership, proximity, and/or a specific role (e.g., the role of the backup cloud controller), but the disclosed techniques are in no way limited by these examples, and peer cache requests may be routed or partitioned based on other factors. For instance, cloud controllers may periodically calculate and share both the network latency and bandwidth between cloud controllers as well as cloud controller load (e.g., how many clients are connected to a given cloud controller, and/or how many current requests are currently queued for disk I/O), and use such information when determining the appropriateness and targets for peer cache requests. For example, a requesting cloud controller may decide not to send a peer cache request if a target cloud controller&#39;s load is above a specified threshold, or only send high-priority requests in such instances. 
     In some embodiments, cloud controllers may be configured to operate in different modes during different times of the day. For instance, a cloud controller may be configured to cache data that is being actively accessed by a set of local clients during a particular time period (e.g., daytime working hours). However, when these clients are idle (e.g., during the night), this cloud controller may be configured to replace some or all of this cached data with data that is being accessed in a different time zone. More specifically, instead of being idle, this cloud controller can be configured to load a different set of data and serve this data to other active cloud controllers via peer cache requests. Such configurations may provide substantial improvements to the efficiency and average response time of the other cloud controllers. For example, depending on the situation, retrieving a small set of data blocks from a distant cloud controller may still be faster than downloading an entire cloud file from a closer cloud storage system. Such configurations may be specified and customized for individual cloud controllers using locality policies (as described in following sections). 
     Note that accessing other cloud controllers as peer caches does not involve having to maintain or check coherency for data blocks. The characteristics of the transactional filesystem ensure that each data block is unique and read-only (e.g., new data is written to new blocks and new cloud files). Thus, requesting cloud controllers only need to be concerned with retrieving needed data blocks as quickly as possible, and do not need to perform checks to determine whether another cloud controller and/or client have modified the contents of the requested data block. Note also that peer cache requests are typically constant-time requests that are sent directly to target cloud controllers, and not multi-hop queries. 
       FIG. 19  presents a flow chart that illustrates the process of accessing cached data from a peer cloud controller in a distributed filesystem. Two or more cloud controllers collectively manage the distributed filesystem data that is stored in one or more cloud storage systems (operation  1900 ); the cloud controllers cache and ensure data consistency for the stored data. Whenever each cloud controller receives new data from a client, it sends an incremental metadata snapshot for the new data to the other cloud controllers and an incremental data snapshot containing the new data to a cloud storage system. During operation, a cloud controller receives a request from a client for a data block of a file stored in the distributed filesystem (operation  1910 ). Upon determining that the requested data block is not currently cached in the cloud controller (operation  1920 ), the cloud controller sends a peer cache request for the requested data block to a peer cloud controller in the distributed filesystem (operation  1930 ). 
     Deduplication 
     A given data block may be stored multiple times in a filesystem; for instance, users may back up or otherwise keep multiple copies of the same file, or may send copies of a file to other users in their organization. Over time, file duplication can lead to an increasing number of duplicate data blocks and a substantial amount of wasted storage space. Data deduplication techniques involve calculating and tracking hash values for previously written data blocks, and comparing the hash values for newly written data blocks against these previous hash values to determine whether new data blocks have already been previously stored in a filesystem (and, if so, referencing the existing data block instead of writing a new, additional data block). 
       FIG. 29A  illustrates the process of writing new data blocks in an exemplary deduplication architecture. During operation, a filesystem receives a set of data blocks to be written  2900 - 2904 . A hash function  2906  is used to determine hash keys  2908  based on the contents of each data block that is being written; this hash function can use one or mechanisms to compute unique, random hash values for each data block. Note that the deduplication architecture is designed to ensure that the probability of duplicate hash keys for two data blocks that contain different data is very small (e.g., smaller than the probability of a disk error). 
     The hash keys  2908  generated by hash function  2906  are used as indices into a hash table of block entries  2910  (which is sometimes also referred to as a deduplication, or “dedup,” table). Each block entry can include metadata identifying the storage location for the data block and a reference count for the data block (e.g., the number of times that the block is currently referenced in the filesystem metadata). During operation, a request to store a new block of data prompts the filesystem to calculate a hash key  2908  for the data block and then use this hash key  2908  as an index into hash table  2910  to determine whether the data block has already been written previously (e.g., determine whether a block entry already exists in hash table  2910  for that specific hash key). If no block entry exists for the hash key, the filesystem: (1) writes the data block to storage; (2) updates the filesystem metadata for the data block to point to the storage location; (3) creates a new block entry for the data block (that points to the storage location and stores an initial reference count of one for the newly written data block); and (4) updates hash table  2910  so that the index for the hash key points to the new block entry. Alternatively, if a block entry already exists for the hash key, the filesystem: (1) accesses the block entry to retrieve the storage location for the previously written data block; (2) updates the filesystem metadata for the current data block to point to the storage location (e.g., updating the metadata with additional references to an existing storage location instead of writing a duplicate data block to another storage location); and (3) increments the reference count in the block entry to reflect the additional reference to the previously written data block. For the example illustrated in  FIG. 29A , copies of data blocks  2900 - 2904  have previously been written, and lookups of the hash keys  2908  for these data blocks in hash table  2910  (operations  1 ,  2 , and  3 , respectively, in  FIG. 29A ) are associated with block entries  2912 - 2916 . Note that a file delete operation in a filesystem that supports deduplication techniques typically involves updating the reference counts for the file&#39;s data blocks in hash table  2910  (and only actually deleting each given data block and its associated block entry when its reference count reaches zero). Note also that deduplication techniques may be used across data blocks of different fixed and/or variable sizes; in some instances, for instance, a hash function may be configured such that two different-sized blocks that contain the same actual data (e.g., with one larger block being partially empty) resolve to the same hash value. 
     In general, deduplication techniques increase the effective write performance of a storage device by reducing the total number of writes that need to be made to the storage device. More specifically, the ability to detect and prevent duplicate data from being written effectively increases write throughput while also reducing the amount of storage space that is actually needed. However, deduplication techniques do typically involve some additional overhead. For instance, if the number of data blocks is large, memory constraints may lead to deduplication structures being stored lower in the memory hierarchy (e.g., on a hard drive). In such situations, the architecture illustrated in  FIG. 29A  may involve additional disk reads to access the hash table  2910  and block entries  2912 - 2916 . For example, because hash values are based on the contents of data blocks, and not storage locations, a set of data blocks  2900 - 2904  from a single file that are being written contiguously to a storage device may have very different hash values, and hence be widely spaced across different locations in hash table  2910 . As a result, a subsequent duplicate write of these same data blocks may involve three distinct hash table lookups (e.g., reading three different disk blocks from potentially different regions of the storage device, with all of the associated seek, rotational, and read latencies), with each of these lookups requiring a further distinct lookup of a corresponding block entry (e.g., reading three additional disk blocks, again from potentially different regions of the storage device). Thus, deduplication operations can sometimes involve substantial lookup delays and overhead. 
