Patent Publication Number: US-2010125598-A1

Title: Architecture for supporting sparse volumes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 11/409,624, filed on Apr. 24, 2006, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/674,641, which was filed on Apr. 25, 2005, by Jason Ansel Lango for an Architecture For Supporting Sparse Volumes and is hereby incorporated by reference. 
     RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 11/409,887, filed on Apr. 24, 2006, entitled SYSTEM AND METHOD FOR SPARSE VOLUMES, by Jason Lango, et al, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to file systems and, more specifically, to a protocol for use with a file system that includes volumes having one or more files with blocks that require a special operation to retrieve data associated therewith from a remote backing store. 
     BACKGROUND OF THE INVENTION 
     A storage system typically comprises one or more storage devices into which information may be entered, and from which information may be obtained, as desired. The storage system includes a storage operating system that functionally organizes the system by, inter alia, invoking storage operations in support of a storage service implemented by the system. The storage system may be implemented in accordance with a variety of storage architectures including, but not limited to, a network-attached storage environment, a storage area network and a disk assembly directly attached to a client or host computer. The storage devices are typically disk drives organized as a disk array, wherein the term “disk” commonly describes a self-contained rotating magnetic media storage device. The term disk in this context is synonymous with hard disk drive (HDD) or direct access storage device (DASD). 
     Storage of information on the disk array is preferably implemented as one or more storage “volumes” of physical disks, defining an overall logical arrangement of disk space. The disks within a volume are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). Most RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of redundant information (parity) with respect to the striped data. The physical disks of each RAID group may include disks configure to store striped data (i.e., data disks) and disks configure to store parity for the data (i.e., parity disks). The parity may thereafter be retrieved to enable recovery of data lost when a disk fails. The term “RAID” and its various implementations are well-known and disclosed in  A Case for Redundant Arrays of Inexpensive Disks  ( RAID ), by D. A. Patterson, G. A. Gibson and R. H. Katz, Proceedings of the International Conference on Management of Data (SIGMOD), June 1988. 
     The storage operating system of the storage system may implement a high-level module, such as a file system, to logically organize the information stored on the disks as a hierarchical structure of directories, files and blocks. For example, each “on-disk” file may be implemented as set of data structures, i.e., disk blocks, configured to store information, such as the actual data for the file. These data blocks are organized within a volume block number (vbn) space. The file system, which controls the use and contents of blocks within the vbn space, organizes the data blocks within the vbn space as a “logical volume”; each logical volume may be, although is not necessarily, associated with its own file system. The file system typically consists of a contiguous range of vbns from zero to n-1, for a file system of size n blocks. 
     A known type of file system is a write-anywhere file system that does not over-write data on disks. If a data block is retrieved (read) from disk into a memory of the storage system and “dirtied” (i.e., updated or modified) with new data, the data block is thereafter stored (written) to a new location on disk to optimize write performance. A write-anywhere file system may also opt to maintain a near optimal layout such that the data is substantially contiguously arranged on disks. The optimal disk layout results in efficient access operations, particularly for sequential read operations, directed to the disks. An example of a write-anywhere file system that is configure to operate on a storage system is the Write Anywhere File Layout (WAFL™) file system available from Network Appliance, Inc., Sunnyvale, Calif. 
     The storage operating system may further implement a storage module, such as a RAID system, that manages the storage and retrieval of the information to and from the disks in accordance with input/output (I/O) operations. The RAID system is also responsible for parity operations in the storage system. Note that the file system only “sees” the is data disks within its vbn space; the parity disks are “hidden” from the file system and, thus, are only visible to the RAID system. The RAID system typically organizes the RAID groups into one large “physical” disk (i.e., a physical volume), such that the disk blocks are concatenated across all disks of all RAID groups. The logical volume maintained by the file system is then “disposed over” (spread over) the physical volume maintained by the RAID system. 
     The storage system may be configure to operate according to a client/server model of information delivery to thereby allow many clients to access the directories, files and blocks stored on the system. In this model, the client may comprise an application, such as a database application, executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. Each client may request the services of the file system by issuing file system protocol messages (in the form of packets) to the storage system over the network. By supporting a plurality of file system protocols, such as the conventional Common Internet File System (CIFS) and the Network File System (NFS) protocols, the utility of the storage system is enhanced. 
     When accessing a block of a file in response to servicing a client request, the file system specifies a vbn that is translated at the file system/RAID system boundary into a disk block number (dbn) location on a particular disk (disk, dbn) within a RAID group of the physical volume. It should be noted that a client request is typically directed to a specific file offset, which is then converted by the file system into a file block number (fbn), which represents an offset into a particular file. For example, if a file system is using 4 KB blocks, fbn 6 of a file represents a block of data starting 24 KB into the file and extending to 28 KB, where fbn 7 begins. The fbn is converted to an appropriate vbn by the file system. Each block in the vbn space and in the dbn space is typically fixed, e.g., 4 k bytes (kB), in size; accordingly, there is typically a one-to-one mapping between the information stored on the disks in the dbn space and the information organized by the file system in the vbn space. The (disk, dbn) location specified by the RAID system is further translated by a disk driver system of the storage operating system into a plurality of sectors (e.g., a  4 kB block with a RAID header translates to 8 or 9 disk sectors of 512 or 520 bytes) on the specified disk. 
     The requested block is then retrieved from disk and stored in a buffer cache of the memory as part of a buffer tree of the file. The buffer tree is an internal representation of blocks for a file stored in the buffer cache and maintained by the file system. Broadly stated, the buffer tree has an Mode at the root (top-level) of the file. An Mode is a data structure used to store information, such as metadata, about a file, whereas the data blocks are structures used to store the actual data for the file. The information contained in an Mode may include, e.g., ownership of the file, access permission for the file, size of the file, file type and references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by pointers, which may further reference indirect blocks that, in turn, reference the data blocks, depending upon the quantity of data in the file. Each pointer may be embodied as a vbn to facilitate efficiency among the file system and the RAID system when accessing the data on disks. 
     The RAID system maintains information about the geometry of the underlying physical disks (e.g., the number of blocks in each disk) in raid labels stored on the disks. The RAID system provides the disk geometry information to the file system for use when creating and maintaining the vbn-to-disk,dbn mappings used to perform write allocation operations and to translate vbns to disk locations for read operations. Block allocation data structures, such as an active map, a snapmap, a space map and a summary map, are data structures that describe block usage within the file system, such as the write-anywhere file system. These mapping data structures are independent of the geometry and are used by a write allocator of the file system as existing infrastructure for the logical volume. Examples of the block allocation data structures are described in U.S. Pat. No. 7,494,445, titled Instant Snapshot, by Blake Lewis et al., issued on Nov. 18, 2008 which application is hereby incorporated by reference. 
     The write-anywhere file system typically performs write allocation of blocks in a logical volume in response to an event in the file system (e.g., dirtying of the blocks in a file). When write allocating, the file system uses the block allocation data structures to select free blocks within its vbn space to which to write the dirty blocks. The selected blocks are generally in the same positions along the disks for each RAID group (i.e., within a stripe) so as to optimize use of the parity disks. Stripes of positional blocks may vary among other RAID groups to, e.g., allow overlapping of parity update operations. When write allocating, the file system traverses a small portion of each disk (corresponding to a few blocks in depth within each disk) to essentially “lay down” a plurality of stripes per RAID group. In particular, the file system chooses vbns that are on the same stripe per RAID group during write allocation using the vbn-to-disk,dbn mappings. 
     During storage system operation, a volume (or other data container, such as a file or directory) may become corrupted due to, e.g., physical damage to the underlying storage devices, software errors in the storage operating system executing on the storage system or an improperly executing application program that modifies data in the volume. In such situations, an administrator may want to ensure that the volume is promptly mounted and exported so that it is accessible to clients as quickly as possible; this requires that the data in the volume (which may be substantial) be recovered as soon as possible. Often, the data in the volume may be recovered by, e.g., reconstructing the data using stored parity information if the storage devices are utilized in a RAID configuration. Here, reconstruction may occur “on-the-fly”, resulting in virtually no discernable s time where the data is not accessible. 
     In other situations, reconstruction of the data may not be possible. As a result, the administrator has several options, one of which is to initiate a direct copy of the volume from a point-in-time image stored on another storage system. In the general case, all volume data and metadata must be copied, prior to resuming normal operations, as a guarantee of application consistency. However, such “brute force” data copying is generally inefficient, as the time required to transfer substantial amounts of data, e.g., terabytes, may be on the order of days. Similar disadvantages are associated with restoring data from a tape device or other offline data storage. Another option that enables an administrator to rapidly mount and export a volume is to generate a hole-filled volume, wherein is the contents of the volume are “holes”. In this context, holes are manifested as entire blocks of zeros or other predefined pointer values stored within the buffer tree structure of a volume. An example of the use of such holes is described in the U. S. Pat. No. 7,457,982, issued on Nov. 25, 2008, entitled WRITABLE READ-ONLY SNAPSHOTS, by Vijayan Rajan, the contents of which are hereby incorporated by reference. 
     In such a hole-filled environment, the actual data is not retrieved from a backing store until requested by a client. However, a noted disadvantage of such a hole-based technique is that repeated write operations are needed to generate the appropriate number of zero-filled blocks on disk for the volume. That is, the use of holes to implement a data container that requires additional retrieval operations to retrieve data further requires that the entire buffer tree of a file and/or volume be written to disk during creation. The time required to perform the needed write operations may be substantial depending on the size of the volume or file. Thus, the creation of a hole-filled volume is oftentimes impractical due to the need for quick data access to a volume. 
