Abstract:
A method and system of providing replica files within a fileset is disclosed. According to one embodiment, a method is provided which comprises associating a source file and a replica file of the source file within a fileset, wherein the replica file of the source file comprises a virtual copy of the source file. In another embodiment wherein the source file comprises a metadata file system object and the replica file of the source file comprises a replica metadata file system object, the associating comprises generating a reference within the replica metadata file system object to the metadata file system object.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to data storage and retrieval generally and more particularly to a method and system of providing replica files within a fileset. 
     2. Description of the Related Art 
     Information drives business. Companies today rely to an unprecedented extent on online, frequently accessed, constantly changing data to run their businesses. Duplicate copies as well as slightly altered versions or editions of such data are commonly made and maintained, for example, to facilitate independent and simultaneous data access by a number of users and/or processes. Providing such copies or versions has until recently required data to be completely replicated for each copy or version made, often resulting in wasted storage space, particularly for infrequently-accessed and/or modified data. Alternatively, links (e.g., hard links or symbolic links) may be used to reference a single data image. Use of such links fails however to provide an exclusive copy of the data to each user or process. In other words, a change or write operation made by one user or process is immediately visible to all users or processes with an associated loss of prior or original data. 
     More recently, methods (e.g., storage checkpoints) have become available for providing persistent frozen or “point-in-time” images of data using copy-on-write technology. In a storage checkpoint, only that data which has been modified or written-to in a checkpointed data image following the storage checkpoint&#39;s creation is stored. In conventional data storage and/or processing systems however, such methods have been used primarily to facilitate off-host processing such as data backups and consequently, storage checkpointing or “cloning” has been typically performed on entire filesets as this is the most common level for performing backups, where a “fileset” is included in a conventional file system such as a Unix File System (UFS), NT File System (NTFS), File Allocation Table (FAT)-based file system or the like within a virtual file system (e.g., the Veritas File System provided by Veritas Software Corporation of Mountain View, Calif.) or a conventional, independent file system where no such virtual file system is present. 
       FIG. 1  illustrates a file system including a fileset and an associated storage checkpoint according to the prior art. File system  100  of  FIG. 1  includes a primary fileset  110  and an associated storage checkpoint  120 . Primary fileset  110  in turn includes namespace components (e.g., files, directories, etc.)  112 ,  114 , and  116  and storage checkpoint  120  includes corresponding namespace components  122 ,  124 , and  126 . Although namespace components  112 - 116  are arranged in a hierarchical manner in the file system of  FIG. 1 , where namespace component  112  references namespace components  114  and  116 , any number of namespace components arranged in any number of configurations could just as easily be implemented according to the prior art. In the illustrated prior art file system, namespace component  114  in turn references or is associated with data within data blocks  115 A- 115 D and namespace component  116  references or is associated with data within data blocks  117 A- 117 D. 
     In the illustrated example, storage checkpoint  120  is a so-called “virtual copy”, logically identical to primary fileset  110  when storage checkpoint  120  is created, but lacks any associated allocated data blocks and initially stores no data. Instead, storage checkpoint  120  initially includes only namespace components  122 - 126  within a hierarchical directory structure identical to that of primary fileset  110  and associated references (e.g., pointers) to data blocks (e.g., data blocks  115  and  117 ) associated with the primary fileset  110 . One of skill in the art will recognize that such references may be implemented as one or more arrays of pointers to individual data blocks associated with primary fileset  110  or as one or more single pointers to a list of pointers to such data blocks. Storage checkpoint  120  is typically created within the free space available to primary fileset  110 , and thereby minimizes the use of storage space. The following example describes creation and use of a storage checkpoint such as storage checkpoint  120  in further detail. 
       FIGS. 2A-2B  illustrate a write operation within a file system including a fileset and an associated storage checkpoint according to the prior art. The file system  200  illustrated in  FIG. 2A  includes a primary fileset  210  and a storage checkpoint  220  similar to those described with respect to  FIG. 1 . Primary fileset  210  includes files  212  and  214  which are associated with data blocks  213 A-E and  215 A-E, respectively, utilizing one or more associated metadata file system objects (e.g., file indices, i-nodes, etc.) each including one or more associated data block maps. In the exemplary file system of  FIG. 2A , file  214  contains/references a replicated copy of the same data contained/referenced by file  212  such that two distinct copies of the data are maintained. Blocks such as data blocks  213  and  215  may include file data directly, or may otherwise identify or reference (e.g., using disk block numbers) the actual data blocks containing such data. 