     In some embodiments, deduplication techniques can be extended to encode time information in deduplication structures. For instance, the data structures used to store block entries can be re-architected so that the information for data blocks is temporally grouped in a manner that reduces the number of disk reads needed during subsequent deduplication operations. 
       FIG. 29B  illustrates an exemplary temporal deduplication architecture that receives write requests for the same three data blocks  2900 - 2904  illustrated in  FIG. 29A . As described previously, hash function  2906  is used to generate hash keys  2908 . However, unlike in  FIG. 29A , occupied indices of hash table  2920  now point to a temporal deduplication structure  2922  that stores temporally related block entries in proximity with each other. For instance, consider an initial write operation for data blocks  2900 - 2904 . Upon determining that no entry exists in hash table  2920  for the hash key for data block  2900 , the filesystem: (1) writes data block  2900  to storage; (2) updates the filesystem metadata for data block  2900  to point to the new storage location; (3) determines a current temporal deduplication structure (TDS)  2922  that is currently being populated; (4) creates a new block entry  2912  for data block  2900  in TDS  2922 ; (5) updates a set of lookup structures  2926  in TDS  2922  to reflect the addition of block entry  2912 ; and then (6) updates hash table  2920  so that the index for the hash key points to TDS  2922 . The subsequent write requests for data blocks  2902  and  2904  proceed substantially similarly, with the filesystem: (1) writing data blocks  2902 - 2904  to storage; (2) updating the filesystem metadata accordingly; (3) adding block entries  2914  and  2916  for data blocks  2902  and  2904  in the locations subsequent to block entry  2912  in TDS  2922 ; (4) updating lookup structures  2926  in TDS  2922  to reflect the addition of block entries  2914 - 2916 ; and (5) updating hash table  2920  so that the indices for the hash keys for data blocks  2902  and  2904  also point to TDS  2922 . 
     While creating and populating TDS  2922  involves some additional overhead, subsequent duplicate writes benefit from lower overhead. For example, consider a subsequent set of duplicate writes for the same three data blocks  2900 - 2904 . Upon determining that an entry exists in hash table  2920  for the hash key for data block  2900 , the filesystem receives a reference to TDS  2922 . The filesystem can then: (1) use lookup structures  2926  to find and access block entry  2912  to retrieve the storage location for the previously written data block; (2) update the filesystem metadata for the current data block to point to the storage location; and (3) increment the reference count in block entry  2912 . However, for the second and third blocks, the hash key lookups in hash table  2920  both return references to the same TDS, TDS  2922 , which was already read from disk and loaded into memory during the preceding lookup for the first data block. Thus, while the duplicate write of the first data block ( 2900 ) still involves two disk reads (to access hash table  2920  and TDS  2922 ), the (duplicate) writes for the second and third data blocks ( 2902 - 2904 ) only involve at most one further disk read each (to access hash table  2920 ), thereby reducing the total number of disk reads needed and substantially improving deduplication performance. Consider the duplicate write of a file with 100 data blocks in a scenario in which each TDS is the size of a disk block and holds 100 block entries. In this scenario, a deduplication operation in the deduplication architecture of  FIG. 29A  would involve 200 disk reads. In contrast, the deduplication architecture of  FIG. 29B  would only involve at most 101 disk reads (e.g., 100 accesses of hash table  2920  for the 100 hash keys, and one read of the TDS storing the block entries for the 100 data blocks), effectively almost halving the maximum number of disk reads required and potentially dramatically improving deduplication performance. 
     In some embodiments, temporal deduplication structures can be optimized to reduce the number of disk reads even further. For instance, the block entries of TDS  2922  can be enhanced so that each block entry includes the hash value for its associated data block and a “next pointer” that points to the block entry following it in TDS  2922  (e.g., block entry pointers  2928  in  FIG. 29B ). Similarly, the last block entry in TDS  2922  can be configured to point to the first block entry in a subsequent TDS (e.g., TDS  2924 ). These block entry pointers  2928  allow block entries to be traversed temporally during deduplication operations. For example, during a duplicate write of data blocks  2900 - 2904 , the attempt to write data block  2900  would still result in the hash table  2920  lookup and the access of TDS  2922 . However, the subsequent writes for data blocks  2902 - 2904  might no longer involve lookups in hash table  2920 , but could instead involve: (1) traversing block entry pointers from block entry  2912  to the next two block entries; (2) determining whether the hash values stored in these block entries match the hash keys for data blocks  2902  and  2904 ; and (3) if so, using these block entries, thereby avoiding the additional disk reads required by additional lookups into hash table  2920  in situations where the block entries for contiguously written data blocks have also been written contiguously into a TDS. Using block entry pointers to link each TDS to a subsequent TDS further facilitates efficiently traversing large numbers of related block entries; for instance, a filesystem may track the use of these pointers and then preemptively pre-fetch the next TDS when approaching the end of a current TDS. In such embodiments, the previous exemplary scenario of a duplicate write of a file with 100 data blocks might involve only two or three disk reads, e.g., one access of hash table  2920  for the first hash key, and one read for each TDS storing the needed block entries (which are then accessed sequentially via the TDS using the block entry pointers). 
     In some embodiments, multiple sets of block entry pointers may be used to form a doubly-linked list of block entries within and between TDSs. Maintaining doubly-linked lists sometimes involves higher overhead, but can also provide performance benefits, for instance in the face of unusual write patterns (e.g., when a client for some reason performs a duplicate write for a set of blocks in the reverse order that the blocks were initially written and ordered in a TDS). Alternative embodiments may also involve loading the contents of TDSs into a memory cache of block entries, thereby speeding up accesses and efficiently supporting atypical access patterns. 
     Note that the concept of temporal deduplication applies not only to individual files, but also to groups of files. Many automated computing tasks are defined processes (e.g., automated scripts) that execute in a very regular, predictable manner. For instance, applications are typically written to execute in the same way across multiple iterations, e.g., compilation jobs typically create files and data in the same order, and directory copy or backup commands will typically traverse a directory hierarchy that is being operated upon in the same order for every invocation. The above-described temporal deduplication techniques leverage the notion that things written with temporal locality are likely to be re-written again in the same temporal order; the disclosed temporal deduplication structures can be used to efficiently detect and handle long, contiguous sets of duplicate data blocks that are being written by an automated process. 