     A storage environment in which there is typically a need to quickly “bring back” a volume involves the use of a near line storage server. As used herein, the term “near line storage server” means a secondary storage system adapted to store data forwarded from one or more primary storage systems, typically for long term archival purposes. The near s line storage server may be utilized in such a storage environment to provide a back up of data storage (e.g., a volume) served by each primary storage system. As a result, the near line storage server is typically optimized to perform bulk data restore operations, but suffers reduced performance when serving individual client data access requests. This latter situation may arise where a primary storage system encounters a failure that damages its volume in such a manner that a client must send its data access requests to the server in order to access data in the volume. This situation also forces the clients to reconfigure with appropriate network addresses associated with the near line storage server to enable such data access. 
     SUMMARY OF THE INVENTION 
     is The present invention overcomes the disadvantages of the prior art by providing a system and method for supporting a sparse volume within a file system of a storage system. As used herein, a sparse volume contains one or more files with at least one data block (i.e., an absent block) that is not stored locally on disk coupled to the storage system. By not storing the data block (or a block of zeros as in a hole environment), the sparse volume may be generated and exported quickly with minimal write operations required. The “missing” data of an absent block is stored on an alternate, possibly remote, source (e.g., a backing store) and is illustratively retrieved using a remote fetch operation. 
     A storage operating system executing on the storage system includes a novel NRV (NetApp Remote Volume) protocol module that implements an NRV protocol. The NRV protocol module interfaces with the file system to provide remote retrieval from the backing store. The NRV protocol module is invoked by an exemplary Load_Block( ) function within the file system that determines whether a block is to be retrieved from the remote backing store. 
     The Load_Block( ) function initiates a series of NRV protocol requests to the backing store to retrieve the data. The NRV protocol module first authenticates the connection and then transmits an initialization request to match the appropriate information required at the beginning of the connection. Once the NRV protocol connection has been initialized and authenticated, various types of data may be retrieved from the backing store including, for example, information relating to volumes, blocks and files or other data containers stored on the backing store. Additionally, the NRV protocol provides a mechanism to remotely lock a persistent consistency point image (PCPI) or snapshot (a lock PCPI request) on the backing store so that the backing store does not modify or delete the PCPI until it is unlocked via an unlock command (an unlock PCPI request). Such locking may be utilized when the backing store is instantiated within a PCPI that is required for a long-lived the application on the storage system, such as a restore on demand application. The novel NRV protocol also includes commands for retrieving status information such as volume information, from the backing store. This may be accomplished by sending a VOLINFO request to the backing store identifying the particular volume of interest 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identical or functionally similar elements: 
         FIG. 1  is a schematic block diagram of an exemplary network environment in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic block diagram of an exemplary storage operating system in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic block diagram of an exemplary inode in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic block diagram of an exemplary buffer tree in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic block diagram of an illustrative embodiment of a buffer tree of a file that may be advantageously used with the present invention; 
         FIG. 6  is a schematic block diagram of an exemplary aggregate in accordance with an embodiment of the present invention; 
         FIG. 7  is a schematic block diagram of an exemplary on-disk layout in accordance with an embodiment of the present invention; 
         FIG. 8  is a schematic block diagram of an exemplary fsinfo block in accordance with an embodiment of the present invention; 
         FIG. 9  is a schematic block diagram of a protocol header data structure in accordance with an embodiment of the present convention; 
         FIG. 10  is a schematic block diagram of a protocol request data structure in accordance with embodiment of the present convention; 
         FIG. 11  is a schematic block diagram of a protocol response data structure in accordance with embodiment of present convention; 
         FIG. 12  is a schematic block diagram of a file handle data structure in accordance with an embodiment of the present convention; 
         FIG. 13  is a schematic block diagram of a file attribute data structure in accordance with embodiment of the present convention; 
         FIG. 14  is a schematic block diagram of an initialization (INIT) request data structure in accordance with embodiment of the present convention; 
         FIG. 15  is a schematic block diagram of an initialization (INIT) response data structure in accordance with embodiment of the present convention; 
         FIG. 16  is a schematic block diagram of a volume information (VOLINFO) request data structure in accordance with embodiment of the present convention; 
         FIG. 17  is a schematic block diagram of a volume information (VOLINFO) response data structure in accordance with embodiment of the present convention; 
         FIG. 18  is a schematic block diagram of a read (READ) request data structure in accordance with embodiment of the present convention; 
         FIG. 19  is a schematic block diagram of a read (READ) response data structure in accordance with embodiment of the present convention; 
         FIG. 20  is a schematic block diagram of a lock PCPI (LOCK_PCPI) request data structure in accordance with an embodiment of the present convention; 
         FIG. 21  is a schematic block diagram of a PCPI information data structure in accordance with embodiment of the present convention; 
         FIG. 22  is a schematic block diagram of a lock PCPI (LOCK_PCPI) response data structure in accordance with an embodiment of the present convention; 
         FIG. 23  is a schematic block diagram of an unlock PCPI (UNLOCK_PCPI) request data structure in accordance with embodiment of the present convention; 
         FIG. 24  is a schematic block diagram of an authentication (AUTH) request data structure in accordance with embodiment of the present convention; 
         FIG. 25  is a schematic block diagram of an authentication (AUTH) response data structure in accordance with an embodiment of the present convention; 
         FIG. 26  is a schematic block diagram of a get holy bitmap (GET_HOLY_BITMAP) request data structure in accordance with an embodiment of the present invention; 
         FIG. 27  is a schematic block diagram of a get holy bitmap 
       (GET_HOLY_BITMAP) response data structure in accordance with an embodiment of the present invention; 
         FIG. 28  is a schematic block diagram of an indirect block map structure in accordance with an embodiment of the present invention; 
         FIG. 29  is a schematic block diagram of a remove (REMOVE) request data structure in accordance with an embodiment of the present invention; 
         FIG. 30  is a schematic block diagram of a remove (REMOVE) response data structure in accordance with an embodiment of the present invention; 
         FIG. 31  is a schematic block diagram of a rename (RENAME) request data structure in accordance with an embodiment of the present invention; 
         FIG. 32  is a schematic block diagram of a rename (RENAME) response data structure in accordance with an embodiment of the present invention; 
         FIG. 33  is a schematic block diagram of a create (CREATE) request data structure in accordance with an embodiment of the present invention; 
         FIG. 34  is a schematic block diagram of a create (CREATE) response data structure in accordance with an embodiment of the present invention 
         FIG. 35  is a flow chart detailing the steps of a procedure for retrieving one or more blocks from a backing store utilizing the NRV protocol in accordance with an embodiment of the present convention; and 
         FIG. 36  is a flow chart detailing the steps of a procedure showing the use of the LOCK_PCPI command in accordance with an embodiment of the present convention. 
     
    
    
     DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT 
     A. Network Environment 
       FIG. 1  is a schematic block diagram of an environment  100  including a storage system  120  that may be advantageously used with the present invention. The storage system is a computer that provides storage service relating to the organization of information on storage devices, such as disks  130  of a disk array  160 . The storage system  120  comprises a processor  122 , a memory  124 , a network adapter  126  and a storage adapter  128  interconnected by a system bus  125 . The storage system  120  also includes a storage operating system  200  that preferably implements a high-level module, such as a file system, to logically organize the information as a hierarchical structure of directories, files and special types of files called virtual disks (hereinafter “blocks”) on the disks. 
     In the illustrative embodiment, the memory  124  comprises storage locations that are addressable by the processor and adapters for storing software program code. A portion of the memory may be further organized as a “buffer cache”  170  for storing certain data structures associated with the present invention. The processor and adapters may, in turn, comprise processing elements and/or logic circuitry configured to execute the software code and manipulate the data structures. Storage operating system  200 , portions of which are typically resident in memory and executed by the processing elements, functionally organizes the system  120  by, inter alia, invoking storage operations executed by the storage system. It will be apparent to those skilled in the art that other processing and memory means, including various computer readable media, may be used for storing and executing program instructions pertaining to the invention described herein. 
     The network adapter  126  comprises the mechanical, electrical and signaling circuitry needed to connect the storage system  120  to a client  110  over a computer network  140 , which may comprise a point-to-point connection or a shared medium, such as a local area network (LAN) or wide area network (WAN). Illustratively, the computer network  140  may be embodied as an Ethernet network or a Fibre Channel (FC) network. The client  110  may communicate with the storage system over network  140  by exchanging discrete frames or packets of data according to pre-defined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). 
     The client  110  may be a general-purpose computer configured to execute applications  112 . Moreover, the client  110  may interact with the storage system  120  in accordance with a client/server model of information delivery. That is, the client may request the services of the storage system, and the system may return the results of the services requested by the client, by exchanging packets  150  over the network  140 . The clients may issue packets including file-based access protocols, such as the Common Internet File System (CIFS) protocol or Network File System (NFS) protocol, over TCP/IP when accessing information in the form of files and directories. Alternatively, the client may issue packets including block-based access protocols, such as the Small Computer Systems Interface (SCSI) protocol encapsulated over TCP (iSCSI) and SCSI encapsulated over Fibre Channel (FCP), when accessing information in the form of blocks. 
     The storage adapter  128  cooperates with the storage operating system  200  executing on the system  120  to access information requested by a user (or client). The information may be stored on any type of attached array of writable storage device media such as video tape, optical, DVD, magnetic tape, bubble memory, electronic random access memory, micro-electro mechanical and any other similar media adapted to store information, including data and parity information. However, as illustratively described herein, the information is preferably stored on the disks  130 , such as HDD and/or DASD, of array  160 . The storage adapter includes input/output (I/O) interface circuitry that couples to the disks over an I/O interconnect arrangement, such as a conventional high-performance, FC serial link topology. 
