Patent Publication Number: US-7904492-B2

Title: Method and apparatus for concurrent read-only access to filesystem

Description:
RELATED APPLICATIONS 
     This application concerns material that may be related to that disclosed in copending application Ser. No. 11/321,431, entitled Method and Apparatus for Cloning Filesystems Across Computing Systems, filed Dec. 28, 2005. 
     FIELD OF THE INVENTION 
     The invention relates to data storage systems. More specifically, the invention relates to increasing the service capacity of a storage facility by providing additional servers to respond to clients that require read-only access to data. 
     BACKGROUND 
     A filesystem is a data structure (or set of data structures) and associated logic that facilitate tasks related to storing data in a computer system: allocating and freeing storage space on mass storage devices; reading and writing data; creating, modifying and deleting files; maintaining hierarchical directory structures; and so forth. Filesystem data structures are typically fairly complex, with interdependent fields that must be maintained consistently to avoid data corruption or loss. Because of this complexity and interdependence, filesystem operations do not lend themselves to parallelization. Consequently, improving the performance of a large file server (e.g. reducing operation latency or increasing client capacity) often entails purchasing new, faster hardware. 
     Methods of providing enhanced access to filesystem data without such expenditure may be of benefit in this field. 
     SUMMARY OF THE INVENTION 
     A storage server can provide additional data service capacity by obtaining metadata to describe a filesystem, processing the metadata to locate a data block in the filesystem, and reading the block from a remote storage subsystem. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
         FIG. 1  shows a sample environment that implements an embodiment of the invention. 
         FIG. 2  shows some representative data structures that may be incorporated in a filesystem. 
         FIG. 3  shows a broader view of data structures in a filesystem. 
         FIGS. 4A-4C  show several ways a file&#39;s contents can be modified. 
         FIG. 5A  outlines the operations of a system performing concurrent access to a read-only filesystem, and  FIG. 5B  gives a concrete example. 
         FIG. 6  shows how a filesystem can be nested within a container file on a lower-level filesystem. 
         FIG. 7  is a block diagram of a system that could implement an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a computing environment that can support an embodiment of the invention. Systems  100  and  104  are storage servers that provide data storage and retrieval services to clients  108 ,  112  and  116 . (A “storage server” may be a traditional file server, or a server to provide data storage in units other than files—for example, in fixed-size blocks. The first type of server is often associated with the acronym “NAS” for Network Attached Storage, while the second type goes by the acronym “SAN” for Storage Area Network. Both types of server functionality are available commercially, for example in the Fabric-Attached Storage or “FAS” product line from Network Appliance, Inc. of Sunnyvale, Calif.) 
     Clients can communicate with storage servers  100  and  104  through local-area networks (“LANs”)  120  and  124 , or over a wide-area network (“WAN”)  128  such as the Internet. Requests to create or delete files, to list directories, to read or write data, or to perform similar filesystem-related operations are transmitted from clients to a storage server, and responses are sent back. 
     Each storage server  100 ,  104  will have at least one mass storage device to store data and instructions for its own use, as well as to store user data files and information about the filesystem within which the files exist. In  FIG. 1 , each storage server has two “private” disks that are accessible only to that server (server  100  has disks  132  and  136 ; server  104  has disks  140  and  144 ). In the exemplary embodiment of the invention shown here, each storage server is also connected to a Fibre Channel (“FC”) switch  148 , which mediates access from the servers to an array of disks  152 - 180 . Each server may be able to read or write to any of the disks in the FC array through switch  148 , but software executing on the servers may cooperate to respect a convention that, for example, disks in group  184  may only be written by server  100  and are read-only with respect to server  104 , while disks in group  188  may only be written by server  104  and are read-only to server  100 . Henceforth, references to “read-only storage systems” will include storage systems on which data cannot be modified (e.g. because the data on the storage media is fixed and unmodifiable); systems on which data modification is prevented (e.g. by a write-locking switch or signal); as well as systems on which data could be written, but is not (e.g. by convention, agreement, or software design). 