     In some embodiments, temporal deduplication structures can also be optimized to accommodate the subsequent addition of data blocks to a previously created file. For instance, a filesystem may be configured to reserve some block entries in each TDS that can then be used at a later time (e.g., to add a few additional data blocks to a file, and have the block entries associated with those new data blocks show up in the same TDS as the rest of the block entries for the file), in contrast with a purely temporal approach that would add the block entries for subsequent data blocks being appended to a previously written file to another, separate TDS that is currently being populated (and, hence, potentially splitting the block entries for the appended file across multiple non-contiguous TDSs, and requiring more TDSs to be read in subsequent deduplication operations that involve the entire appended file). Note, however, that such additional capabilities may interfere with some of the advantages of the above-described temporal deduplication structures; for instance, all of the block entries for a given file may still be in the same TDS (e.g., after an additional block entry has been added to the TDS), but an efficient stream of contiguous temporal block entry lookups may have been disrupted as a result. Hence, the decision of whether to reserve some block entries in a TDS for subsequent additions may be limited to certain situations (e.g., files and/or filesystem regions that are more likely to be accessed in isolation) and/or be selectively specified using a locality policy. 
     Global Deduplication for a Distributed Filesystem 
     Deduplication techniques can be applied across a range of scopes. For instance, the above-described deduplication techniques can be performed on (individual) single- or multi-user workstations and/or servers to conserve storage space and increase user-perceived write performance. However, deduplication techniques can be even more effective and beneficial when leveraged across a distributed filesystem that encompasses a large number of users and data files. More specifically, the types of regular operations that typically cause file duplication (e.g., duplicate compilation jobs, recurring backup jobs, the sharing of files between users, etc.) quite often increase as the size of the filesystem and user base grow, leading to substantial storage space and bandwidth savings for a distributed filesystem. 
     In some embodiments, cloud controllers use data deduplication techniques to reduce the amount of duplicate data that is stored in a distributed filesystem. For instance, supporting deduplication techniques across the distributed filesystem may involve leveraging incremental metadata snapshots to update deduplication information in the cloud controllers that manage the data in the distributed filesystem. Each cloud controller then monitors the writes made by its clients and performs deduplication operations to avoid storing duplicate copies of data blocks in the cloud storage system. Note that the disclosed deduplication techniques are distributed across the cloud controllers; attempts to perform centralized deduplication operations for a large distributed filesystem on a single server typically limit the scalability and performance of deduplication operations. 
       FIG. 29C  illustrates the process of using incremental metadata snapshots to update deduplication information. During operation, a cloud controller  2950  receives new data  2952  (e.g., a new file containing one or more data blocks) from a client  2954 . Cloud controller  2950  uses a local deduplication table  2956  to determine whether these received data block(s) have already been written previously. If so, cloud controller  2950 : (1) creates new metadata for new data blocks  2952  that references the previously created data block(s); (2) updates the reference counts in deduplication table  2956  appropriately; and (3) distributes an incremental metadata snapshot  2958  that reflects the additional references and the existence of the new file. If not, cloud controller  2950 : (1) allocates new CVA addresses for the new data block(s); (2) writes an incremental data snapshot  2960  containing the new data block(s) to cloud storage system  302 ; (3) creates one or more new block entries for the new data block(s) in a TDS in deduplication table  2956  and updates the deduplication hash table accordingly; (4) creates new metadata for the distributed filesystem that references (the CVA addresses and offsets for) the new data block(s); and (5) distributes an incremental metadata snapshot  2958  that reflects these metadata updates to the other cloud controllers of the distributed filesystem. Note that both types of operations may occur simultaneously; for instance, some of the new data  2952  received from a client may have been previously written already, while other parts may be completely new. For example, a client may take an existing file and append new material; in this case, a first set of data blocks for the file may be identified as duplicate data (and result in incremented reference counts), while a second set of data blocks for the file may be identified as new data, and handled accordingly. As a result, incremental metadata snapshot  2958  may include metadata changes that encompass both additional references to existing data as well as new data being written in incremental data snapshot  2960 . 
     Other cloud controllers can make use of the metadata updates propagated via incremental metadata snapshots to update their own local deduplication tables. For instance, metadata updates may specify a set of data blocks that are being referenced, created, and/or deleted, and include the hash values and locations (e.g., CVA address and offset) for each new or newly referenced data block. Hence, in the context of  FIG. 29C , a second cloud controller  2962  receiving incremental metadata snapshot  2958  will: (1) update its local metadata hierarchy to reflect the updates to the distributed filesystem that were made by cloud controller  2950 ; and (2) update its own local deduplication table  2964  to reflect any new data blocks and/or reference changes described in incremental metadata snapshot  2958 . At this point, duplicate writes of new data  2952  that are received by cloud controller  2962  can be detected using deduplication table  2964 . Note that incremental metadata snapshots can also propagate information related to deletion requests, which can lead to the decrement of reference counts in deduplication tables and the eventual deletion of data blocks; file deletion operations are described in more detail in a subsequent section. 
       FIG. 30  presents a flow chart that illustrates the process of performing deduplication in a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems (operation  3000 ); the cloud controllers cache and ensure data consistency for the stored data. During operation, a cloud controller receives an incremental metadata snapshot that references new data that was added to the distributed filesystem by a remote cloud controller (operation  3010 ). The cloud controller extracts a set of deduplication information from this incremental metadata snapshot (operation  3020 ). Upon receiving a client write request (operation  3030 ), the cloud controller uses the extracted deduplication information to determine that one or more data blocks in the client write request have already been written to the distributed filesystem (operation  3040 ). 
     The regular exchange of incremental metadata snapshots between the cloud controllers of the distributed filesystem ensures that deduplication information is shared and that each cloud controller&#39;s local deduplication table remains up to date, thereby allowing duplicate writes to be detected and avoided. Note that temporal deduplication information can also be shared between the cloud controllers. Consider, for example, a cloud controller that receives from a client a write request for a file with ten data blocks that have not previously been written to the distributed filesystem. The cloud controller: (1) takes hash values for these ten data blocks, and determines that the blocks are new; (2) allocates new CVAs and offsets for the new data blocks; (3) updates a TDS in its local deduplication table, creating temporally related block entries for the new data blocks; (4) updates the local metadata hierarchy appropriately; and (5) sends out incremental data and metadata snapshots. Other cloud controllers receiving the incremental metadata snapshot detect the creation of the ten new blocks, and write the information for these ten blocks into their own deduplication tables in a temporal manner (e.g., updating TDSs in their local deduplication tables in a manner that creates temporally related block entries for the new data blocks). Hence, if a client of one of these other cloud controllers attempts to write the same file, the deduplication operation in that cloud controller benefits from the same temporal lookup aspects described previously (e.g., a reduced number of disk reads in the deduplication operation due to the temporal similarities). 