     Storage of information on array  160  is preferably implemented as one or more storage “volumes” that comprise a collection of physical storage disks  130  cooperating to define an overall logical arrangement of volume block number (vbn) space on the volume(s). Each logical volume is generally, although not necessarily, associated with its own file system. The disks within a logical volume/file system are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). Most RAID implementations, such as a RAID-4 level implementation, enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of parity information with respect to the striped data. An illustrative example of a RAID implementation is a RAID-4 level implementation, although it should be understood that other types and levels of RAID implementations may be used in accordance with the inventive principles described herein. 
     Additionally, a second storage system  120   b  is operatively interconnected with the network  140 . The second storage system  120   b  may be configured as a remote backing store server or, illustratively, a near line storage server. The storage system  120   b  generally comprises hardware similar to storage system  120   a ; however, it may alternatively execute a modified storage operating system that adapts the storage system for use as a near line storage server. It should be noted that in alternate embodiments, multiple storage systems  120   b  may be utilized. 
     B. Storage Operating System 
     To facilitate access to the disks  130 , the storage operating system  200  implements a write-anywhere file system that cooperates with virtualization modules to “virtualize” the storage space provided by disks  130 . The file system logically organizes the information as a hierarchical structure of named directories and files on the disks. Each “on-disk” file may be implemented as set of disk blocks configure to store information, such as data, whereas the directory may be implemented as a specially formatted file in which names and links to other files and directories are stored. The virtualization modules allow the file system to further logically organize information as a hierarchical structure of blocks on the disks that are exported as named logical unit numbers (luns). 
     In the illustrative embodiment, the storage operating system is preferably the NetApp® Data ONTAP™ operating system available from Network Appliance, Inc., Sunnyvale, Calif. that implements a Write Anywhere File Layout (WAFL™) file system. However, it is expressly contemplated that any appropriate storage operating system may be enhanced for use in accordance with the inventive principles described herein. As such, where the term “WAFL” is employed, it should be taken broadly to refer to any file system that is otherwise adaptable to the teachings of this invention. 
       FIG. 2  is a schematic block diagram of the storage operating system  200  that may be advantageously used with the present invention. The storage operating system comprises a series of software layers organized to form an integrated network protocol stack or, more generally, a multi-protocol engine that provides data paths for clients to access information stored on the storage system using block and file access protocols. The protocol stack includes a media access layer  210  of network drivers (e.g., gigabit Ethernet drivers) that interfaces to network protocol layers, such as the IP layer  212  and its supporting transport mechanisms, the TCP layer  214  and the User Datagram Protocol (UDP) layer  216 . A file system protocol layer provides multi-protocol file access and, to that end, includes support for the Direct Access File System (DAFS) protocol  218 , the NFS protocol  220 , the CIFS protocol  222  and the Hypertext Transfer Protocol (HTTP) protocol  224 . A VI layer  226  implements the VI architecture to provide direct access transport (DAT) capabilities, such as RDMA, as required by the DAFS protocol  218 . 
     An iSCSI driver layer  228  provides block protocol access over the TCP/IP network protocol layers, while a FC driver layer  230  receives and transmits block access requests and responses to and from the storage system. The FC and iSCSI drivers provide FC-specific and iSCSI-specific access control to the blocks and, thus, manage exports of luns to either iSCSI or FCP or, alternatively, to both iSCSI and FCP when accessing the blocks on the storage system. In addition, the storage operating system includes a storage module embodied as a RAID system  240  that manages the storage and retrieval of information to and from the volumes/disks in accordance with I/O operations, and a disk is driver system  250  that implements a disk access protocol such as, e.g., the SCSI protocol. 
     The storage operating system  200  further comprises an NRV protocol layer  295  that interfaces with file system  280 . The NRV protocol is generally utilized for remote fetching of data blocks that are not stored locally on disk. However, as described further below, the NRV protocol may be further utilized in storage appliance-to-storage appliance communication to fetch absent blocks in a sparse volume in accordance with the principles of the present invention. 
     Bridging the disk software layers with the integrated network protocol stack layers is a virtualization system that is implemented by a file system  280  interacting with virtualization modules illustratively embodied as, e.g., vdisk module  290  and SCSI target module  270 . The vdisk module  290  is layered on the file system  280  to enable access by administrative interfaces, such as a user interface (UI)  275 , in response to a user (system administrator) issuing commands to the storage system. The SCSI target module  270  is disposed between the FC and iSCSI drivers  228 ,  230  and the file system  280  to provide a translation layer of the virtualization system between the block (lun) space and the file system space, where luns are represented as blocks. The UI  275  is disposed over the storage operating system in a manner that enables administrative or user access to the various layers and systems. 
     The file system is illustratively a message-based system that provides logical volume management capabilities for use in access to the information stored on the storage devices, such as disks. That is, in addition to providing file system semantics, the file system  280  provides functions normally associated with a volume manager. These functions include (i) aggregation of the disks, (ii) aggregation of storage bandwidth of the disks, and (iii) reliability guarantees, such as mirroring and/or parity (RAID). The file system  280  illustratively implements the WAFL file system (hereinafter generally the “write-anywhere file system”) having an on-disk format representation that is block-based using, e.g., 4 kilobyte (kB) blocks and using index nodes (“inodes”) to identify files and file attributes (such as creation time, access permissions, size and block location). The file system uses files to store metadata describing the layout of its file system; is these metadata files include, among others, an inode file. A file handle, i.e., an identifier that includes an inode number, is used to retrieve an inode from disk. 
     Broadly stated, all inodes of the write-anywhere file system are organized into the inode file. A file system (fs) info block specifies the layout of information in the file system and includes an inode of a file that includes all other inodes of the file system. Each logical volume (file system) has an fsinfo block that is preferably stored at a fixed location within, e.g., a RAID group. The inode of the root fsinfo block may directly reference (point to) blocks of the inode file or may reference indirect blocks of the inode file that, in turn, reference direct blocks of the inode file. Within each direct block of the inode file are embedded inodes, each of which may reference indirect blocks that, in turn, reference data blocks of a file. 
     Operationally, a request from the client  110  is forwarded as a packet  150  over the computer network  140  and onto the storage system  120  where it is received at the network adapter  126 . A network driver (of layer  210  or layer  230 ) processes the packet and, if appropriate, passes it on to a network protocol and file access layer for additional processing prior to forwarding to the write-anywhere file system  280 . Here, the file system generates operations to load (retrieve) the requested data from disk  130  if it is not resident “in core”, i.e., in the buffer cache  170 . Illustratively this operation may be embodied as a Load_Block( )function  284  of the file system  280 . If the information is not in the s cache, the file system  280  indexes into the inode file using the inode number to access an appropriate entry and retrieve a logical vbn. The file system then passes a message structure including the logical vbn to the RAID system  240 ; the logical vbn is mapped to a disk identifier and disk block number (disk,dbn) and sent to an appropriate driver (e.g., SCSI) of the disk driver system  250 . The disk driver accesses the dbn from the specified disk  130  and loads the requested data block(s) in buffer cache  170  for processing by the storage system. Upon completion of the request, the storage system (and operating system) returns a reply to the client  110  over the network  140 . 
     The file system  280  illustratively provides the Load_Block( ) function  284  to retrieve one or more blocks of data from disk. A block may be retrieved in response to a is read request or may be retrieved in response to an exemplary read ahead algorithm. The illustrative Load_Block( ) function  284  attempts to load a requested block of data. The Load_Block( ) function  284  initiates transfer of a fetch operation to an appropriate backing store using the illustrative NRV protocol  295  if any blocks require data to be remotely retrieved. Once the data has been retrieved, the Load_Block( ) function  284  returns with the requested data. Sparse volumes and ABSENT block pointers are further described in the above-referenced U.S. Patent Application, entitled SYSTEM AND METHOD FOR SPARSE VOLUMES, by Jason Lango et al. It should be noted that the use of the NRV protocol for remote retrieval of data for sparse volumes is exemplary and that the novel NRV protocol described herein may be utilized for other types of remote data retrieval. As such, the illustrative embodiment of utilizing the NRV protocol for retrieving sparse volumes data should be taken as exemplary only and should not limit the scope of the present invention. 
     Additionally, in the illustrative embodiment, the file system  280  provides a Load_Inode ( ) function  286  to retrieve an inode from disk. In the illustrative embodiment, the Load_Inode ( ) function  286  is adopted to obtain appropriate file geometry information, as described further below. In the illustrative embodiment, a sparse configuration metadata file is stored on the storage system. The sparse configuration metadata file includes appropriate configuration information to enable data retrieval from a backing store. Such information may include identification information of the remote backing store along with an identification of what data container(s) on the backing store are to be utilized as the backing store. In the illustrative embodiment, a sparse volume may be supported by a plurality of backing stores. 
     It should be further noted that the software “path” through the storage operating system layers described above needed to perform data storage access for the client request received at the storage system may alternatively be implemented in hardware. That is, in an alternate embodiment of the invention, a storage access request data path may be implemented as logic circuitry embodied within a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). This type of hardware implementation increases the performance of the storage service provided by storage system  120  in response to a request issued by client  110 . Moreover, in another alternate embodiment of the invention, the processing elements of adapters  126 ,  128  may be configure to offload some or all of the packet processing and storage access operations, respectively, from processor  122 , to thereby increase the performance of the storage service provided by the system. It is expressly contemplated that the various processes, architectures and procedures described herein can be implemented in hardware, firmware or software. 
