Method and apparatus for identifying and eliminating duplicate data blocks and sharing data blocks in a storage system

A method for sharing data blocks in a hierarchical file system in a storage server includes allocating a plurality of data blocks in the file system, and sharing data blocks in the file system, without using a persistent point-in-time image, to avoid duplication of data blocks. A method for identifying data blocks that can be shared includes computing a fingerprint for each of multiple data blocks to be written to a storage facility and storing the fingerprint with information identifying the data block in an entry in a set of metadata. The set of metadata is used to identify data blocks which are duplicates.

FIELD OF THE INVENTION

At least one embodiment of the present invention pertains to data storage systems, and more particularly, to a technique for identifying and eliminating duplicate data blocks and for sharing data blocks managed by a file system.

BACKGROUND

In a data storage system it is desirable to use storage space as efficiently as possible, to avoid wasting storage space. One type of system in which this concern can be particularly important is a storage server, such as a file server. File servers and other types of storage servers often are used to maintain extremely large quantities of data. In such systems, efficiency of storage space utilization is critical.

Files maintained by a file server generally are made up of individual blocks of data. A common block size is four kilobytes. In a large file system, it is common to find duplicate occurrences of individual blocks of data. Duplication of data blocks may occur when, for example, two or more files have some data in common or where a given set of data occurs at multiple places within a given file has. Duplication of data blocks results in inefficient use of storage space.

A technique which has been used to address this problem in the prior art is referred to as “file folding”. The basic principle of file folding is to allow new data of a file in the active file system to share a disk block with the old data of the file in a persistent image if the new data are identical to the old data. By using file folding, ideally only one occurrence of each unique data block will exist in a file system. This technique has been implemented in file servers, known as Filers, made by Network Appliance, Inc., of Sunnyvale, Calif. Specifically, Network Appliance Filers are capable of acquiring a Snapshot™ of a specified set of data. A “Snapshot” is a persistent, read-only image of the storage system, and more particularly, of the active file system, at a particular instant in time. If a block within a file that has been “Snapshotted” is modified after the Snapshot, rather than creating another complete (modified) copy of the file in the active file system, the Filer only creates the modified block for that file in the active file system; for each unmodified block, the Filer simply gives the file a pointer to the corresponding block in the Snapshot. In this way, the unmodified blocks in the Snapshot become shared between the Snapshot and the active file system. This technique is described in greater detail in U.S. Patent Application Publication no. 2003/0182317, entitled, “File Folding Technique,” filed on Mar. 22, 2002 by A. Kahn et al., and assigned to the assignee of the present application.

File folding does help to more efficiently use storage space. However, it is desirable to reduce data duplication in an active file system without having to rely upon a persistent point-in-time image (e.g., a Snapshot). It is also desirable to reduce data duplication regardless of the location of the data in the file system.

Another prior art approach to avoiding duplication of data in a storage system involves computing a hash value for every file that is stored. For example, in one known prior art system, which does not use a traditional (hierarchical) file system approach, a storage server is used to store data on behalf of an application server or other client. When the application server wants the storage server to store a particular file, the application server computes a hash value for the file and sends the storage server a write request containing the file and the hash value.

The storage server uses hash values of files to help reduce data duplication. More specifically, the storage server maintains a database containing a mapping of all of the stored files to their respective hash values. When the storage server receives a write request with a hash value, it searches for a match of that hash value in its database. If no match is found, the storage server concludes that it does not have a copy of that file already stored, in which case the storage server requests the file from the application server. If a match of the hash value is found, however, the storage server concludes that it already has a copy of that file stored and, therefore, does not have to request the file from the application server.

This method of using hash values employs a proprietary set of protocols and semantics, which are very different from those used in a traditional (hierarchical) file system. Further, the need to compute a hash value for every read or write and for every data block adversely affects performance, particularly during reads. In addition, every time a file is modified, the file has to be stored as a new file with a new hash value associated with it. Moreover, this approach involves complicated cleanup issues with regard to determining when particular blocks can be freed.