     Note, however, that some challenges can arise for temporal deduplication techniques in a distributed filesystem environment. For instance, consider a cloud controller that is receiving incremental metadata snapshots (from another cloud controller) while simultaneously locally writing a set of new data blocks in response to a client request. Unfortunately, such scenarios can lead to situations in which sets of block entries from the remote deduplication information and from local writes (e.g., two sets of unrelated write operations) are written into the same TDS in an interleaved manner. As described above, temporal deduplication techniques benefit from grouping block entries for temporally related data blocks together in TDSs; however, while these two sets of writes occur in the same approximate temporal timeframe, they originate on different cloud controllers, and are most likely not related. In general, interleaving two unrelated sets of block entries in a TDS typically reduces the temporal nature and efficiency of the deduplication table. 
     Another problem may arise when block references are not sent between cloud controllers in the order in which they were written. For instance, incremental metadata snapshots may send block references between cloud controllers in a “filesystem:file:block” format, instead of the order in which the blocks were actually written (and the order in which their respective block entries were written to a TDS in the originating cloud controller). Such reordering of write information can affect the ordering of a temporal deduplication table; for example, a second cloud controller may end up with a significantly different block entry order in its local deduplication table. Consider a client subsequently writing the same file to this second cloud controller—the deduplication operation will still determine that all of the data blocks for the file have been previously written in the distributed filesystem, but the deduplication operation may be highly inefficient. Because of the difference in the write order of the block entries, the cloud controller may not be able to take benefit from temporal locality in the deduplication table, and instead may have to perform individual hash lookups for each data block in the file. For example, in the worst case, the original write order for the initial file may have been “data block 1, 2, 3, 4, . . . , N−2, N−1, N,” but the temporal deduplication table may have been populated in reverse order (e.g., “data block N, N−1, N−2, . . . , 2, 1”) or some other non-temporal order, in which case a subsequent write of the same file received by the second cloud controller might result in a backwards walk through and/or random accesses into the temporal deduplication table, which are typically less efficient. Note, however, that the worst-case behavior for temporal deduplication is still no worse than non-temporal deduplication techniques (e.g., the deduplication techniques described in  FIG. 29A ); thus, as long as temporal deduplication is beneficial in at least some scenarios, it is generally likely to improve performance. 
     In some embodiments, cloud controllers reorder write information to improve the temporal locality of block entries for writes performed both locally and on remote cloud controllers. For instance, write information can be sorted based on one or more secondary qualifiers (e.g., the data&#39;s source cloud controller, the data&#39;s source filesystem, a snapshot identifier, a file ID, and/or a block offset) to separate write information received directly from local clients and from remote clients (via other cloud controllers&#39; incremental metadata updates) back into individual segments that are “temporally accurate” and facilitate efficient temporal deduplication. For example, one specific scenario may involve sorting write operations by CVA and offset; cloud controllers may use the CCID in CVAs to differentiate which cloud controller is the source for each given write, and may then use other portions of the CVA (e.g., the FSID, SSID, FileID and/or offset) to determine the exact order in which data blocks were written on each cloud controller and/or block entries should be written into a TDS. Each cloud controller can then use such determinations to ensure that any desired aspects of the original write order (e.g., in the originating cloud controller) are maintained for corresponding block entries in each local deduplication table, thereby avoiding interference between remote and local writes and ensuring improved temporal deduplication performance for local writes. 
     As mentioned above, a range of sorting (or any other form of reordering) techniques may be used to ensure high temporal locality on both local and remote filesystems and to facilitate efficient global deduplication efforts across all of the cloud controllers of the distributed filesystem. In some embodiments, one or more parameters for a secondary sort operation may be specified using a locality policy. For instance, a locality policy may be used to tune temporal deduplication behavior on a given cloud controller to match the specific needs of the cloud controller&#39;s expected clients and client load. For example, a cloud controller expected to receive a large amount of new client data may be configured to use a specific sort order. Consider as a specific example a cloud controller that is configured to gather data from multiple other cloud controllers and then perform backups of the full data set. In such a scenario, if the host cloud controller is configured to traverse and write this backup data on a per-cloud-controller basis, a sensible approach might be to ensure that the block entries in the temporal deduplication table are also grouped together similarly; a locality policy can be used to ensure that this is indeed the case. 
     In some embodiments, a locality policy may also be used to specify that the block entries for one or more files should be written in a different order than the data blocks for the files. Files are quite frequently accessed (e.g., copied, or re-written) in the same manner that they were initially written, but in some instances it may be known that a certain set (or type) of files will have a specific unusual future re-write pattern; in such scenarios, ordering the block entries for these files to anticipate these future write patterns may improve the performance of subsequent deduplication operations for those files. 
     In some embodiments, a cloud controller may be configured to maintain multiple simultaneous sets of TDSs that are all referenced by the same top-level hash table. For instance, a cloud controller may maintain one set of TDSs that store block entries for locally written data, while a second set of TDSs store block entries for remote data written by other cloud controllers. The appropriate TDS is determined based on the initial hash value for a given deduplication operation. Separating the stream of local and remote writes completely ensures that block entries for local and remote writes are never interleaved. Note that the block entries for remote writes are automatically grouped at the snapshot level already (e.g., if incremental metadata snapshots from different cloud controllers are processed serially), and thus are less likely to become interleaved. 
     The above sections describe how temporal deduplication techniques can be managed and synchronized across cloud controllers in a manner that facilitates maintaining temporal accuracy and locality, thereby providing a range of benefits for a distributed filesystem. These benefits can be broken down into benefits of deduplication “at rest” (e.g., specific storage space savings gained from reducing duplicate stores) as well as benefits of deduplication “in transit” (e.g., specific network bandwidth savings gained from avoiding network transfers of duplicate data). However, as also mentioned above, deduplication techniques also involve a certain level of overhead (e.g., extra reads and writes for the deduplication tables as well as additional complexity and compute overhead). The actual value of deduplication efforts in a distributed filesystem depends on the amount of duplicate data being written (e.g., the “dedup ratio,” or ratio between the average number of references per stored data block). For instance, a 10:1 at-rest dedup ratio indicates that there are on average ten references for each actual stored data block, e.g., deduplication efforts have on average saved nine additional copies of each data block from being written. A 10:1 in-transit dedup ratio indicates that on average ten metadata references are sent for every copy of a data block that is actually transferred between a cloud controller and a cloud storage system; because metadata is much smaller than actual data, deduplication efforts can save substantial network bandwidth. 