     Mass storage devices that may be written by a server are said to be “local” to the server or “owned by” the server. Devices that may be read but not written by a server are considered to be “remote,” and the reading server is said to be a “guest.” The owner and one or more guests share access to the storage system. In  FIG. 1 , server  100  owns disks  152 ,  156 ,  160 ,  168  and  172  in group  184  and shares access to remote disks  164 ,  176  and  180  in group  188 . Server  104  owns the local disks in group  188  and is a guest with respect to remote disks in group  184 . 
     The servers may manage the raw data storage provided by the disks shown in  FIG. 1  as a Redundant Array of Independent Disks (“RAID”), or some intermediate or lower-level hardware, firmware, or software entity (not shown) could provide reliable storage without any special arrangements by the servers. Embodiments of the invention operate logically at a higher level of abstraction, so the specific details of the storage subsystems will not be considered further. Instead, embodiments will be described with reference to one or more “storage systems” that provide either read-only or read-write access to a series of data blocks in a storage volume, each data block having a unique, sequential identifying number from zero to the total number of blocks in the volume. An Integrated Device Electronics (“IDE”) or Small Computer System Interface (“SCSI”) hard disk provides such an interface, and a RAID array can be operated in this way also. However, note that a data file in a filesystem can also be viewed as a sequence of read-only or read-write blocks, so embodiments of the invention can be nested: a filesystem as described here may be constructed in a data file, which is itself a file on a lower-level filesystem—even another filesystem contained in a data file on an even lower-level filesystem. Applications for one or two levels of nesting will be mentioned below; greater numbers of levels are possible, but may not be particularly useful. 
       FIG. 2  shows a simplified representation of some of the data structures that may be included in a filesystem. A first structure called an “inode”  210  is a metadata container to contain metadata about a file in the filesystem (metadata may include, for example, the file&#39;s size  220 , owner  222 , permissions  224 , creation time  226 , modification time  228 , and other information  230 ). The inode may also contain data block numbers  235  so that the file contents can be located on the storage volume. Every file is associated with an inode. The file associated with inode  210  is 176 bytes long, and those bytes are stored in data blocks  240 ,  247  and  252 . (In this simple illustration, data blocks of only 64 bytes are used, but in practical systems, larger blocks—usually in sizes that are powers of two—may be used.) This simple filesystem also contains a block map structure  260 , which indicates, for each data block in the storage volume, whether the block is in use. For example, in addition to blocks  240 ,  247  and  252  which are in use by the data file corresponding to inode  210 , blocks  001  and  236  are marked “in use” in block map  260 . Inodes themselves and the block map are data that may be stored in some of the data blocks of a storage system. 
     Note that neither of the data structures described above contains the file&#39;s name. The filesystem can implement named files and a hierarchical directory structure by placing the names and corresponding inode numbers in a file (which is itself associated with an inode), and treating that file specially as a directory. One inode in a filesystem is typically designated as the “root directory” of the filesystem; all other files should be accessible through an inode associated with a filename in the root directory or in a hierarchical descendant of the root directory. 
       FIG. 3  presents a broader view of a filesystem to illustrate the relationships between files, directories and inodes. Data structure  310  contains filesystem metadata—control information about the filesystem as a whole. For example, it may contain the date the filesystem was created  312 , the date the file system was last mounted (placed in use on a computer system)  314 , the last mount location  316 , and the date of the last backup  318 . Other information  320  may also be stored in the filesystem metadata structure. One important piece of information is the identity of the root inode  322 , which permits filesystem processing logic to find other files and directories stored in the filesystem. 
     In the filesystem shown here, the root inode is inode  2  ( 325 ); as described with reference to  FIG. 2 , the inode provides block numbers for data blocks containing information on contents of the root directory. This information has been formatted as a table  330  containing a directory flag column  332 , file (or directory) name column  335 , and an inode number  338 . Directory entries (rows in the table) may contain other information as well, but it is not relevant to descriptions of embodiments of the invention. 