SUMMARY OF THE INVENTION

The present invention includes a method and apparatus for sharing data blocks in a hierarchical file system in a storage server. In at least one embodiment, the method includes allocating a plurality of data blocks in the hierarchical file system, and sharing data blocks in the hierarchical file system, without using a persistent point-in-time image, to avoid duplication of data blocks.

Another aspect of the invention is a method and an apparatus for identifying data blocks that can be shared. In at least one embodiment, the method includes computing a fingerprint for each of multiple data blocks to be written to a storage facility and storing the fingerprint with information identifying the data block in an entry in a set of metadata. The method further includes using the set of metadata to identify data blocks which are duplicates.

Other aspects of the invention will be apparent from the accompanying figures and from the detailed description which follows.

DETAILED DESCRIPTION

A method and apparatus for identifying and eliminating duplicate data blocks and for sharing data blocks in a hierarchical file system are described. As explained in greater detail below, the technique introduced herein allows sharing of data blocks in a hierarchical file system of a storage server to avoid duplication of data blocks anywhere in the file system, without using any persistent point-in-time image (PPI), such as a Snapshot. The technique also does not require any client-side behavioral/design changes or appreciably impact read/write performance in a negative way.

In certain embodiments of the invention, the sharing of data blocks involves maintaining a reference count file that contains, for each data block in the file system, a reference count indicating the number of references to the data block. Shared data blocks are blocks which are the target of two or more references. Data blocks that can be shared are identified by computing a fingerprint for each data block to be written to a storage facility and storing the fingerprint with information identifying the data block in an entry in a metadata file, as the data block is being written to the storage facility. A “fingerprint”, as the term is used herein, is any information derived from the content of a data block, that might uniquely identify the data block. The entries in the metadata file are then sorted by fingerprint, and the sorted metadata file is used to identify data blocks which are duplicates. Duplicate data blocks are freed for reuse, and their remaining counterparts which are not freed are shared to the extent possible.

One example of a system in which this technique can be implemented is a file server system.FIG. 1shows an example of a network environment in which such a system can be employed. InFIG. 1, a file server2is coupled locally to a storage subsystem4which includes multiple mass storage devices, and to a set of clients1through a network3, such as a local area network (LAN). Each of the clients1may be, for example, a conventional personal computer (PC), workstation, or the like. The storage subsystem4is managed by the file server2. The file server2receives and responds to various read and write requests from the clients1, directed to data stored in or to be stored in the storage subsystem4. The mass storage devices in the storage subsystem4may be, for example, conventional magnetic disks, optical disks such as CD-ROM or DVD based storage, magneto-optical (MO) storage, or any other type of non-volatile storage devices suitable for storing large quantities of data. The storage devices in storage subsystem4can be organized as a Redundant Array of Inexpensive Disks (RAID), in which case the file server2accesses the storage subsystem4using one or more well-known RAID protocols.

The file server2may be a file server designed for use in a network attached storage (NAS) mode (otherwise referred to as a “filer”), as is henceforth assumed in this description. The technique introduced herein can be implemented in the file server2, or in other devices, as described below. The technique can also be adapted for use in other types of storage systems, however, such as storage servers which provide clients with block-level access to stored data, or processing systems other than storage servers.

FIG. 2is a block diagram showing an example of the architecture of the file server2at a high level. Certain standard and well-known components which are not germane to the present invention are not shown. The file server2includes one or more processors21and memory22coupled to a bus system23. The bus system23shown inFIG. 2is an abstraction that represents any one or more separate physical buses and/or point-to-point connections, connected by appropriate bridges, adapters and/or controllers. The bus system23, therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus (sometimes referred to as “Firewire”).

The processors21are the central processing units (CPUs) of the file server2and, thus, control its overall operation. In certain embodiments, the processors21accomplish this by executing software stored in memory22. A processor21may be, or may include, one or more programmable general-purpose or special-purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), or the like, or a combination of such devices.