     As used herein, the term “storage operating system” generally refers to the computer-executable code operable to perform a storage function in a storage system, e.g., that manages data access and may, in the case of a file server, implement file system semantics. In this sense, the ONTAP software is an example of such a storage operating system implemented as a microkernel and including the WAFL layer to implement the WAFL file system semantics and manage data access. The storage operating system can also be implemented as an application program operating over a general-purpose operating system, such as UNIX® or Windows NT®, or as a general-purpose operating system with configurable functionality, which is configure for storage applications as described herein. 
     In addition, it will be understood to those skilled in the art that the inventive technique described herein may apply to any type of special-purpose (e.g., file server, filer or multi-protocol storage appliance) or general-purpose computer, including a standalone computer or portion thereof, embodied as or including a storage system  120 . An example of a multi-protocol storage appliance that may be advantageously used with the present invention is described in U.S. patent application Ser. No. 10/215,917 titled MULTI-PROTOCOL STORAGE APPLIANCE THAT PROVIDES INTEGRATED SUPPORT FOR FILE AND BLOCK ACCESS PROTOCOLS, filed on Aug. 8, 2002 and published as U.S. Patent Application Publication No. 2004/0030668 A1 on Feb. 12, 2004. Moreover, the teachings of this invention can be adapted to a variety of storage system architectures including, but not limited to, a network-attached storage environment, a storage area network and disk assembly directly-attached to a client or host computer. The term “storage system” should therefore be taken broadly to include such arrangements in addition to any subsystems configure to perform a storage function and associated with other equipment or systems. 
     C. File System Organization 
     In the illustrative embodiment, a file is represented in the write-anywhere file system as an inode data structure adapted for storage on the disks  130 .  FIG. 3  is a schematic block diagram of an inode  300 , which preferably includes a metadata section  310  and a data section  350 . The information stored in the metadata section  310  of each inode  300  describes the file and, as such, includes the type (e.g., regular, directory, virtual disk)  312  of file, the size  314  of the file, time stamps (e.g., access and/or modification)  316  for the file and ownership, i.e., user identifier (UID  318 ) and group ID (GID  320 ), of the file. The contents of the data section  350  of each inode, however, may be interpreted differently depending upon the type of file (inode) defined within the type field  312 . For example, the data section  350  of a directory inode contains metadata controlled by the file system, whereas the data section of a regular inode contains file system data. In this latter case, the data section  350  includes a representation of the data associated with the file. 
     Specifically, the data section  350  of a regular on-disk inode may include file system data or pointers, the latter referencing 4 kilobyte (KB) data blocks on disk used to store the file system data. Each pointer is preferably a logical vbn to facilitate efficiency among the file system and the RAID system  240  when accessing the data on disks. Given the restricted size (e.g., 128 bytes) of the inode, file system data having a size that is less than or equal to 64 bytes is represented, in its entirety, within the data section of that inode. However, if the file system data is greater than 64 bytes but less than or equal to 64 KB, then the data section of the inode (e.g., a first level inode) comprises up to 16 pointers, each of which references a 4 KB block of data on the disk. 
     Moreover, if the size of the data is greater than 64 KB but less than or equal to 64 megabytes (MB), then each pointer in the data section  350  of the inode (e.g., a second level inode) references an indirect block (e.g., a first level block) that contains 1024 pointers, each of which references a 4 KB data block on disk. For file system data having a size greater than 64 MB, each pointer in the data section  350  of the inode (e.g., a third level inode) references a double-indirect block (e.g., a second level block) that contains 1024 pointers, each referencing an indirect (e.g., a first level) block. The indirect block, in turn, that contains 1024 pointers, each of which references a 4 KB data block on disk. When accessing a file, each block of the file may be loaded from disk  130  into the buffer cache  170 . 
     When an on-disk inode (or block) is loaded from disk  130  into buffer cache  170 , its corresponding in core structure embeds the on-disk structure. For example, the dotted line surrounding the inode  300  ( FIG. 3 ) indicates the in core representation of the on-disk inode structure. The in core structure is a block of memory that stores the on-disk structure plus additional information needed to manage data in the memory (but not on disk). The additional information may include, e.g., a “dirty” bit  360 . After data in the inode (or block) is updated/modified as instructed by, e.g., a write operation, the modified data is marked “dirty” using the dirty bit  360  so that the inode (block) can be subsequently “flushed” (stored) to disk. The in core and on-disk format structures of the WAFL file system, including the inodes and inode file, are disclosed and described in the previously incorporated U.S. Pat. No. 5,819,292 titled METHOD FOR MAINTAINING CONSISTENT STATES OF A FILE SYSTEM AND FOR CREATING USER-ACCESSIBLE READ-ONLY COPIES OF A FILE SYSTEM by David Hitz et al., issued on Oct. 6, 1998. 
       FIG. 4  is a schematic block diagram of an embodiment of a buffer tree of a file that may be advantageously used with the present invention. The buffer tree is an internal representation of blocks for a file (e.g., file  400 ) loaded into the buffer cache  170  and maintained by the write-anywhere file system  280 . A root (top-level) inode  402 , such as an embedded inode, references indirect (e.g., level 1) blocks  404 . Note that there may be additional levels of indirect blocks (e.g., level 2, level 3) depending upon the size of the file. The indirect blocks (and inode) contain pointers  405  that ultimately reference data blocks  406  used to store the actual data of the file. That is, the data of file  400  are contained in data blocks and the locations of these blocks are stored in the indirect blocks of the file. Each level 1 indirect block  404  may contain pointers to as many as 1024 data blocks. According to the “write anywhere” nature of the file system, these blocks may be located anywhere on the disks  130 . 
     A file system layout is provided that apportions an underlying physical volume into one or more virtual volumes (vvols) of a storage system. An example of such a file system layout is described in U.S. Pat. No. 7,409,494 titled EXTENSION OF WRITE ANYWHERE FILE SYSTEM LAYOUT, by John K. Edwards et al., issued on Aug. 5, 2008. The underlying physical volume is an aggregate comprising one or more groups of disks, such as RAID groups, of the storage system. The aggregate has its own physical volume block number (pvbn) space and maintains metadata, such as block allocation structures, within that pvbn space. Each vvol has its own virtual volume block number (vvbn) space and maintains metadata, such as block allocation structures, within that vvbn space. Each vvol is a file system that is associated with a container file; the container file is a file in the aggregate that contains all blocks used by the vvol. Moreover, each vvol comprises data blocks and indirect blocks that contain block pointers that point at either other indirect blocks or data blocks. 
     In one embodiment, pvbns are used as block pointers within buffer trees of files (such as file  400 ) stored in a vvol. This “hybrid” vvol embodiment involves the insertion of only the pvbn in the parent indirect block (e.g., Mode or indirect block). On a read path of a logical volume, a “logical” volume (vol) info block has one or more pointers that reference one or more fsinfo blocks, each of which, in turn, “points to” an Mode file and its corresponding Mode buffer tree. The read path on a vvol is generally the same, following pvbns (instead of vvbns) to find appropriate locations of blocks; in this context, the read path (and corresponding read performance) of a vvol is substantially similar to that of a physical volume. Translation from pvbn-to-disk,dbn occurs at the file system/RAID system boundary of the storage operating system  200 . 
     In an illustrative “dual vbn” hybrid (“flexible”) vvol embodiment, both a pvbn and its corresponding vvbn are inserted in the parent indirect blocks in the buffer tree of a file. That is, the pvbn and vvbn are stored as a pair for each block pointer in most buffer tree structures that have pointers to other blocks, e.g., level 1(L1) indirect blocks, Mode file level 0 (L0) blocks.  FIG. 5  is a schematic block diagram of an illustrative embodiment of a buffer tree of a file  500  that may be advantageously used with the present invention. A root (top-level) Mode  502 , such as an embedded Mode, references indirect (e.g., level 1) blocks  504 . Note that there may be additional levels of indirect blocks (e.g., level 2, level 3) depending upon the size of the file. The indirect blocks (and Mode) contain pvbn/vvbn pointer pair structures  508  that ultimately reference data blocks  506  used to store the actual data of the file. 
     The pvbns reference locations on disks of the aggregate, whereas the vvbns reference locations within files of the vvol. The use of pvbns as block pointers  508  in the indirect blocks  504  provides efficiencies in the read paths, while the use of vvbn block pointers provide efficient access to required metadata. That is, when freeing a block of a file, the parent indirect block in the file contains readily available vvbn block pointers, which avoids the latency associated with accessing an owner map to perform pvbn-to-vvbn translations; yet, on the read path, the pvbn is available. 
     As noted, each inode has 64 bytes in its data section that, depending upon the size of the inode file (e.g., greater than 64 bytes of data), function as block pointers to other blocks. For traditional and hybrid volumes, those 64 bytes are embodied as 16 block pointers, i.e., sixteen (16) 4 byte block pointers. For the illustrative dual vbn flexible volume, the 64 bytes of an inode are embodied as eight (8) pairs of 4 byte block pointers, wherein each pair is a vvbn/pvbn pair. In addition, each indirect block of a traditional or hybrid volume may contain up to 1024 (pvbn) pointers; each indirect block of a dual vbn flexible volume, however, has a maximum of 510 (pvbn/vvbn) pairs of pointers. 
     Moreover, one or more of pointers  508  may contain a special ABSENT value to signify that the object(s) (e.g., an indirect block or data block) referenced by the pointer(s) is not locally stored (e.g., on the volume) and, thus, must be fetched (retrieved) from an alternate backing store. In the illustrative embodiment, the Load_Block ( ) function interprets the content of the each pointer and, if a requested block is ABSENT, initiates transmission of an appropriate request (e.g., a remote fetch operation) for the data to a backing store using, e.g. the novel NRV protocol of the present invention. 