     Performing temporal deduplication for a distributed filesystem can provide substantial at-rest and in-transit benefits. While certain minimum dedup ratios are needed to break even (e.g., justify the additional deduplication overheads), a system that profits from both at-rest and in-transit benefits begins to reap those benefits at lower dedup ratios than systems that only provide one or the other of the two benefits. Deduplication benefits also tend to scale with the number of users and amount of data in the system; distributed filesystems tend to be large and encompass a larger number of users, and hence tend to have higher natural levels of duplication that in turn result in higher dedup ratios. Hence, tuning deduplication efforts to optimize a distributed filesystem can result in definite performance improvements and reduced system costs. For example, effectively reducing network load can have direct economic benefits, because upgrading network links often involves considerable delay and cost, and may not even be feasible in some situations. 
     Locality Policies 
     In some embodiments, a set of locality policies specify how data in the distributed filesystem should be managed. Such locality policies may define and/or control how data is managed at different levels of scope. For instance, locality policies may be defined at a global level (e.g., for the entire distributed filesystem) as well as at a range of finer granularities (e.g., on a per-cloud-controller, per-filesystem, per-user, per-file-type, or per-directory basis). The ability to define arbitrary locality policies that span a range of granularities allows the behavior of the distributed filesystem to be customized to meet the needs of an organization, specific users, and/or specific applications or data sets. 
     In some embodiments, a locality policy can specify that one or more files should be “pinned” in the cache(s) of one or more cloud controllers. For instance, a locality policy may specify that a specific set of files that are being actively collaborated on by users throughout an organization should be pinned into the cache of every cloud controller. For example, this configuration may be achieved by specifying a locality policy that identifies one or more directories whose contents (and sub-contents) should be pinned in all cloud controllers; users can explicitly ensure that a file will be immediately distributed to and actively cached in all of the cloud controllers by storing it in such directories. In another similar example, the home directory of an important user (e.g., a chief executive officer) who frequently travels throughout an organization may be pinned so that that person can access their data with the fastest possible access times no matter where they are currently located. Alternatively, a project directory for a single site in an organization may be pinned in only the cloud controller for that site, to ensure that the data is quickly available locally (e.g., even if there is a network failure/partition between the cloud controller and the cloud storage system storing the cloud file containing that data). In another example of per-cloud-controller pinning, a locality policy may specify that certain blocks that are frequently read by specific applications (e.g., internet browsers or local file browsers) be pinned for users accessing the cloud controller to ensure that common, frequently used user applications execute as quickly as possible. Note that in such situations part of the cache is fixed (e.g., may not follow normal cache replacement policies), while the remaining portions of the cache may be managed using traditional cache policies (e.g., “best effort” cache replacement policies). 
     In some embodiments, a locality policy can specify storage behavior for different classes and/or types of files. For instance, a locality policy may specify that certain files should be written together (e.g., putting a project file and all of its supporting files in the same cloud file), or that certain file types should not be written into the same cloud file. For example, image and video files typically consume a substantial amount of space, and are often only viewed for a brief initial time window. Hence, a locality policy may specify that image and video files (e.g., files with “.jpg” and “.avi” extensions) should be considered low-priority (e.g., in comparison with other project file types), and should immediately be stored in a second-level (e.g., cheaper) cloud storage system instead of in a costlier high-speed, highly replicated cloud storage system that is used for high-priority files. Conversely, other file types may be identified as being high-priority, and as a result be pinned in cloud controllers and/or stored (and kept) in the first-level cloud storage system even if they are infrequently accessed. Note that while an initial user accessing low-priority files may experience a slightly higher latency as the files are loaded from the second-level cloud storage system, subsequent users accessing these files fairly soon thereafter from the same location (e.g., before they are flushed out of the cloud controller&#39;s cache) will be able to access the files quickly from the cache of the cloud controller. Note also that filling drive files with files of the same type can facilitate performing uniform operations on certain classes of files at a later time (e.g., archiving all of the image files in a portion of the filesystem hierarchy) without having to split cloud files or duplicate portions of cloud files. 
     In some embodiments, a locality policy may include time-based specifications and/or time-outs. For instance, a certain project directory may be specified to only be pinned for the lifetime of a project, or a training video may be pinned for an initial period (e.g., 15 days), and then subject to normal cache replacement policies. For example, an alternative locality policy for image files may specify that image files should be kept in a first-level cloud storage system for 30 days (or 30 days after the last access), and then moved to a second-level cloud storage system. 