Memory22includes the main memory of the file server2. Memory22represents any form of random access memory (RAM), read-only memory (ROM), flash memory, or the like, or a combination of such devices. Memory22stores (among other things) the storage server's operating system24, which can implement the technique introduced herein.

Also connected to the processors21through the bus system23are one or more internal mass storage devices25, a storage adapter26and a network adapter27. Internal mass storage devices25may be or include any conventional medium for storing large volumes of data in a non-volatile manner, such as one or more magnetic or optical based disks. The storage adapter26allows the file server2to access the storage subsystem4and may be, for example, a Fibre Channel adapter or a SCSI adapter. The network adapter27provides the file server2with the ability to communicate with remote devices, such as the clients1, over a network and may be, for example, an Ethernet adapter, a Fibre Channel adapter, or the like.

FIG. 3shows an example of the architecture of the operating system24of the file server2. As shown, the operating system24includes several software modules, or “layers”. These layers include a file system31. The file system31is application-layer software which imposes a hierarchical structure on the data stored in the storage subsystem4(i.e., a hierarchy of files and directories) and which services read/write requests from clients1. Logically “under” the file system31, the operating system24also includes a protocol layer32and an associated network access layer33, to allow the file server2to communicate over the network3(e.g., with clients1). The protocol32layer implements one or more of various higher-level network protocols, such as Network File System (NFS), Common Internet File System (CIFS), Hypertext Transfer Protocol (HTTP) and/or Transmission Control Protocol/Internet Protocol (TCP/IP), and serves to translate read/write requests from clients into a format understandable by the file system31. The network access layer33includes a driver or drivers to implement one or more lower-level protocols to communicate over the network, such as Ethernet, Fibre Channel or iSCSI.

Also logically under the file system31, the operating system24includes a storage access layer34and an associated storage driver layer35, to allow the file server2to communicate with the storage subsystem4. The storage access layer34implements a higher-level disk storage protocol, such as RAID, while the storage driver layer35implements a lower-level storage device access protocol, such as Fibre Channel Protocol (FCP) or small computer system interface (SCSI). Also shown inFIG. 3is the path37of data flow, through the operating system24, associated with a read or write operation.

The operating system24also includes a user interface module36and a de-duplication module39logically on top of the file system31. The user interface module may implement a command line interface and/or a graphical user interface, which may be accessed by a network administrator from an attached administrative console or through the network3. The de-duplication module39is an application layer which identifies and eliminates duplicate data blocks (“de-duplication”) and triggers data block sharing in accordance with the technique introduced herein.

The operating system24also includes, or has access to, three databases that are used to implement the sharing technique, namely, a reference count file41, an Active map42and a fingerprints database43. The reference count file41contains an entry for each data block maintained by the file server2, wherein each entry includes a value, REFCOUNT, indicating the number of references to that data block. For example, a data block which is shared by two files would have a REFCOUNT value of 2. A data block can be shared by more than two files (or other entities), in which case the REFCOUNT value would reflect this accordingly. A data block which is allocated but not shared would have a REFCOUNT value of 1. A data block which is not yet allocated would have a REFCOUNT value of 0. In certain embodiments of the invention, the REFCOUNT value for each data block is a two-byte binary value, which allows each data block to be the target of up to 216−1 references.

The Active map42is a bitmap of all data blocks managed by the file server2, i.e., one bit per data block. The bit for a given data block is set in the Active map42if the data block is allocated and cleared if the data block is free to be used. The Active map42is used during allocation of blocks to determine whether a block is free or not. The Active map42helps to improve performance by avoiding the need to read the reference count file41to identify free blocks; the reference count file41is much larger (and therefore takes longer to read) than the Active map42. In an alternative embodiment, however, the Active map42and the reference count file41could be combined into a single file to identify each free block as well as to indicate the number of references to the data block.

The fingerprints database43is used by the de-duplication module39to store a fingerprint for each data block that is written to the storage subsystem4. In this context, a “fingerprint” may be a checksum, for example. The fingerprints are used in a process for efficiently identifying duplicate data blocks, i.e., to identify data blocks that can be shared. This process is described below in detail.