       FIG. 6  is a schematic block diagram of an embodiment of an aggregate  600  that may be advantageously used with the present invention. Luns (blocks)  602 , directories  604 , qtrees  606  and files  608  may be contained within vvols  610 , such as dual vbn flexible vvols, that, in turn, are contained within the aggregate  600 . The aggregate  600  is illustratively layered on top of the RAID system, which is represented by at least one RAID plex  650  (depending upon whether the storage configuration is mirrored), wherein each plex  650  comprises at least one RAID group  660 . Each RAID group further comprises a plurality of disks  630 , e.g., one or more data (D) disks and at least one (P) parity disk. 
     Whereas the aggregate  600  is analogous to a physical volume of a conventional storage system, a vvol is analogous to a file within that physical volume. That is, the aggregate  600  may include one or more files, wherein each file contains a vvol  610  and wherein the sum of the storage space consumed by the vvols is physically smaller than (or equal to) the size of the overall physical volume. The aggregate utilizes a “physical” pvbn space that defines a storage space of blocks provided by the disks of the physical volume, while each embedded vvol (within a file) utilizes a “logical” vvbn space to organize those blocks, e.g., as files. Each vvbn space is an independent set of numbers that corresponds to locations within the file, which locations are then translated to dbns on disks. Since the vvol  610  is also a logical volume, it has its own block allocation structures (e.g., active, space and summary maps) in its vvbn space. 
     A container file is a file in the aggregate that contains all blocks used by a vvol. The container file is an internal (to the aggregate) feature that supports a vvol; illustratively, there is one container file per vvol. Similar to a pure logical volume in a file approach, the container file is a hidden file (not accessible to a user) in the aggregate that holds every block in use by the vvol. The aggregate includes an illustrative hidden metadata data root directory that contains subdirectories of vvols:
         WAFL/fsid/filesystem file, storage label file       

     Specifically, a “physical” file system (WAFL) directory includes a subdirectory for each vvol in the aggregate, with the name of subdirectory being a file system identifier (fsid) of the vvol. Each fsid subdirectory (vvol) contains at least two files, a filesystem file and a storage label file. The storage label file is illustratively a 4 kB file that contains metadata similar to that stored in a conventional raid label. In other words, the storage label file is the analog of a raid label and, as such, contains information about the state of the vvol such as, e.g., the name of the vvol, a universal unique identifier (uuid) and fsid of the vvol, whether it is online, being created or being destroyed, etc. 
       FIG. 7  is a schematic block diagram of an on-disk representation of an aggregate  700 . The storage operating system  200 , e.g., the RAID system  240 , assembles a physical volume of pvbns to create the aggregate  700 , with pvbns 1 and 2 comprising a “physical” volinfo block  702  for the aggregate. The volinfo block  702  contains block pointers to fsinfo blocks  704 , each of which may represent a snapshot of the aggregate. Each fsinfo block  704  includes a block pointer to an inode file  706  that contains inodes of a plurality of files, including an owner map  710 , an active map  712 , a summary map  714  and a space map  716 , as well as other special metadata files. The inode file  706  further includes a root directory  720  and a “hidden” metadata root directory  730 , the latter of which includes a namespace having files related to a vvol in which users cannot “see” the files. The hidden metadata root directory also includes the WAFL/fsid/ directory structure that contains filesystem file  740  and storage label file  790 . Note that root directory  720  in the aggregate is empty; all files related to the aggregate are organized within the hidden metadata root directory  730 . The hidden metadata root directory  730  also illustratively includes a sparse configuration file  732  that contains appropriate configuration metadata for use with a sparse volume. Such metadata includes, e.g., the identification of the backing store associated with a particular sparse volume. 
     In addition to being embodied as a container file having level 1 blocks organized is as a container map, the filesystem file  740  includes block pointers that reference various file systems embodied as vvols  750 . The aggregate  700  maintains these vvols  750  at special reserved inode numbers. Each vvol  750  also has special reserved inode numbers within its vvol space that are used for, among other things, the block allocation bitmap structures. As noted, the block allocation bitmap structures, e.g., active map  762 , summary map  764  and space map  766 , are located in each vvol. 
     Specifically, each vvol  750  has the same inode file structure/content as the aggregate, with the exception that there is no owner map and no WAFL/fsid/filesystem file, storage label file directory structure in a hidden metadata root directory  780 . To that end, each vvol  750  has a volinfo block  752  that points to one or more fsinfo blocks  800 , each of which may represent a snapshot, along with the active file system of the vvol. Each fsinfo block, in turn, points to an inode file  760  that, as noted, has the same inode structure/content as the aggregate with the exceptions noted above. Each vvol  750  has its own inode file  760  and distinct inode space with corresponding inode numbers, as well as its own root (fsid) directory  770  and subdirectories of files that can be exported separately from other vvols. 
     The storage label file  790  contained within the hidden metadata root directory  730  of the aggregate is a small file that functions as an analog to a conventional raid label. A raid label includes “physical” information about the storage system, such as the volume name; that information is loaded into the storage label file  790 . Illustratively, the storage label file  790  includes the name  792  of the associated vvol  750 , the online/offline status  794  of the vvol, and other identity and state information  796  of the associated vvol (whether it is in the process of being created or destroyed). 
     A sparse volume is identified by a special marking of an on-disk structure of the volume (vvol) to denote the inclusion of a file with an absent block.  FIG. 8  is a schematic block diagram of the on-disk structure, which illustratively is an exemplary fsinfo block  800 . The fsinfo block  800  includes a set of PCPI pointers  805 , a sparse volume flag field  810 , an inode for the inode file  815  and, in alternate embodiments, additional fields  820 . The PCIP pointers  805  are “dual vbn” (vvbn/pvbn) pairs of pointers to PCPIs associated with the file system. The sparse volume flag field  810  identifies whether the vvol described by the fsinfo block is sparse. In the illustrative embodiment, a flag is asserted in field  810  to identify the volume as sparse. The sparse volume flag field  810  may be embodied as a type field identifying the type of a vvol associated with the fsinfo block. The inode for the inode file  815  includes the inode containing the root-level pointers to the inode file  760  ( FIG. 7 ) of the file system associated with the fsinfo block. 
     Appropriate block pointer(s) of the file are marked (labeled) with special ABSENT value(s) to identify that certain block(s), including data and/or indirect blocks, within the sparse volume are not physically located on the storage system serving the volume. The special value further alerts the file system that the data is to be obtained from the alternate source, namely a remote backing store, which is illustratively near line storage server  120   b . In response to a data access request, the Load_Block( ) function  284  of the file system  280  detects whether an appropriate block pointer of a file is marked as ABSENT and, if so, transmits a remote fetch (e.g., read) operation from the storage system to the remote backing store to fetch the required data. The fetch operation illustratively requests one or more file block numbers of the file stored on the backing store. 
     The backing store retrieves the requested data from its storage devices and returns the requested data to the storage system, which processes the data access request and stores the returned data in its memory. Subsequently, the file system “flushes” (writes) the data stored in memory to local disk during a write allocation procedure. In accordance with an illustrative write anywhere policy of the procedure, the file system assigns pointer values (other than ABSENT values) to indirect block(s) of the file to thereby identify location(s) of the data stored locally within the volume. Thus, the remote fetch operation is no longer needed to access the data. 
     An example of a write allocation procedure that may be advantageously used with the present invention is described in U.S. Pat. No. 7,430,571, titled Extension of Write Anywhere File Layout Write Allocation, by John K. Edwards and assigned to Network Appliance, Inc., issued on Sep. 30, 2008, which application is hereby incorporated by reference. Broadly stated, block allocation proceeds in parallel on the flexible vvol and aggregate when write allocating a block within the vvol, with a write allocator process  282  selecting an actual pvbn in the aggregate and a vvbn in the vvol. The write allocator adjusts block allocation bitmap structures, such an active map and space map, of the aggregate to record the selected pvbn and adjusts similar structures of the vvol to record the selected vvbn. A vvid of the vvol and the vvbn are inserted into owner map  710  of the aggregate at an entry defined by the selected pvbn. The selected pvbn is also inserted into a container map (not shown) of the destination vvol. Finally, an indirect block or inode file parent of the allocated block is updated with one or more block pointers to the allocated block. The content of the update operation depends on the vvol embodiment. For the dual vbn hybrid vvol embodiment, both the pvbn and vvbn are inserted in the indirect block or inode as block pointers. 
     D. NRV Protocol 
     In the illustrative embodiment, the storage operating system utilizes the novel NRV protocol to retrieve ABSENT blocks from a remote storage system configured to act as a backing store for a sparse volume. It should be noted that the novel NRV protocol may also be utilized to retrieve non-ABSENT blocks from the backing store. Thus, the NRV protocol may be utilized to retrieve data in a file system that utilizes holes as described above. The NRV protocol typically utilizes the TCP/IP protocol as a transport protocol and all NRV messages (both requests and responses) are prefixed with a framing header identifying the length of the NRV message in bytes (exclusive of this length of the initial length header itself). 
       FIG. 9  is a schematic block diagram of an NRV protocol header data structure  900  in accordance with an embodiment of the present invention. The header data structure  900  includes a transaction identifier (ID) field  905 , a checksum field  910 , a call field  915  and, in alternate embodiments, additional fields  920 . The transaction ID field  905  contains a unique transaction ID utilized by the protocol to pair requests and responses. Thus a NRV response from the backing store will identify which NRV request it is associated with by including the transaction ID of the request. The transaction ID is unique per request per connection. In the illustrative embodiment, the first transaction ID utilized per connection is a random value, which is thereafter incremented with each transaction. The checksum field  910  is utilized for storing checksum information to ensure that the response/request has not been corrupted. 