     In some embodiments, locality policies can specify the synchronization and management of metadata and data. For example:
         a locality policy for a specific cloud controller may be configured such that the cloud controller, upon receiving an incremental metadata snapshot, “warms its cache” (e.g., preemptively downloads some of the data referenced in the incremental metadata snapshot from a cloud storage system); for instance, the cloud controller may be configured to preemptively download and cache any files created by a certain user or related to a certain project.   a locality policy may be configured to manage how data is packaged into cloud files; for instance, a locality policy may ensure that files of a similar type (e.g., image files) or other criteria are packed into the same cloud file, so that the set of associated files can later be managed uniformly (e.g., all image files can be easily sent to a cloud storage system that is specified for image files, or a group of backup files are grouped into a common set of cloud files using a locality policy so that they can be archived or deleted as a unit with minimal effort at a later time).   a locality policy can specify a mapping of data to cloud storage providers and/or cloud storage systems; for instance, a locality policy may specify a preferred cloud storage provider (e.g., based on performance or cost), when data should be migrated between cloud storage systems, that backup data should immediately be sent to an archival cloud storage system, and/or that certain cloud storage providers should not store portions of the distributed filesystem (e.g., for geopolitical reasons).   a locality policy may specify whether a cloud controller should send peer cache requests, and if so, may specify one or more peer cloud controllers that should be targeted by peer cache requests; for instance, the locality policy may specify a priority list for target peer cloud controllers, and/or may identify specific target peer cloud controllers based on a type of file, owner, project, etc.   a locality policy may specify when global deduplication techniques should be used; for instance, the locality policy may specify that deduplication should be disabled when a cloud controller is writing encrypted files (which frequently do not benefit from deduplication efforts but would still consume entries in the deduplication tables, thereby negatively impacting overall deduplication performance).   a locality policy may specify the aggressiveness of pre-fetching decisions (e.g., aggressively attempt to reduce user latency at a cost of additional network pre-fetch transfers vs. reducing the aggressiveness of pre-fetching to save network bandwidth at the expense of higher user latency).   a locality policy may indicate files and/or portions of the distributed filesystem hierarchy that should not be uploaded to a cloud storage system (e.g., files that are known to be temporary and short-lived, or files that users have explicitly specified to not be shared).   a locality policy may allow a cloud file&#39;s block size to be set on a per-file basis. For instance, database dumps typically use 8 KB block sizes, and hence a locality policy may specify that all files of that file type should be 8 KB in size to improve the deduplication hit rate; because database dumps often have a high level of redundancy, such a policy may substantially improve the deduplication hit rate for that set of blocks.   a locality policy may specify the target location for data and/or metadata on a set of storage drives; for instance, the locality policy may specify that a set of data that is expected to be accessed very frequently should be cached near the outer edge of a disk platter (e.g., to improve performance).   a locality policy may specify a set of parameters that affect delete operations and delays for the distributed filesystem.   a locality policy may specify that some or all accesses (e.g., create, rename, read, write, and/or stat) for one or more files should be tracked and/or timed; such access data can be used to audit file usage, to determine the proportion of cache hits in the cloud controller (in contrast with accesses that require cloud files to be downloaded to the cloud controller), and to help tune the performance of the cloud controller and/or distributed filesystem.   a locality policy may be used to specify defragmentation parameters (e.g., time intervals, file types and access patterns to monitor and track, etc.) for file data blocks cached in the local storage of a cloud controller.       

       FIG. 25  illustrates an exemplary distributed filesystem environment in which the characteristics of a distributed filesystem are guided and/or adjusted by a set of locality policies  2510 - 2516  that influence the operation of cloud controllers  2500 - 2508 . For instance, a global locality policy  2510  may specify that a set of data that is frequently accessed through an organization should be pinned into the cache of all of cloud controllers  2500 - 2508 . Cloud controller  2500  may be located at the organization&#39;s headquarters, and hence be configured with a local locality policy  2512  that specifies aggressive pre-fetching to minimize the access latency experienced by the organization&#39;s executives. Cloud controller  2504  may include a partial filesystem locality policy  2514  that identifies that a portion of the distributed filesystem hierarchy contains highly sensitive data, and should not be stored on a certain set of less-trusted cloud storage providers. Multiple cloud controllers  2506  and  2508  may be co-located at a site that has a particularly large number of clients and client data requests to ensure adequate data throughput; a site locality policy  2516  specifies that cloud controllers  2506  and  2508  should send peer cache requests to each other to effectively double the amount of data that is cached at that site. 
     Note that some locality policies can be modified at any time without incurring substantial overhead. For instance, decisions to no longer pin certain portions of the distributed filesystem and/or to pin a different set of data may simply result in the formerly pinned data now being subject to normal cache replacement policies and the new data being downloaded into the cloud controller. Some policies, however, are primarily applied at the time new data is written (e.g., data policies that group certain types of files into common cloud files); changing the organization of cloud files that were written based on a previous locality policy to reflect a new locality policy may involve walking the metadata of the distributed filesystem and rewriting previously stored data into new cloud files that reflect the new policy. 
       FIG. 26  presents a flow chart that illustrates the process of adjusting the characteristics of a distributed filesystem using a locality policy. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems (operation  2600 ); the cloud controllers cache and ensure data consistency for the stored data. During operation, a cloud controller receives a locality policy that specifies one or more management policies for the cloud controller (operation  2610 ), after which the portion of the distributed filesystem&#39;s data that is managed, created, and/or cached at the cloud controller is then managed based on this locality policy (operation  2620 ). Locality policies facilitate customizing and optimizing data management for the distributed filesystem to fit the needs of an organization (e.g., specific sets of users, applications, and/or datasets). 
     Deleting Files from a Distributed Filesystem 
     A distributed filesystem based upon cloud storage systems can efficiently store and access extremely large data sets. At some point, however, some of this stored data will no longer be needed, and hence will need to be deleted from the distributed filesystem. Deletion techniques may involve substantial complexity, computation, and potential delay, and hence need to be carefully managed to ensure that deletions do not negatively impact perceived user file access performance. 
     Consider as an example an organization that regularly backs up the data on all of its client machines to the distributed filesystem. For instance, all of the clients of the distributed system may be configured to periodically (e.g., on a daily, weekly and/or monthly basis) write backups of their local data to the distributed filesystem. All of this backup data may be collected into a single “tarball” (e.g., a single tape archive file that encompasses the full collection of backed up files while preserving important file system information, such as user permissions, dates, and directory structures). These tarballs may have only a limited lifespan—for instance, a tarball for a given backup date may only be preserved for a given timeframe (e.g., a month, or 60 days), and then be deleted to conserve storage space and/or make way for future backups. Depending, however, on the number of clients and the amount of data on each client, the collected data size may be substantial; for instance, performing backups may involve creating and deleting files that consume on the order of terabytes (or larger) of data in the distributed system, and hence may involve considerable time and effort. 
     In some embodiments, deletion from the distributed filesystem involves a series of steps that first hide a deleted file from users (to provide a user perspective of instant response) and then perform a number of background operations that traverse the file&#39;s metadata and then actually delete the data from the distributed filesystem over time in a manner that does not affect the performance of other simultaneous data accesses in the distributed filesystem. 
       FIGS. 27A-27C  illustrate the deletion of a file from a distributed filesystem.  FIG. 27A  illustrates the state of a cloud controller  2700  prior to a delete request. As described previously, each cloud controller in the distributed filesystem actively manages and updates its view of the distributed transaction filesystem; more specifically, cloud controller  2700  downloads incremental metadata snapshots that are distributed by other cloud controllers, and creates and uploads new cloud files containing incremental metadata and data snapshots whenever new data is written by local clients. The collected metadata for the distributed filesystem is maintained in a snapshot hierarchy  2702 . Snapshot hierarchy  2702  includes a set of metadata that tracks all of the data that is currently stored in the distributed filesystem; snapshot hierarchy  2702  also includes samples over time of the metadata tree that indicate how the metadata has changed historically (e.g., across multiple snapshots). The metadata in snapshot hierarchy  2702  is used to determine and access the set of data blocks  2708  that are currently being cached by cloud controller  2700 . Additional layers of metadata can be layered on top of snapshot hierarchy  2702  to present different views into the distributed filesystem. For instance, a set of active filesystem metadata  2704  tracks the portion of the distributed filesystem that is currently visible to the clients (e.g., users) of the distributed filesystem. Another set of overlay metadata  2706  pinpoints the set of modified data in a cloud controller that needs to be uploaded in a subsequent snapshot and facilitates creating cloud files in place (as described earlier). 