FIG. 4shows the overall process of the technique introduced herein at a high-level, according to certain embodiments of the invention. The first phase401is identifying duplicate data blocks. Once the duplicate data blocks are identified, the duplicates are eliminated at402so as to leave only one instance of each unique data block. This phase402includes sharing the remaining instance of each data block that was duplicated and freeing the (no longer used) duplicate data block(s). Phase402also involves updating the reference count file41, Active map42and fingerprints database43. This whole process may be triggered automatically at predetermined intervals or at predetermined times, or it may be triggered manually or in response to pre-specified events (such as deletion of a file) or in response to a pre-specified policy (such as a given number of new blocks having been collected).

The sharing technique will now be further described with reference toFIGS. 5,6and7, according to certain embodiments of the invention. Assume for purposes of explanation that the active file system of a file server maintains two simple files, named Foo and Bar, shown inFIG. 5. File Foo contains two data blocks, and file Bar contains two data blocks. Each data block is identified in the file system by (among other things) its volume block number (VBN). A VBN identifies the logical block where the data is stored (since RAID aggregates multiple physical drives as one logical drive), as opposed to a physical block. A VBN should be distinguished from a disk block number (DBN) which identifies the physical block number within a disk in which the block is stored, or a file block number (FBN) which identifies the logical position of the data within a file. The two blocks of file Foo have VBN values of 1 and 2. VBN 1 contains the data, “A”, while VBN 2 contains the data, “B”. The two blocks of file Bar have VBN values of 3 and 4. VBN 3 contains the data, “C”, while VBN 4 contains the data, “D”.

For each VBN maintained by the file system, the reference count file41includes a value, REFCOUNT, indicating the number of references to that VBN. In the example ofFIG. 5, for VBNs [1,2,3,4] the REFCOUNT values are [1,1,1,1], respectively, indicating that each VBN is the target of one reference.

Refer now toFIG. 6, which is a variation of the example ofFIG. 5, in which VBNs 3 and 4 of file Bar have the same data (“A”) as VBN 1 of file Foo. That is, VBNs 3 and 4 are duplicates of VBN 1 and of each other. Initially, when a data block is allocated by the file system, its REFCOUNT value in the reference count file41is set equal to 1. Accordingly, before duplicate data blocks are identified in the example ofFIG. 6, the REFCOUNT values for the example ofFIG. 6are the same as inFIG. 5, i.e., [1,1,1,1], as shown.

In contrast,FIG. 7shows what the example ofFIG. 6would look like after duplicate data blocks has been identified and sharing is implemented. Sharing involves giving, to each entity which owns a shareable data block, a pointer to that data block. Accordingly, in the example ofFIG. 7this involves giving file Bar two pointers to VBN 1 (file Foo already had a pointer to VBN 1). The process also involves eliminating the duplicate data blocks, VBNs 3 and 4, and freeing them for reuse. Once this process is completed, the REFCOUNT values for VBNs [1,2,3,4] are adjusted to be [3,1,0,0], respectively, to reflect the fact that VBNs 3 and 4 have been freed and VBN 1 now has three references to it (i.e., VBN 1 is shared).

The reference count file41is continually updated to reflect events that affect these blocks. For example, if file Foo is now deleted, the REFCOUNT values for VBNs [1,2,3,4] would be adjusted to be [2,0,0,0], respectively, reflecting that VBN 2 has been freed in addition to VBNs 3 and 4. Note that VBN 1 has not been freed (i.e., its REFCOUNT value is not zero), since VBN 1 is still in use by file Bar; instead the REFCOUNT value for VBN 1 has been decremented from 3 to 2. If file Bar is now deleted, the REFCOUNT values for VBNs [1,2,3,4] would be adjusted to be [0,0,0,0], respectively.