       FIG. 10  is a schematic block diagram of an exemplary protocol request data structure  1000  in accordance with embodiment of the present invention. The request data structure  1000  includes protocol header  900 , a type field  1005  and, in alternate embodiments, additional fields  1010 . The type field  1005  identifies one of the remote file system operations supported by the protocol. These types include, inter alia, INIT, VOLINFO, READ, LOCK_PCPI, UNLOCK_PCPI and AUTH, each of which is described in detail further below in reference to type-specific data structures. Each of these types of requests has a data structure associated therewith. The type-specific data structure is appended to the request data structure  1000  when transmitted to the backing store. 
     A response to the protocol request is in the format of a protocol response data structure  1100 , which is illustratively shown as a schematic block diagram in  FIG. 11 . The response data structure  1100  includes header  900 , a NRV_Status field  1105 , a protocol status field  1110  and, in alternate embodiments, additional fields  1115 . The NRV_Status field  1105  may include one of the protocol specific status indicators such as OK, NOINIT, VERSION, CANTSEND, LS, and FS_VERSION. It should be noted that in alternate embodiments, other and/or differing status indicators may be utilized. The OK status indicator signifies that the request was successful and that there is no error condition. The NOINIT indicator is sent in response to a request being transferred prior to beginning a session. In the illustrative embodiment, an INIT request, described further below, must be the first request in a session after any authentication (AUTH) requests. The VERSION indicator is utilized when there are mismatched versions of the NRV protocol, e.g., the storage system and backing store are utilizing incompatible versions of the NRV protocol. The CANTSEND indicator indicates a failure of the underlying transport protocol in transmitting a particular request or response. The LS status indicator is used by the backing store to indicate that a PCPI was not able to be locked in response to a LOCK_PCPI request, described further below. The FS_VERSION indicator means that the storage system and the backing store are utilizing incompatible versions of a file system so that data may not be retrieved from the backing store. 
     The protocol status field  1110  includes a file system error value. Thus, the protocol status field  1110  may be utilized to transfer a WAFL file system or other file system error value between the backing store and the storage appliance. Each of the NRV protocol operations that includes a response data structure includes a type-specific data structure that is appended to the end of a protocol response data structure  1100 . 
     Many NRV protocol requests and/or responses include a file handle identifying a file to which an operation is directed.  FIG. 12  is a schematic block diagram of a file handle data structure  1200  in accordance with an embodiment of the present invention. The file handle data structure  1200  includes a file system ID field  1205 , a PCPI ID field  1210 , a file ID field  1215 , a generation field  1220  and, in alternate embodiments, additional fields  1225 . The file system ID field  1205  identifies the particular file system containing the file of interest. This may be a particular virtual volume or physical volume associated with the backing store. This field  1205  typically contains the fsid of the desired volume The PCPI ID field  1210  identifies the appropriate PCPI associated with the file. Thus, the NRV protocol permits access to a file stored within a particular PCPI. File ID field  1215  identifies the unique file ID associated with the file. The generation field  1220  contains a value identifying a particular generation of the inode associated with the file. 
     Additionally, many NRV requests and responses contain a set of file attributes that are contained within an exemplary file attribute data structure  1300  as shown in a schematic block diagram of  FIG. 13 . The file attribute data structure  1300  includes a blocks field  1305 , a size field  1310 , a type field  1315 , a subtype field  1320 , a generation field  1325 , a user identifier (UID) field  1330 , a group identifier field (GID)  1335 , a creation time field  1340  and, in alternate embodiments, additional fields  1345 . The blocks field  1305  identifies the number of blocks utilized by the file. The size field  1310  contains the size of the file in bytes. The type and subtype fields  1315 ,  1320  identify the type and, if necessary, a subtype of the file. The generation field  1325  identifies the current generation number associated with the inode of the file. The UID field  1330  identifies the owner of the file, whereas the GID field  1335  identifies the current group that is associated with the file. 
     In accordance with the illustrative embodiment of the protocol, the first request sent over a connection, after any authentication requests described further below, is an initialization request. This initialization request (i.e. an INIT type of type field  1005 ) comprises an initialization data structure  1400 , which is exemplary shown as a schematic block diagram in  FIG. 14 . The initialization data structure  1400  includes a protocol request data structure  1000 , a protocol version field  1405 , an application field  1410 , a byte order field  1415  and, in alternate embodiments, additional fields  1420 . The request data structure  1000  is described above in reference to  FIG. 10 . The protocol version field  1405  contains a protocol “minor” version in use at the client (storage appliance initiating the connection) that identifies clients utilizing different versions of the protocol. The application field  1410  identifies the application utilizing the NRV protocol; such applications may include restore on demand (ROD) or proxy file system (PFS). Restore on demand techniques are further described in U.S. patent application Ser. No. 11/409,626 entitled SYSTEM AND METHOD FOR RESTORING DATA ON DEMAN FOR INSTANT VOLUME RESTORATION by Jason Lango et al., now published as U.S. Patent Application Publication No. 2007/0124341 A1, and proxy file systems are further described in U.S. patent application Ser. No. 11/409,625, filed on Apr. 24, 2006, entitled SYSTEM AND METHOD FOR CACHING NETWORK FILE SYSTEMS by Jason Lango et al. The byte order field  1415  identifies the client&#39;s native byte order, e.g., big or little endian. 
     In response to the initialization request data structure  1400 , the backing store transmits an initialization response data structure  1500 , which is illustratively shown in a schematic block diagram of  FIG. 15 . The initialization response data structure  1500  includes a protocol response data structure  1100 , a file system version field  1505 , a byte order field  1510  and, in alternate embodiments, additional fields  1515 . The response data structure  1100  is described above in reference to  FIG. 11 . The file system version field  1505  identifies the maximum file system version supported by the backing store. The byte order field  1510  identifies the backing store&#39;s native byte order. In the protocol specification, if the storage system&#39;s and backing store&#39;s byte orders differ, all future communication occurs using the backing store&#39;s of byte order as defined in field  1510 . 
     To retrieve information pertaining to a particular volume, the storage appliance may transmit a volume information (VOLINFO) request data structure  1600 , which is shown as a schematic block diagram of  FIG. 16 . The volume information data structure  1600  includes a protocol request data structure  1000 , a name length field  1605 , a volume name field at  1610  and, in alternate embodiments, additional fields  1615 . The name length field  1605  identifies length of the volume name field while the volume name field  1610  comprises a text string of the volume name. The VOLINFO request is utilized to obtain volume information, which may be used to, e.g., ensure that a volume on the storage system is sufficiently sized to accommodate all data located on a volume on the backing store. 
     In response to a volume information request, the backing store will issue a volume information response data structure  1700 , of which an exemplary schematic block diagram is shown in  FIG. 17 . The volume information response data structure  1700  comprises a protocol response data structure  1100 , a root file handle field  1705 , a maximum volume block number field  1710 , a number of inodes used field  1715 , a number of inodes field  1720  and, in alternate embodiments, additional fields  1725 . The root file handle field  1705  contains a conventional file handle for the root directory of the specified volume. The maximum volume block number field  1710  is set to the greatest allowable volume block number in the file system of the specified volume. The value of this field plus one is the size of the volume in blocks as, in the illustrative embodiment, volume block numbers begin with vbn  0 . Thus, in the illustrative embodiment of the WAFL file system, which utilizes 4 KB blocks, the value of this field plus one is the size of the volume is in 4 KB blocks. The number of inodes used field  1715  contains number of inodes in use in the active file system of the specified volume, whereas the number of inodes field  1720  holds the total number of allocable inodes in the active file system of the specified volume. 
       FIG. 18  is a schematic block diagram of an exemplary read (i.e.; a READ type of field  1005 ) request  1800  in accordance with an embodiment of the present intention. The read request data structure  1800  includes protocol request data structure  1000 , file handle  1200 , a file block number field  1805 , a number of blocks field  1810  and, in alternate embodiments, additional fields  1815 . The request data structure  1000  is described above in reference to  FIG. 10 , whereas the file handle data structure  1200  is described above in reference to  FIG. 12 . The file block number field  1805  identifies the first file block to be read. The file block number represents an offset of 4 KB blocks into the file. In alternate embodiments, where the file system utilizes differing sizes for file blocks, the file block number is the offset in the appropriate block size into the file. The number of blocks field  1810  identifies the number of file blocks to be read. 
     A read request response data structure  1900  is illustratively shown in  FIG. 19 . The read response data structure  1900  includes response data structure  1100 , an end of file field  1905 , a data field  1910  and, in alternate embodiments, additional fields  1915 . The response structure  1100  is described above in reference to  FIG. 11 . The end of file field  1905  identifies whether there is additional data to be read from the file and, if not, its content may be set to a FALSE value. Alternatively, the field  1905  may be set to a TRUE value if the end of the file has been reached by the requested read operation. The data field  1910  is a variable number of bytes of data from the file, starting at the requested file block number. 