       FIG. 27B  illustrates the initiation of a delete operation. Client  2710  sends a request  2712  to cloud controller  2700  to delete a specific file, ‘File X.’ In response, cloud controller  2700  modifies active filesystem metadata  2704  to no longer refer to File X (operation  1  in  FIG. 27B ) and writes a snapshot update to the other cloud controllers reflecting this change. At this point, the file is effectively deleted from the user viewpoint (e.g., the file is no longer visible to users browsing through the active filesystem), but no data has actually been deleted from the distributed filesystem; a data block is not actually deleted until it is no longer referenced in snapshot hierarchy  2702 . 
       FIG. 27C  illustrates the execution of the delete process in response to the delete request of  FIG. 27B . As described above, the metadata for “deleted” (from the user perspective) File X is still available in snapshot hierarchy  2702 , and can now be traversed (e.g., in the background, when system load is low). More specifically, the cloud controller walks the metadata for each data block of the file and adds the deduplication hash value for the data block (which is stored in the metadata for the data block) to a list of deduplication updates  2714  (operation  2 ). After all of the metadata for the file has been traversed, the metadata for File X can be removed from snapshot hierarchy  2702  (and a corresponding metadata snapshot reflecting this change can be propagated to the other cloud controllers). 
     The set of hash values in the list of deduplication updates  2714  indicate data blocks in the deduplication tables  2716  that need to be decremented (e.g., to reflect the fact that File X no longer refers to these data blocks). These updates are applied to deduplication tables  2716  (operation  3 ); during this process, cloud controller  2700  determines whether any of these data blocks are no longer referenced in the distributed filesystem (e.g., the reference count for a given data block is decremented to zero). If this is the case, cloud controller  2700  further determines from cloud file status-tracking structures whether these changes result in any cloud files whose data blocks have all been deleted; if so, these cloud files are added to a list of cloud files that are queued for deletion  2718  (operation  4 ). The cloud controller can then subsequently send requests to cloud storage system  302  to actually delete (or archive) these cloud files (operation  5 ); only at this point, after the cloud storage system has executed and confirmed the deletion request, has the data actually been deleted from the distributed filesystem. Note that each cloud controller maintains and updates its own internal deduplication table. For example, the process by which the other cloud controllers of the distributed filesystem update their deduplication tables in response to the above-described file deletion on cloud controller  2700  may comprise performing the following on each of the other cloud controllers: (1) receiving an incremental metadata snapshot uploaded by cloud controller  2700  that indicates that the metadata for File X should be removed; (2) using the metadata for File X (e.g., the deduplication hash values for the blocks of File X) to properly update (e.g., decrement references in) the cloud controller&#39;s local deduplication table; and (3) updating the locally stored metadata to reflect the removal of File X. 
     In some embodiments, operations  2 - 5  as described for  FIG. 27C  may execute in parallel as simultaneous background jobs. For instance, some deduplication updates may already be processed at the same time that other portions of the metadata for the file are still being walked and other cloud files that contained unused data blocks are already being deleted. Alternatively, during peak load times some or all of these operations may be delayed to reduce system load. Performing delete operations as low-priority background jobs ensures that intensive delete operations do not adversely affect user experience with the distributed filesystem. 
     In some embodiments, a cloud controller may also intentionally delay some of the steps of the deletion process for a time interval to ensure that the deletion was intended. For instance, upon receiving user file delete request  2712 , cloud controller  2700  may initially delete the view of the file from active filesystem metadata  2704  (operation  1 ), but then delay performing the subsequent deletion operations for a specified time interval to ensure that the deletion request was not accidental. This delay (and other deletion parameters) may be configured using a locality policy. For example, a locality policy influencing deletions might specify: whether deletions should be delayed, and if so, for how long of a time interval; whether other cloud controllers, administrators, and/or clients should be notified of pending deletes; and/or whether deletion requests for certain (or all) files should to be moderated (e.g., approved) by an administrator and/or file owner before being executed. A locality policy may also be used to group backup files into a distinct set of cloud files (e.g., not mixed with other data) to simplify the deletion of the backup files at a later time. 
     Note that the point at which cached data blocks for a file that is being deleted should be flushed from cloud controller caches may vary. If deduplication is enabled, a data block referenced by the file being deleted may also still be referenced (and actively used) by other files, and hence should not be flushed from the cloud controller&#39;s cache. If, however, the cloud controller determines that a data block is no longer referenced, it can check whether the block is still cached and, if so, delete it from the cache. Alternatively, instead of being explicitly flushed, the cache controller may instead leave the data block in the cache and let it be flushed out eventually (due to disuse) via normal cache replacement policies. Note also that deduplication techniques can help to reduce the overhead of deletion operations by reducing the amount of redundant data that is stored and will later need to be deleted; deletion operations that delete unique data involve somewhat more computation and overhead (e.g., additional operations  4  and  5  in  FIG. 27C ) than deletion operations that primarily decrement references to still-used data blocks. 
       FIG. 28  presents a flow chart that illustrates the process of deleting a file from a distributed filesystem. Two or more cloud controllers collectively manage distributed filesystem data that is stored in one or more cloud storage systems (operation  2800 ); the cloud controllers cache and ensure data consistency for the stored data. During operation, a cloud controller receives a request from a client to delete a file in the distributed filesystem (operation  2810 ). The cloud controller updates a user view of the distributed filesystem to present the appearance of the target file being deleted to the client (operation  2820 ), and then initiates a background deletion operation to delete the target file without negatively affecting the performance of the other users of the distributed filesystem (operation  2830 ). 