     Storage checkpoint  220  of  FIGS. 2   a  and  2   b  includes sparse files  222  and  224  corresponding to files  212  and  214  of primary fileset  210  each in turn comprising one or more references (e.g., pointers) to data blocks associated with the primary fileset  210  and/or allocated blocks associated with persistent storage. It should be noted that there is a one to one correspondence between files within the primary fileset and sparse files or similar namespace components within storage checkpoint  220 . The blockmaps of sparse files  222  and  224  may initially include references  223 A- 223 E and  225 A- 225 E, as illustrated, respectively corresponding to each block (e.g., data blocks  213  and  215 ) of files  212  and  214 . In  FIG. 2B , the file system of  FIG. 2A  is illustrated following write operations to data blocks  213 B,  213 E, and  215 C of primary fileset  210 . In this example, prior to performing any write operation(s), data block(s) are allocated as needed within sparse files  222  and  224  to store the prior contents of data blocks  213 B,  213 E, and  215 C of files  212  and  214 . 
     After the prior data contents have been stored (e.g., “pushed” or “pulled”) within the newly allocated data blocks  223 B,  223 E, and  225 C of sparse files  222  and  224 , new data may be written to blocks  213 B,  213 E, and  215 C of primary fileset  210  and the write operation may be completed. This process may be continued as additional write operations are performed on files  212  and  214  until all of the original data contents of the files has been transferred such that sparse files  222  and  224  become complete and independent files themselves. 
     SUMMARY 
     Disclosed is a method and system of providing replica files within a fileset. Embodiments of the present invention may be used to reduce the replication of data and storage space requirements by extending existing storage checkpoint or “clone” facilities and associated copy-on-write techniques to individual files within a fileset. 
     According to one embodiment, a method is provided which comprises associating a source file and a replica file of the source file within a fileset, wherein the replica file of the source file comprises a virtual copy of the source file. In another embodiment wherein the source file comprises a metadata file system object and the replica file of the source file comprises a replica metadata file system object, the associating comprises generating a reference within the replica metadata file system object to the metadata file system object. 
     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings in which: 
         FIG. 1  illustrates a file system including a fileset and an associated storage checkpoint according to the prior art; 
         FIGS. 2A-2B  illustrate a write operation within a file system including a fileset and an associated storage checkpoint according to the prior art; 
         FIG. 3  illustrates a fileset including a source file and a replica file according to an embodiment of the present invention; 
         FIGS. 4   a - 4   c  illustrate a fileset including a source file and a number of replica files arranged within a chain structure according to an embodiment of the present invention; 
         FIG. 5   a  illustrates a first portion of a process for forming one or more replica files of a source file using a chain structure according to an embodiment of the present invention; 
         FIG. 5   b  illustrates a second portion of a process for forming one or more replica files of a source file using a chain structure according to an embodiment of the present invention; 
         FIG. 5   c  illustrates a process for maintaining data coherency within a fileset including a source file and a number of replica files formed using a chain structure according to an embodiment of the present invention; 
         FIG. 5   d  illustrates a process for retrieving data stored within a fileset including a source file and a number of replica files formed using a chain structure according to an embodiment of the present invention; 
         FIGS. 6   a - 6   d  illustrate a fileset including a source file and a number of replica files arranged within a tree structure according to an embodiment of the present invention; 
         FIG. 7   a  illustrates a first portion of a process for forming one or more replica files of a source file using a tree structure according to an embodiment of the present invention; 
         FIG. 7   b  illustrates a second portion of a process for forming one or more replica files of a source file using a tree structure according to an embodiment of the present invention; 
         FIG. 7   c  illustrates a process for maintaining data coherency within a fileset including a source file and a number of replica files formed using a tree structure according to an embodiment of the present invention; and 
         FIG. 7   d  illustrates a process for retrieving data stored within a fileset including a source file and a number of replica files formed using a tree structure according to an embodiment of the present invention. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     Although the present invention has been described in connection with one embodiment, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims. 
     In the following detailed description, numerous specific details such as specific method orders, structures, elements, and connections have been set forth. It is to be understood however that these and other specific details need not be utilized to practice embodiments of the present invention. In other circumstances, well-known structures, elements, or connections have been omitted, or have not been described in particular detail in order to avoid unnecessarily obscuring this description. 
     References within the specification to “one embodiment” or “an embodiment” are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. The appearance of the phrase “in one embodiment” in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
       FIG. 3  illustrates a fileset including a source file and a replica file according to an embodiment of the present invention. Fileset  300  of the illustrated embodiment includes namespace components (e.g., files, directories, etc.)  302 - 306 . 
     Although namespace components  302 - 306  are arranged in a hierarchical manner in the fileset of  FIG. 3 , where namespace component  302  (/root) references namespace components  304  (source) and  306  (replica), any number of namespace components arranged in any number of configurations are contemplated by alternative embodiments of the present invention. In the embodiment illustrated by  FIG. 3 , unlike in prior art file systems in which namespace components of a given fileset must each reference independent groups of data blocks, source and replica files such as namespace components  304  and  306  are provided which reference a single group of data blocks  308 A- 308 D using copy-on-write technology. According to one embodiment of the present invention, one or more similar or duplicate file indices (e.g., i-nodes) are created within fileset  300  to provide the described file replica functionality. 