     Another type of remote file system operation supported by the novel NRV protocol is the lock PCPI operation (i.e., a LOCK_PCPI type field  1005 ) that is used to prevent a PCPI from being deleted on the backing store. The Lock PCPI operation is typically utilized when the PCPI is necessary for a “long-lived” application, such as restore on demand. In the illustrative embodiment, the locked PCPI command is an inherently stateful is request that instructs the backing store to prevent deletion of the PCPI until either the client disconnects or unlocks the PCPI (the latter with the unlocked PCPI command described further below). An exemplary LOCK_PCPI request data structure  2000  is illustratively shown as a schematic block diagram in  FIG. 20 . The LOCK_PCPI request data structure  2000  includes a request data structure, a file system ID field  2005 , a lock default PCPI field  2010 , a checked PCPI configuration field  2015 , a PCPI name length field  2020 , a PCPI information field  2100 , a PCPI name field  2030  and, in alternate embodiments, additional fields  2035 . The request data structure  1000  is described above in conjunction with  FIG. 10 . The file system ID field  2005  identifies the volume containing the PCPI to be locked. The lock default PCPI field  2010  may be set to a value of TRUE or FALSE. If it is set to TRUE, then the backing store locks the default PCPI for the volume identified and ignores the name and information fields  2030 ,  2100 . If the value if FALSE then the values of these fields  2030 ,  2100  are utilized in identifying the PCPI. In certain embodiments, the backing store may be configured to have a default PCPI for use in serving NRV protocols. This default PCPI may be selected by the use of the lock default PCPI field  2010 . The check PCPI configuration field  2015  may also be set to a value of TRUE or FALSE. If TRUE then the server verifies that the specified volume is an acceptable secondary volume for use in a sparse volume application. The PCPI name length field  2020  is set to the length of the PCPI name field, which holds a string comprising the name of the PCPI to be locked. 
     The PCPI information field  2100  comprises a PCPI information data structure  2100  illustratively shown as a schematic block diagram of  FIG. 21 . The PCPI information data structure  2100  includes an identifier field  2105 , a consistency point count field  2110 , a PCPI creation time field  2115 , a PCPI creation time in microseconds field  2120  and, in alternate embodiments additional fields  2125 . The identifier field  2105  is a PCPI identifier that uniquely identifies a particular PCPI. The consistency point count field  2110  identifies a particular CP count associated with the PCPI. Illustratively, at each CP, the CP count is incremented, thereby providing a unique label for the PCPI created at that point in time. Similarly, the PCPI creation time fields  2115 ,  2120  are utilized to uniquely identify the particular PCPI by identifying its creation time in seconds and microseconds, respectively. 
     In response the server sends a lock_PCPI response data structure  2200 , of which a schematic block diagram of which s shown in  FIG. 22 . The lock PCPI response data structure  2200  includes a response data structure  1100 , PCPI information data structure  2100 , a blocks used field  2210 , a blocks_holes field  2215 , a blocks_overwrite field  2220 , a blocks_holes_CIFS field  2225 , an inodes used field  2230 , a total number of inodes field  2235  and, in alternate embodiments, additional fields  2240 . The response data structure  1100  is described above in reference to  FIG. 11 . The PCPI information data structure  2100  is described above in reference to  FIG. 21 . The blocks used field  2210  contains a value identifying the number of blocks that are utilized by the PCPI on the backing store. The blocks_holes field  2215  identifies the number of blocks in the PCPI that are reserved for holes within the PCPI. The blocks_overwrite field  2220  contains a value identifying the number of blocks that are reserved for overwriting in the PCPI. The inodes field  2230  contains a value identifying the number of inodes used in the PCPI and the total number of inodes field  2235  contains a value identifying the total number of allocable inodes in the PCPI. 
     Once a client no longer requires a PCPI to be locked, it may issue an unlock PCPI command (of type UNLOCK_PCPI in field  1005 ) to the backing store. The client issues such a command by sending an unlock PCPI request data structure  2300  as illustratively shown in  FIG. 23 . The unlock PCPI command data structure  2300  includes a request data structure  1000 , a file system ID field  2305 , a PCPI ID field  2310  and, in alternate embodiments additional fields  2315 . The requested data structure  1000  is described above conjunction with  FIG. 10 . The file system identifier field  2305  identifies the volume containing the PCPI to the unlocked. The PCPI identifier field  2310  identifies the PCPI previously locked using LOCK_PCPI request. In accordance with the protocol, the server must unlocked the PCPI prior to responding to this command. The response to an unlock PCPI request is illustratively a zero length message body. 
     As noted above, the first request issued over a protocol connection is a series of authentication requests (i.e., a AUTH type of field  1005 ). The authentication request is utilized for NRV session authentication and, in the illustrative embodiment, is preferably the first request issued over an NRV connection. The backing store and storage appliance may negotiate with any number of authentication request/response pairs. An illustrative schematic block diagram of an authentication request data structure  2400  is shown in  FIG. 24 . The AUTH request data structure  2400  includes a request data structure  1000 , a length field  2405 , a type field  2410 , an application field  2415 , a data field  2420  and, in alternate embodiments, additional fields  2425 . The requested data structure  1000  is described above in conjunction with  FIG. 10 . The length field  2405  identifies the number of bytes contained within the data field  2420 . Type field  2410  identifies a type of authentication to be utilized. The application field  2415  identifies one of a plurality of applications that utilizes the protocol. The application utilizing the protocol is identified so that, for example, the backing store may impose higher or lower authentication and standards depending on the type of application utilizing the protocol. The data field  2420  contains authentication data. 
     In response, the backing store sends an authentication response data structure  2500  as shown in  FIG. 25 . The authentication response data structure  2500  includes response data structure  1100 , a status field  2505 , a data field  2510  and, in alternate embodiments, additional fields  2515 . The response data structure  1100  is described above in reference to  FIG. 11 . The status field  2505  identifies the current status of the authentication e.g., OK, signifying that authentication is complete, or NEED_AUTHENTICATION, signifying that the backing store requests that the storage system transmit a higher level of authentication. The status field  2505  may also hold a value of CONTINUE, which may be utilized if multiple exchanges are required to authenticate the session. The data field  2510  contains the authentication response data. 
     The NRV protocol also supports a get holy bitmap function (i.e., a GET_HOLY_BITMAP type of field  1005 ) that identifies which, if any, blocks on a backing store are not present, e.g., either absent or a hole.  FIG. 26  is a schematic block diagram of an exemplary GET_HOLY_BITMAP request data structure  2600  in accordance with an embodiment of the present invention. The request  2600  includes a protocol request data structure  1000 , a file handle  2605 , a cookie value  2610  and, in alternate embodiments, additional field  2615 . The protocol request data structure  1000  is described above in reference to  FIG. 10 . The file handle field  2605  contains a protocol file handle that identifies the file system ID, snapshot ID and file ID of the file for which the bitmap is to be obtained. The cookie field  2610  contains one of two values. The first value is a predetermined value utilized for an initial request. The second value is the value of the last cookie value received from the backing store to be utilized for continued retrieval of bitmaps. 
       FIG. 27  is a schematic block diagram of an exemplary GET_HOLY_BITMAP response data structure  2700  in accordance with an embodiment of the present invention. The response data structure  2700  includes a protocol response data structure  1100 , and attributes field  2705 , a cookie field  2710 , an array of maps  2715  and, in alternate embodiments, additional fields  2720 . The protocol response data structure  1100  is described above in reference to  FIG. 11 . The attributes of field  2705  contains the most up to date file attributes of the identified file at the time the GET_HOLY_BITMAP request is processed. The cookie field  2710  contains a cookie that is of one of two values. The first value is a predefined value utilized for the final response. The second value is a new cookie value to be utilized by the storage system for continued retrieval operations. The maps array  2715  it is a variable length array of indirect block map structures  2800 . 
       FIG. 28  is a schematic block diagram of an exemplary indirect block map structure  2800 . The indirect block map structure  2800  comprises of a file block number field to a  2805 , a level field  2810 , a map field  2815 , and, in alternate embodiments, additional fields  2820 . The file block number field  2805  in conjunction with the level field  2810  identifies an indirect block in a buffer tree of the specified file. The map field  2815  is a bitmap wherein every bit that is set in the bitmap represents a missing block (absent or hole) at the index in the indirect block. That is, for any block that is missing (absent or a hole) in the identified indirect block, a bit will be set. In the illustrative embodiment, the response from the request is utilized to ensure that appropriate space reservations are made when first accessing a file. 
     E. Pre/Post Operation Attributes 
     Network file system protocols typically provide information within the protocol so that clients may cache data to provide an accurate and consistent view of the file system. For example, in the Network File System (NFS) Version 2, file attributes are sometimes returned along with operations, thereby permitting clients to cache data as long as the attributes have not been modified. This was further improved in version  3  of NFS where many operations that modify the file system return attributes from before the operation as well as after the operation. This feature allows a client to recognize if its cached content was up-to-date before the operation was executed. If the cache content was accurate, the client may update its cache by doing the update locally without invalidate its own cached content. This technique is known as pre/post operation attributes. 
     Most file systems cache content based on a file&#39;s unique file handle. While most network operations in protocols that modify the file system have the necessary file handle in attributes allow the client to correctly update its cache, there are some operations that do not include sufficient information. These operations typically reference files using a directory file handle and a file name, which results in the client receiving a response from which it cannot determine which file was referenced and potentially modified. As a client cannot determine which file was referenced and/or modified, it is unable to ensure that its cache is consistent with the state of the file system. One advantage of the present invention is that the novel NRV protocol provides sufficient information to permit proper caching of any object modified on the origin server using any of these operations. 
       FIG. 29  is a schematic block diagram of a remove request data structure  2900  (i.e., a REMOVE type of field  1005 ) in accordance with an embodiment of the present invention. The remove request data structure  2900  includes a protocol request data structure  1000 , a directory file handle field  2905 , a filename field  2910  and, in alternate embodiments, additional fields  2915 . The request data structure  1000  is described above in reference to  FIG. 10 . The directory file handle field  2905  comprises a file handle associated with a particular directory within the file system. The filename field  2910  contains the filename of the file to be removed. 