     In some embodiments, a distributed filesystem can be tuned to balance a range of performance and cost goals. For a scenario in which performance is the only factor and cost is no issue, the data for the distributed filesystem might be stored in the cloud storage system that provides the highest availability, redundancy, and performance, and each client site may include multiple peer cloud controllers as well as backup cloud controllers. In a more typical scenario, however, performance may need to be balanced by cost factors. For instance, the distributed filesystem may leverage multiple cloud service providers (including archival cloud service providers) to minimize the storage costs for infrequently accessed data while maintaining high performance for frequently accessed data. For example, cloud controllers may be configured to track and consider a range of parameters (e.g., the most commonly used data, the most frequently transferred data, access frequencies, typical access sizes, the costs of storing data in different cloud storage providers, the network cost of leveraging distributed cloud controllers as peer caches, etc.) and attempt to automatically rebalance and optimize system behavior to maximize performance for a given fixed cost. 
     In summary, embodiments of the present invention facilitate storing and accessing data in a distributed filesystem. A set of distributed cloud controllers manage data stored in a cloud-based storage system to provide a high-capacity, high-reliability storage system that ensures data consistency. These cloud controllers cache the set of data that is being used by their respective clients, store updates in cloud files on the cloud storage system, and forward updates to each other via incremental snapshots. The data capacity of the system can be easily extended as needed by leasing additional space for the cloud storage system. Hence, the disclosed embodiments present an abstraction of one global, extensible filesystem while preserving the abstraction of high-speed local data access. 
     Computing Environment 
     In some embodiments of the present invention, techniques for managing and/or accessing a distributed filesystem can be incorporated into a wide range of computing devices in a computing environment. For example,  FIG. 9  illustrates a computing environment  900  in accordance with an embodiment of the present invention. Computing environment  900  includes a number of computer systems, which can generally include any type of computer system based on a microprocessor, a mainframe computer, a digital signal processor, a portable computing device, a personal organizer, a device controller, or a computational engine within an appliance. More specifically, referring to  FIG. 9 , computing environment  900  includes clients  910 - 912 , users  920  and  921 , servers  930 - 950 , network  960 , database  970 , devices  980 , appliance  990 , and cloud-based storage system  995 . 
     Clients  910 - 912  can include any node on a network that includes computational capability and includes a mechanism for communicating across the network. Additionally, clients  910 - 912  may comprise a tier in an n-tier application architecture, wherein clients  910 - 912  perform as servers (servicing requests from lower tiers or users), and wherein clients  910 - 912  perform as clients (forwarding the requests to a higher tier). 
     Similarly, servers  930 - 950  can generally include any node on a network including a mechanism for servicing requests from a client for computational and/or data storage resources. Servers  930 - 950  can participate in an advanced computing cluster, or can act as stand-alone servers. For instance, computing environment  900  can include a large number of compute nodes that are organized into a computing cluster and/or server farm. In one embodiment of the present invention, server  940  is an online “hot spare” of server  950 . 
     Users  920  and  921  can include: an individual; a group of individuals; an organization; a group of organizations; a computing system; a group of computing systems; or any other entity that can interact with computing environment  900 . 
     Network  960  can include any type of wired or wireless communication channel capable of coupling together computing nodes. This includes, but is not limited to, a local area network, a wide area network, or a combination of networks. In one embodiment of the present invention, network  960  includes the Internet. In some embodiments of the present invention, network  960  includes phone and cellular phone networks. 
     Database  970  can include any type of system for storing data in non-volatile storage. This includes, but is not limited to, systems based upon magnetic, optical, or magneto-optical storage devices, as well as storage devices based on flash memory and/or battery-backed up memory. Note that database  970  can be coupled: to a server (such as server  950 ), to a client, or directly to a network. In some embodiments of the present invention, database  970  is used to store information that may later be stored in unused bits of a memory pointer. Alternatively, other entities in computing environment  900  (e.g., servers  930 - 950 ) may also store such data. 
     Devices  980  can include any type of electronic device that can be coupled to a client, such as client  912 . This includes, but is not limited to, cell phones, personal digital assistants (PDAs), smartphones, personal music players (such as MP3 players), gaming systems, digital cameras, portable storage media, or any other device that can be coupled to the client. Note that, in some embodiments of the present invention, devices  980  can be coupled directly to network  960  and can function in the same manner as clients  910 - 912 . 
     Appliance  990  can include any type of appliance that can be coupled to network  960 . This includes, but is not limited to, routers, switches, load balancers, network accelerators, and specialty processors. Appliance  990  may act as a gateway, a proxy, or a translator between server  940  and network  960 . 
     Cloud-based storage system  995  can include any type of networked storage devices (e.g., a federation of homogeneous or heterogeneous storage devices) that together provide data storage capabilities to one or more clients. 
     Note that different embodiments of the present invention may use different system configurations, and are not limited to the system configuration illustrated in computing environment  900 . In general, any device that includes computational and storage capabilities may incorporate elements of the present invention. 
       FIG. 10  illustrates a computing device  1000  that includes a processor  1002  and a storage mechanism  1004 . Computing device  1000  also includes a receiving mechanism  1006  and a storage management mechanism  1008 . 
     In some embodiments, computing device  1000  uses receiving mechanism  1006 , storage management mechanism  1008 , and storage mechanism  1004  to manage data in a distributed filesystem. For instance, storage mechanism  1004  can store metadata for a distributed filesystem, and computing device  1000  can use receiving mechanism  1006  to receive a request to access a data block for a file. Program instructions executing on processor  1002  can traverse the stored metadata to identify a metadata entry that is associated with the data block. Storage management mechanism  1008  can use this metadata entry to download a cloud file containing the data block from a cloud storage system. 
     In some embodiments of the present invention, some or all aspects of receiving mechanism  1006 , storage management mechanism  1008 , and/or a filesystem device driver can be implemented as dedicated hardware modules in computing device  1000 . These hardware modules can include, but are not limited to, processor chips, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), memory chips, and other programmable-logic devices now known or later developed. 
     Processor  1002  can include one or more specialized circuits for performing the operations of the mechanisms. Alternatively, some or all of the operations of receiving mechanism  1006 , storage management mechanism  1008 , and/or a filesystem device driver may be performed using general-purpose circuits in processor  1002  that are configured using processor instructions. Thus, while  FIG. 10  illustrates receiving mechanism  1006  and/or storage management mechanism  1008  as being external to processor  1002 , in alternative embodiments some or all of these mechanisms can be internal to processor  1002 . 
     In these embodiments, when the external hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. For example, in some embodiments of the present invention, the hardware module includes one or more dedicated circuits for performing the operations described above. As another example, in some embodiments of the present invention, the hardware module is a general-purpose computational circuit (e.g., a microprocessor or an ASIC), and when the hardware module is activated, the hardware module executes program code (e.g., BIOS, firmware, etc.) that configures the general-purpose circuits to perform the operations described above. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.