     An entry in the root directory (or in any other directory) may identify a data file or another directory. For example, entry  340  identifies a file named “vmunix” whose data  350  can be located through inode  112  ( 345 ). Entry  355  identifies a second-level directory named “home” that is associated with inode  823  ( 360 ). Home  365  contains four third-level directory entries, including one named “elaine”  370  that is associated with inode  302  ( 375 ). Directory elaine  380  contains entries for two files and one sub-directory. Entry  385  identifies a file called “dissertation” associated with inode  98  ( 390 ) whose data  395  can be located through the inode. 
     The filesystem data structures described with reference to  FIGS. 2 and 3  support an operational mode that is used by embodiments of the invention. Consider a filesystem that contains various files and directories.  FIG. 4A  shows one of those files: inode  410  contains information to locate data blocks  420 ,  425  and  430 . If the file contents are modified by a write operation, the new data might simply be placed into the currently-allocated data blocks, overwriting some of the existing contents as shown by before-and-after  FIG. 4B . However, it may be useful to preserve the state of the file at a particular time, so instead of overwriting the existing file contents, a new inode  460  might be allocated and configured to refer to a new sequence of data blocks. Data blocks that are not modified can be shared between the original inode  410  and the new inode  460 . This is illustrated in  FIG. 4C , where original inode  410  continues to list data blocks  320 ,  325  and  330 , while inode  460  lists data blocks  420 ,  470  and  430 . Data block  470  contains the data of block  425  as modified by the write operation. The original version of the file is available through inode  410 , while the modified version is available through inode  460 . Thus, inode  410  describes the file at a point in time just before modifications began to be made through another inode. Eventually, inode  460  may be preserved as a second point-in-time version, and further modifications may be made within a third sequence of data blocks located by a third inode. The versions persist as long as the inodes describing them are maintained. They are read-only, because some of the data blocks from a file image may be shared with other file images (or with the active file), so modifications made through an image inode might cause unwanted changes in other files as well. For example, if a previously-saved image from a first time and a second image from a later time happened to share a data block, and the shared block was allowed to be modified through an inode from the second image, the same change would appear in the file in the first image. The change might be unexpected by software that referred to the first image, and could cause incorrect operation. The images described above will be referred to as read-only, persistent point-in-time images (“RPPI”). RPPIs are like the Snapshot® functionality available in storage server products from Network Appliance, Inc. of Sunnyvale, Calif. 
     RPPIs can be made of directories, too, since they are simply files that are treated specially for some purposes. Thus, the filesystem data structures shown in  FIGS. 2 and 3  can support an RPPI facility to preserve the state of any file, directory, or complete hierarchy at a point in time. Future modifications to the files and directories occur within data block sequences identified by new inodes, while inodes and blocks that are not modified can be shared with one or more RPPIs. An RPPI of a complete filesystem may have a metadata structure to describe the RPPI and to locate the root directory; from that metadata, filesystem logic can descend the filesystem hierarchy to locate and read any file in the RPPI. 
     Although an RPPI is an image of a file system and consequently has all the complexity and data interdependencies of such a structure, the fact that its elements do not change can be exploited by an embodiment of the invention. Consider the following scenario: a storage server (such as server  100  in  FIG. 1 ) creates a filesystem on storage devices in group  184 , then makes an image of its contents at a point in time as an RPPI with a filesystem metadata structure. Server  104 , which has read-only access to the storage devices in group  184 , can access the RPPI according to the method of  FIG. 5A . The accesses can occur concurrently with any activities of storage server  100 , because neither storage server will modify the RPPI filesystem. In particular, both storage servers (or any of the storage servers, if more than one “remote” server is configured to serve the data on the shared storage subsystem) can retrieve the same data at substantially the same time. The servers can respond to co-pending requests from a number of clients without intra-request blocking, serialization, or sequencing restrictions. 