     A remove response data structure  3000  is illustratively shown in  FIG. 30 . The remove response data structure  3000  illustratively includes a protocol response data structure  1100 , a directory pre/post attributes field  3005 , a removed file handle field  3010 , a removed file pre/post attributes field  3015  and, in alternate embodiments, additional fields  3020 . The protocol response data structure  1100  is described above in reference to  FIG. 11 . The directory pre/post attributes field  3005  contains the attributes for the directory both before and after the removal. These attributes permit clients to properly maintain their caches. The removed file handle field  3010  contains the file handle for the file that was removed while processing the remove operation. The removed file pre/post attributes contains the attributes for the file prior to and following the removal operation. 
       FIG. 31  is a schematic block diagram of an exemplary rename request  3100  (i.e., a RENAME type of field  1005 ) in accordance with an embodiment of the present invention. The rename request data structure  3100  includes a protocol request data structure  1000 , a source directory file handle  3105 , a source file name field  3110 , a destination directory file handle field  3115 , a destination file name field  3120 , and in alternate embodiments additional fields  3125 . The protocol request data structure  1000  is described above in reference to  FIG. 10 . The source directory file handle field  3105  contains the file handle identifying the source directory of the file to be renamed. The source filename field  3110  contains the filename of a file within the source directory identified by the source directory file handle field  31051 . The destination directory file handle field  3115  contains a file handle for the directory to which she file is to be renamed. The destination file name field  3120  contains the filename of the resulting file. 
       FIG. 32  is a schematic block diagram of an exemplary of a rename response data structure  3200  in accordance with an embodiment of the present invention. The rename response data structure  3200  includes a protocol response data structure  1100 , a source directory pre-post attributes field  3205 , a source file handle field  3210 , a source file pre/post attributes field  3215 , a destination directory pre/post attributes field  3220 , a destination file handle field  3225 , a destination file pre/post attributes field  3230  and, in alternate embodiments additional fields  3235 . The protocol response data structure  1100  is described above in reference to  FIG. 11 . The source directory pre/post attributes field  3205  contains the attributes for the source directory before and after the rename operation. The source file handle field  3210  contains a file handle associated with the file prior to the rename operation. The source file pre/post attributes field  3215  contains the attributes associated with the file prior to and immediately following the rename operation. The destination directory pre/post attributes field contains the attributes associated with the directory of the directory in which the file is being renamed. The destination file handle field  3225  contains the file handle for the newly renamed file, while the destination file pre-/post attributes field  3230  contains the file attributes for the destination file both before and after the rename operation. 
       FIG. 33  is a schematic block diagram of an exemplary create request  3300  in accordance with an embodiment of the present invention. The create request data structure  3300  includes a protocol request data structure  1000 , a directory file handle field  3305 , a file name field  3310  and, in alternate embodiments additional fields  3315 . The protocol request data structure  1000  is described above in reference to  FIG. 10 . The directory file handle field  3305  contains a file handle identifying the directory in which the file is to be created. The filename field  3310  identifies the name to be utilized for the creation of the file. 
       FIG. 34  is a schematic block diagram of a create response data structure  3400  in accordance with an embodiment of the present invention. The create response data structure  3400  includes a protocol response data structure  1100 , a directory pre/post attributes field  3405 , a created file handle field  3410 , a created pre/post attributes field  3415  and, in alternate embodiments, additional fields  3420 . The protocol response data structure  1100  is described above in relation to  FIG. 11 . The directory pre/post attributes field  3405  contains the attributes for the directory containing the newly created file both before and after the creation of the file. The created file handle field  3410  contains the file handle for the newly created file. The created file pre/post attributes field  3415  contains the attributes for the file prior to and following the file creation. 
     E. Retrieval of Data Using The NRV Protocol 
       FIG. 35  is a flow chart detailing the steps of a procedure  3500  for retrieving one or more blocks from a backing store utilizing the novel NRV protocol in accordance with an embodiment of the present invention. The procedure begins in step  3502  and continues to step  3504  where a storage appliance identifies one or more blocks to be retrieved from a backing store. This identification may be made by determining that the blocks are marked ABSENT, as in the case of a sparse volume, or may be determined by other, alternate means. In response, the storage system sends an AUTH request to the backing store to authenticate the connection in step  3506 . The backing store responds with an AUTH response in step  3508  and in step  3510 , a the storage system determines whether the connection has been authenticated. If it has not been authenticated, the procedure branches back to step  3506  and the storage appliance sends another AUTH request to the backing store. However, if the connection has been authenticated in step  3510 , the procedure continues to step  3512  where the storage appliance sends an INIT request to the backing store. In response, the backing store sends an INIT reply to the storage appliance in step  3514 . At this point, the protocol connection between the storage appliance and backing store has been initialized and authenticated, thereby enabling issuance of additional commands including, for example a VOLINFO command. 
     In this illustrated example, the storage appliance sends a READ request to the backing store in step  3516 . In response the backing store retrieves the requested data from its storage devices in step  3518  by, for example, retrieving the data from disk. The backing store then sends a READ response including the requested data to the storage to appliance in step  3520 . Upon receiving the requested data, the storage appliance processes the retrieved data in step  3522 . The process then completes in step  3524 . 
       FIG. 36  is a flow chart detailing the steps of a procedure  3600  for using the lock PCPI command with a long-lived application. The procedure begins in step  2702  and continues to step  3604  were the storage system initiates a long-lived application that requires one or more blocks to be retrieved from the backing store. The long-live application may comprise a restore on demand application or any other application that may require continued use of a particular file or PCPI on the backing store. The storage appliance then sends an AUTH request (step  3606 ) to the backing store to authenticate the connection. In response, the backing store transmits an AUTH response to the storage appliance in step  3608 . In step  3610 , a determination is made as to whether the connection is authenticated. If not, the procedure loops back to step  3606 . Otherwise, the procedure continues to step  3612  where the storage system transmits an INIT request to the backing store, which responds (in step  3614 ) by sending an INIT response. Once the communication has been authenticated and initialized, the storage system sends a lock PCPI request to the backing store in step  3616  that identifies the appropriate PCPI to be locked. In response, the backing store locks the requested PCPI and send a lock PCPI reply to the storage appliance in step  3618 . 
     The storage appliance may then send a READ request to the backing store in step  3620 . In response, the backing store retrieves the requested data from its storage devices in step  3622  and a sends a READ reply, including the requested data, to the storage appliance in step  3624 . It should be noted that during the course of the long-lived application, steps to  3620 - 3624  may be repeated a plurality of times. Additionally, alternate commands other than a READ request may be issued by the storage appliance to the backing store. In response to such alternate commands, the backing store processes the received commands in accordance with the protocol specification as described above. At some point in time, when the long-lived application no longer requires the use of the particular PCPI, the storage appliance sends an unlock PCPI request to the backing store (step  3626 ). In response, the backing store unlocks the identified PCPI and sends an unlock PCPI reply to the storage appliance in step  3628 . The procedure then completes in step  3630 . 
     To again summarize, the present invention is directed to system and method for supporting a sparse volume within a file system of a storage system. In accordance with the illustrative embodiment a storage operating system executing on a storage appliance includes a novel NRV protocol module that implements the NRV protocol. The NRV protocol module interfaces with the file system to provide remote retrieval of data from a backing store. The NRV protocol illustratively utilizes the TCP/IP protocol as a transport protocol. The NRV protocol module is invoked by an exemplary Load_Block( ) function within a file system that determines whether a block is to be retrieved from the remote backing store. If so, the Load_Block( ) function initiates a series of NRV protocol requests to the backing store to retrieve the data. 
     The NRV protocol module first authenticates the connection and then transmits an initialization request to match the appropriate information required at the beginning of the connection. Once the NRV protocol connection has been initialized and authenticated, various types of data may be retrieved from the backing store including, for example, information relating to volumes, blocks and files or other data containers stored on the backing store. Additionally, the NRV protocol provides a mechanism to remotely lock a PCPI (a lock PCPI request) on the backing store so that the backing store does not modify or delete the PCPI until it is unlocked via an unlock command (an unlock PCPI request) sent via the NRV protocol. Such locking may be utilized when the backing store is instantiated within a PCPI that is required for a long-lived the application on the storage appliance, such as a restore on demand application. The novel NRV protocol also includes commands for retrieving status information such as volume information, from the backing store. This may be accomplished by sending a VOLINFO request to the backing store identifying the particular volume of interest. 
     The present invention provides a NRV protocol that provides several noted advantages over using conventional open protocols. One noted advantage is the transparency of operations. Existing open protocols such as the network file system protocol (NFS) do not expose side effects file system operations, such as that generated a rename operation, which implicitly deletes a target file. Conventional protocols do not inform a client that the file handle of the file that has been deleted. However, certain applications is of the NRV protocol, such as that described in U.S. patent application Ser. No. 11/409,625, entitled Proxy File System, by Jason Lango, or other file caching mechanisms is interested in such information to ensure that cache contents can be invalidated at the appropriate times. A second noted advantage is that the novel NRV protocol of the present invention exposes file system metadata. Conventional protocols, such as NFS. do not expose file system-specific metadata, but rather normalizes the information into a standard format, which may be lossy in that it does not convey some file system specific information. In one alternate embodiment of the present invention, certain features of the NRV protocol may be implemented using a conventional open protocol coupled with an extension protocol that provides the desired functionality necessary for implementing sparse volumes. In such an environment, an open protocol, such as the NFS protocol would be coupled to the NRV protocol. In such an environment the NRV  295  would be configured to utilize the NFS protocol for certain file system operations directed to a backing store. 
     The foregoing description has been directed to specific embodiments of this invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. For instance, it is expressly contemplated that the teachings of this invention can be implemented as software, including a computer-readable medium having program instructions executing on a computer, hardware, firmware, or a combination thereof. Accordingly this description is to be taken only by way of example and not to otherwise limit the scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.