The approach just described assumes that the file system adheres to the copy-on-write principle; that is, anytime a data block is modified, it is written to a different VBN, rather than modifying the data in place. Referring back to the example ofFIG. 7, therefore, assume that a write request from a client causes the data “A” in file Bar to be changed to “F”. In this case, VBN 1, which contains the data “A”, is not modified. However, since the new data, “F”, is written to a new logical and physical block, the REFCOUNT value for VBN 1 must still be updated. Hence, the REFCOUNT value for VBN 1 in this case would be decremented by one. In addition, the REFCOUNT value for whichever VBN is allocated to store the new data, “F”, would be incremented by one.

It is possible to implement this technique even in a file system which does not impose copy-on-write in all instances. For example, the technique can be employed by requiring copy-on-write only when the REFCOUNT value for given data block is greater than one.

To avoid data inconsistencies, when a file which contains one or more shared blocks is modified, its REFCOUNT values and block pointers are updated in a single atomic transaction. This updating may be done, for example, during a “consistency point”, i.e., when a set of accumulated write transactions are committed from temporary storage to persistent storage.

It is possible for data in the reference count file41to become corrupted, for any of various reasons. Therefore, it is prudent to have some way of ensuring consistency between the reference count file41and the actual state of the file system. One way this can be accomplished is scanned the entire active file system for consistency with the reference count file41before boot-up of the file system. This can include creating a separate, temporary reference count file in main memory of the file server2, scanning all data blocks in the file system to identify shared data blocks, and updating the temporary reference count file to reflect any shared data blocks. The temporary reference count file is then compared to the regular (persistent, on-disk) reference count file41to determine whether they match. If they do not match, an inconsistency is identified, and appropriate corrective action is taken.

In addition, it may be desirable to allow this type of consistency check to be run while the file system is in operation. This can be done by creating the temporary reference count file on disk, so as not to consume main memory in the file server2. In that case, however, if the user modifies a particular block while the consistency check is running, it is necessary to update both the temporary and the persistent reference count files.

Various other optimizations can be added to the above described technique. For example, a SHARED flag can be provided for each file in the file system, to indicate whether the file contains any shared blocks. The SHARED flag can be stored in a convenient location, such as in the file's inode (a container of metadata about the file, used by the file system), to allow fast determination of whether it is necessary to read the reference count file41when modifying a block. This avoids unnecessarily having to read the (large) reference count file41when the file includes no shared blocks. Similarly, another flag can be implemented for each volume in the storage system, to indicate whether the volume is allowed to implement block sharing. The benefit, as in the previous example, is avoiding the need to read the reference count file41in all cases.

Further, one or more counters can be implemented in the file system to track the total number of shared blocks. These counters can be used to provide an output to a user (e.g., a storage network administrator) indicating the amount of disk space being saved as a result of block sharing.

There are many possible advantageous uses of the above-described technique. For example, this technique allows any individual file to be copied by simply copying its block pointers and incrementing the REFCOUNT values of all of its data blocks by one, without actually creating copies of the data blocks. In this way, it is possible to create a single-file PPI, i.e., a persistent point-in-time image of a particular file. In the event the file is modified after the PPI is taken, it is possible to revert the particular file back to the PPI as long as the PPI remains available.

Another advantage of the above-described technique is “file cloning” functionality. By copying the block pointers from one file to another file, it is possible to make a “clone” of the original file. At the time the clone is created, both the clone and the original share the same set of pointers. However, to users they are two completely independent files. If a user modifies a block, call it block A, in the original file (which is shared by the clone), the clone will be modified at the same time. Assuming copy-on-write is being used, the original file will get a new block, block A′, and the clone will get another new block, block A″, through two simultaneous copy-on-write operations.

Yet another advantage of the above-described technique is that a file system with shared data blocks can be easily converted to a file system without shared blocks, if desired. For example, when such an operation is desired, any data blocks whose REFCOUNTs are greater than one are simply copied out to new locations, the RECOUNTs of those data blocks are then each decremented by one, and block pointers are adjusted as necessary to point to the new instances of those blocks.

Still another advantage is that the above-described technique allows for efficient replication. When replicating a file system or a portion thereof, for example, shared blocks only need to be sent from the source to the destination once, thus making replication more bandwidth-efficient. In addition, the replicated file system will use less space than it would otherwise, just as with the original volumes.