     First, server  104  obtains metadata that describes the filesystem ( 510 ). The metadata may simply be read from the remote storage subsystem, but in one embodiment, server  104  may request the metadata from server  100  so that server  100  will be aware that the RPPI is in use and that its data should not be discarded. As an extension of this idea, in some embodiments server  104  will request a lock of the RPPI from server  100 , and transmit “heartbeat” signals while the RPPI is still needed. After server  104  completes its use of the RPPI, it can release the lock. If server  104  crashes, server  100  may notice the failed heartbeat and discard the RPPI. 
     Returning to  FIG. 5A , server  104  can process the metadata to locate a data block in the filesystem ( 520 ). This processing can include, for example, retrieving the root inode number from the metadata ( 522 ), locating the root inode on the remote volume ( 525 ), and reading a list of block numbers from the root inode ( 528 ). After the data block has been located, server  104  can read the data in the block from the remote volume ( 530 ). 
     Operations  520  and  530  may be repeated as necessary to find and read any data block in any file in the filesystem described by the metadata ( 540 ). For example,  FIG. 5B  shows the operations of storage server  104  processing an RPPI of the filesystem shown in  FIG. 3  to obtain data from the file named “/home/elaine/dissertation”. The root inode number is obtained from filesystem metadata  310  ( 545 ) and inode  2  ( 325 ) is read ( 550 ) to find the data blocks comprising the root directory. The root directory  330  is read ( 555 ) and the “home” entry is located ( 560 ) to obtain the next inode number,  823 . Inode  823  is read ( 565 ) to find the data blocks comprising the home directory  365 , and those data blocks are read ( 570 ). The “elaine” entry is located ( 575 ) and its inode number  302  is obtained ( 580 ). The data blocks comprising the “elaine” directory are read ( 585 ) and the “dissertation” entry  385  is located ( 590 ). Finally inode number  98  is read to find the data blocks that make up the “dissertation” file ( 595 ), and the contents of the file itself may be read ( 599 ). Note that, while storage server  104  is searching for and reading the file “/home/elaine/dissertation”, server  100  may also be searching for and reading files in the filesystem. In fact, server  100  may be reading the very same file on behalf of another client system. The servers provide parallel access paths to the same data, thus increasing the amount of processing power and network bandwidth available to serve the data from the shared storage subsystem. 
     The concurrent, read-only access to the RPPI filesystem that is provided by storage server  104  may be of use to several types of clients. For example, external clients that do not need (or do not have) write access to the filesystem can receive service from server  104  instead of relying on server  100 . Also, server  104  may be provided with backup facilities such as a tape drive or tape library, and may use its read-only access to perform backups of the RPPI filesystem without affecting server  100 &#39;s response to its clients. 
     As mentioned in paragraph [0018], data for a filesystem and the files and directories it organizes may be stored directly on a mass storage subsystem such as a hard disk or RAID array, or in a file in a lower-level filesystem. (A file that contains a filesystem is called a “container file.”) The latter arrangement may require additional processing to find and read data in a file, since the data block must be located once by reference to the filesystem data in the container file, and again by reference to the data of the filesystem where the container file resides. Each level of nesting requires filesystem parsing logic to locate data blocks by block numbers or addresses within the filesystem&#39;s storage. One level of nesting provides certain advantages to offset the extra processing, but multiple levels of nesting are less likely to be useful. 
       FIG. 6  shows some portions of a filesystem  600 : filesystem metadata  610 , root inode  620 , and data blocks containing the root directory data  630 ,  633  and  636 . Filesystem parsing logic can compute the block number of a root directory data block by following the method outlined in  FIG. 5A . If filesystem  620  is stored directly on a storage subsystem, then the block number thus computed may be presented to the storage subsystem and the data in the block read. The block number identifies a physical data block on the underlying storage system volume, so it may be described as a physical volume block number (“PVBN”). 
     However, if filesystem  600  has been constructed within a container file, then the block number computed above does not identify a physical data block. Instead, it identifies a block at a particular offset within the container file, and may be described as a virtual volume block number (“VVBN”). To obtain the desired data, the filesystem parsing logic must examine the filesystem where the container file resides. For example, element  640  shows a second filesystem including metadata  650 , root inode  660  and root directory  670  with container file name  673  and inode number  676 . From the container file inode  680 , filesystem parsing logic can obtain the block numbers of container file data blocks  682 ,  684 ,  686  and  688 . 
     If the virtual volume block number computed by processing data in filesystem  600  is 3, then the desired data might be located in container file data block  686 . The block number of container file data block  686  may be determined through the examination of container filesystem  640  and, if the container filesystem is stored directly on a storage subsystem, the block number may be used as a PVBN to read the desired data. If the container filesystem has been constructed within a lower-level container file, the filesystem parsing must be repeated. 
     The process of locating a desired data block explained with reference to  FIG. 6  may be concisely expressed as translating a virtual volume block number (“VVBN”) to a physical volume block number (“PVBN”). If multiple layers of container files are in use, the translation will have an equal number of steps from VVBN to lower-level-WBN and so on, until a PVBN is obtained. Note that although the guest storage server has access to the storage subsystem where the container file resides, the container file&#39;s filesystem may not be in a read-only state, so the guest storage server may not be able to perform the VVBN-to-PVBN translation itself because of changes to the container file&#39;s filesystem that have not yet been written to the storage system. Therefore, in some embodiments, the guest storage server may present the VVBN to the owner storage server and receive a corresponding PVBN calculated by the owner. The owner storage server will have access to any cached data necessary to locate the desired block of the container file within the container file&#39;s filesystem. 
     Although the filesystem parsing and VVBN-to-PVBN translation methods appear to be tedious and time-consuming, storage server  104  may cache data to improve its performance. If it is desired that server  104  begin serving data from a new, later RPPI, it can freeze its operations, discard cached data, load new filesystem control information from the filesystem metadata, and then “thaw” the filesystem by resuming operations with the new RPPI. 
     Hardware, firmware and software to implement the methods described above may be incorporated in a system like that shown in  FIG. 7 . One or more central processing units (“CPUs” or “processors)  710  may execute instructions contained in memory  720 . These instructions may perform portions of methods of an embodiment. For example, filesystem parsing logic  722  may locate data blocks in a filesystem on a remote storage subsystem based on control information about the remote filesystem and data from the remote storage subsystem. The control information may be stored on the remote storage subsystem or on a local storage system. Memory  720  may also contain protocol processing logic  725  to interpret protocol requests from clients, and control logic  728  to coordinate the activities of filesystem parsing logic  722  and protocol processing logic  725  so that the system can read a data block corresponding to a protocol request and return the content of the data block to the requesting client. In some embodiments, filesystem parsing, protocol processing and control logic may be implemented as hardware circuits  732 ,  735  and  738  instead of software instructions. Logic to translate VVBNs to PVBNs may be incorporated within filesystem parsing logic  722  or implemented as a separate hardware or software module. 
     A system may also include one or more communication interfaces  750  to exchange requests and data with clients and storage adapters  760  and  770  to communicate with mass storage subsystems such as local disks  140  and  144  or remote (read-only) disks  152 ,  156 ,  160 ,  168  and  172 . 
     An embodiment of the invention may be a machine-readable medium having stored thereon instructions which cause a processor to perform operations as described above. In other embodiments, the operations might be performed by specific hardware components that contain hardwired logic. Those operations might alternatively be performed by any combination of programmed computer components and custom hardware components. 
     A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer), including but not limited to Compact Disc Read-Only Memory (CD-ROMs), Read-Only Memory (ROMs), Random Access Memory (RAM), Erasable Programmable Read-Only Memory (EPROM), and a transmission over the Internet. 
     The applications of the present invention have been described largely by reference to specific examples and in terms of particular allocations of functionality to certain hardware and/or software components. However, those of skill in the art will recognize that systems to provide concurrent read-only access to filesystems can also be implemented by software and hardware that distribute the functions of embodiments of this invention differently than herein described. Such variations and implementations are understood to be encompassed by the following claims.