FIG. 8illustrates elements of the de-duplication module39, according to certain embodiments of the invention. The elements include a de-duplication engine81, a gatherer module82, a fingerprint manager83, a fingerprint handler84and a block sharing engine85. The fingerprints database43stores the fingerprints of all data blocks that have been written to the storage subsystem4, as noted above. In certain embodiments, each fingerprint is a checksum, such as an MD5 checksum. Each block has a separate entry in the fingerprints database43. Each such entry includes the fingerprint of the block, the inode number of the file to which the block belongs, and the FBN of the block.

The fingerprint handler84is responsible for computing the fingerprints of data blocks. The gatherer module82is responsible for identifying new data blocks and for triggering the fingerprint handler84to compute fingerprints of the new data blocks and return them to the gatherer module82. The fingerprint manager83receives the fingerprints of the new data blocks from the gatherer module82and updates the fingerprints database43to include entries for those data blocks. The fingerprint manager83is also responsible for identifying potentially duplicate data blocks, by finding entries with matching fingerprints in the fingerprints database43. The block sharing engine85is responsible for comparing potentially duplicate data blocks identified by the fingerprint manager83to identify actual duplicate data blocks, and for calling functions of the file system31when appropriate to eliminate duplicate data blocks and implement block sharing. For performance reasons, multiple block share operations may be ongoing at any given time. The de-duplication engine81schedules and triggers operations of the other modules. In particular, the de-duplication engine81triggers operation of the gatherer module82, which may be done according to a pre-specified schedule, timing algorithm, or in response to a manual input.

According to certain embodiments of the invention, at any particular point in time a block will be in one of the following states: free, in-use, fingerprinted, shared, and zombie. A free block is a block that is not being used (not allocated). An in-use block is a block that is being used and has not yet been processed by the de-duplication operation. A fingerprinted block is a block that has been processed by the de-duplication operation, and for which an entry has been added into the fingerprints database to track the block. A shared block is a block that has become shared and for which one or more duplicates of this block have been identified and eliminated. A zombie is a block that was shared but now is no longer used by any files, but the block has not yet been freed.FIG. 9illustrates how a block can transition through the various states in response to various events.

FIG. 10shows a process performed by the de-duplication module39when a block is written to disk. In response to a request to write a block at1001, the fingerprint handler84computes a fingerprint for the block at1003. The fingerprint is passed to the fingerprint manager83, which writes an entry for the block into the fingerprints database43at1004including the fingerprint, the FBN, the inode number of the block, and other relevant context information that is specific to this block, such as the value of a consistency point counter at the time the block was written to disk. These operations1003and1004are performed concurrently with the operation of writing1002the blocks to the storage subsystem4. In alternative embodiments, however, computing the fingerprint (1003) and writing the fingerprint to the fingerprints database43may not be performed concurrently with writing the block to disk, although doing so is believed to be more efficient.

As noted above, the information saved in the fingerprints database43for each block includes context information, such as the value of a consistency point counter at the time the block was written to disk. This context information can be used to detect and delete “stale” fingerprints from the fingerprints database43. Stale fingerprints are fingerprints that corresponds to blocks that have been deleted or overwritten. Entries with higher consistency point counter values are more recent than entries with lower consistency point counter values. Complementary to this functionality, information on the deleted files and/or blocks in the deletion code path is also logged and used to clean up stale entries.

FIG. 11illustrates the process of eliminating duplicate data blocks. In certain embodiments of the invention, the process ofFIG. 11is performed only during idle time of the file server2(unlike the process ofFIG. 10, which is performed during writes), to avoid consuming processing resources and thereby adversely impacting system performance. At1101the fingerprints database is sorted, by fingerprint. Sorting the fingerprints database is optional but allows faster identification of duplicate blocks, since the entries for any duplicates will reside adjacent to each other in the fingerprints database after the sort operation is complete. At1102the process determines from the fingerprints database whether there are any entries with identical fingerprints. The blocks represented by any entries which have identical fingerprints are considered to be potential duplicate blocks, rather than actual duplicates, since there is always a possibility that two non-identical blocks could have the same fingerprint, regardless of the fingerprint scheme being used. If there are no entries with identical fingerprints, the process ends. If there are any entries with identical fingerprints, the process proceeds to1103, in which the process compares the potential duplicate blocks with each other to determine if they are in fact identical. In an alternative embodiment, operation1103could be eliminated if an approximate verification of comparing fingerprints is deemed sufficient in determining that two blocks are identical.

If the blocks are not identical (1104), the process proceeds to1108, described below. If the blocks are identical, then at1105the process frees the duplicate block or blocks so that only one instance remains of each unique block, and shares the remaining instance of the block to the extent possible. The process then updates the reference count file41and the Active map42at1106to reflect the newly shared and freed blocks. At1107the process deletes the entries for any freed blocks from the fingerprints database43. Following either1107or a negative outcome of1102, the process determines whether the entire fingerprints database43has been examined. If the entire fingerprints database43has not been examined, the process loops back to1102and continues as described above, until the entire fingerprints database43has been examined.

Note that the process ofFIG. 11can be executed in separate stages, for performance reasons. For example, the first stage can be determination1102of whether any entries have identical fingerprints; the second stage can be1103to1106, to perform deduplication on all identified duplicate blocks. In a third stage,1107can be done to remove all stale entries in one pass. As such, operations within each stage can be reordered, if desired, to optimize performance.

In certain embodiments of the invention, sorting of the fingerprints database43(1101inFIG. 11) is done in the following manner, as illustrated inFIG. 12. First, the fingerprints database is divided into some number, N, of approximately equal-sized chunks121. Each of the N chunks121is then independently sorted by fingerprint value, using any conventional sorting algorithm, such as Quicksort, for example. The algorithm then compares the fingerprints in the entries of the same rank in all of the N chunks121(e.g., the top entry in each of the N chunks121) and copies the entry which has the smallest fingerprint value from among those into the next available slot in the sorted output file122. The output file122becomes the sorted fingerprints database43when the sorting operation is complete. This process is then repeated until all of the entries in the N sorted chunks121have been copied into the output file122.

FIG. 13illustrates in greater detail the operation of freeing a data block, such as a duplicate block. At1301the process determines whether the SHARED flag is set for the file which contains the block to be freed. If the SHARED flag is not set (meaning that no blocks in the file are shared), the process proceeds to1304, in which the bit corresponding to the block is cleared in the Active map42. The process then ends. If the SHARED flag is not set at1301, then at1302the process decrements the REFCOUNT value for the block by one in the reference count file41. After decrementing the REFCOUNT value, the process determines at1303whether the REFCOUNT value is zero. If the REFCOUNT value is zero (meaning that the block is no longer used), the process clears the corresponding bit in the Active map42and then ends. If the REFCOUNT value is determined to be non-zero at1303, the process simply ends.

In certain embodiments, the system also maintains a change log44(FIG. 3) to identify blocks that are new or modified since the last time the process ofFIG. 11was executed. The change log44contains information of the same type as the fingerprints database43(i.e., fingerprint of the block, inode number of the file to which the block belongs, and the FBN of the block), but only for new or modified blocks. From time to time, the system then re-executes the sorting process ofFIG. 12on both the fingerprints database43and the change log44, to merge the change log44into the fingerprints database43.

In alternative embodiments, rather than using the techniques ofFIGS. 11 and 12to identify and eliminate duplicate data blocks, the system could simply from time to time scan the entire file system, compute the fingerprints of all data blocks, and eliminate duplicates at essentially the same time. However, the technique described above is believed to be more efficient.

Thus, a method and apparatus for identifying and eliminating duplicate data blocks and for sharing data blocks have been described. Note that references throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics being referred to may be combined as suitable in one or more embodiments of the invention, as will be recognized by those of ordinary skill in the art.