     In the described fileset, no additional independent data blocks must be allocated and associated with namespace component  306  until such time as data is to be modified within the source file or replica file corresponding to namespace components  304  and  306 , respectively. Additionally, the amount of storage (e.g., data storage blocks) which must be allocated is reduced to more accurately reflect the number and location of the modifications which occur. While data block-level granularity has been illustrated with respect to the embodiment of  FIG. 3 , all granularity levels (e.g., extent-level, file-level, etc.) are contemplated in alternative embodiments of the invention and the number of storage units (e.g., blocks, extents, etc.) which are allocated or transferred in response to a request to modify data within namespace components of fileset  300  may be varied according to other embodiments of the present invention. The described file-replica functionality may also be extended in a variety of other ways. For example, a replica of another replica or multiple replicas of a single file may be generated to provide increased operational and/or storage efficiencies. 
     According to still more embodiments of the present invention, the described functionality may be implemented in any number of ways. For example, according to one embodiment, the described functionality is provided through a file system application programming interface (API) which intercepts or overwrites copy file system requests and commands such that a “copy” command used to copy a file within a single fileset results in a source-replica relationship being established. Alternatively, a file system kernel may be modified to detect and re-direct file copy commands in a similar manner. 
     In another embodiment, a daemon system process may be used, as just one example, to determine and maintain file checksums within a centralized or distributed index. The index could then be used by the daemon or another process to scan for identical copies of files within a fileset so that a source-replica relationship could be established there between. According to the described embodiment, when a new file is created, its checksum is determined and compared to that of existing files to identify potential file copies. In another embodiment, the described daemon acts as a background process or utility, checking the index for previously created file copies or files which became similar or identical by operation of writes occurring after their creation. In yet another embodiment of the present invention, a file change log (e.g., a bitmap having a bit location corresponding to each block or region within a file to indicate whether that block or region has been modified since the log was created or reset) is used to quickly determine which files have recently changed and consequently need to be scanned for potential source-replica coalescing. 
     Such checksums may be periodically or constantly determined or updated and then used, for example, when a file is closed to identify such file copies. In on embodiment of the present invention, an extendible hash index is used to store the described checksums. In yet another embodiment, each file or other namespace component within a file set maintains a list of its replicas and to the source from which it depends. 
       FIGS. 4   a - 4   c  illustrate a fileset including a source file and a number of replica files arranged within a chain structure according to an embodiment of the present invention. The fileset  400  of the illustrated embodiment of  FIG. 4   a  includes a source file  402  and a number of replica files  406 ,  410 ,  414  and  418  as shown. While replica files such as replica files  406 ,  410 ,  414  and  418  of  FIG. 4   a  may be described as copies of source file  402 , the particular designation or relationship between replica files (e.g., whether replica file  410  is a copy of source file  402  or of replica file  406 ) should not be construed as limiting to the invention embodiments described herein. 
     Source file  402  of the described embodiment includes a number of associated data blocks  403   a - 403   e  which are referenced by replica files  406 ,  410 ,  414  and  418 . As shown in  FIG. 4   a , replica files  406 ,  410 ,  414  and  418  are arranged in a chain structure and according to one embodiment, each replica file includes a file index (e.g., one or more i-nodes) with a pointer or reference to that replica file&#39;s immediate predecessor in the chain. It is to be appreciated that both the number of replica files and blocks within source file  402  depicted within  FIG. 4   a  are completely arbitrary and could comprise any numbers in alternative embodiments of the invention. Similarly, while blocks such as blocks  403   a - 403   e  have been illustrated herein, other storage units or designators (e.g., extents) may be used interchangeably. 
     In the fileset embodiment illustrated by  FIG. 4   b , a write operation to a portion of replica file  406  has been depicted with the associated changes to the dependency chain structure shown. More specifically, a block  411   c  is allocated within replica file  410  and used to store data pulled from block  403   c  of source file  402 . Consequently, extents  412   a  and  412   b  are created (e.g., replacing a single prior extent linking replica file  410  and replica file  406 ) to link portions of replica file  410  not associated with independent allocated blocks and therefore dependent on preceding replica file  406  with corresponding portions of that replica file. Similarly a block  407   c , used to store data associated with the described write operation, and extents  408   a  and  408   b  are created or allocated within replica file  406 . 
     In the fileset embodiment illustrated by  FIG. 4   c , a write operation to a portion of replica file  414  has been depicted with the associated changes to the dependency chain structure shown. More specifically, a block  419   a  is allocated within replica file  418  and used to store data pulled from block  403   a  of source file  402 . Consequently, extent  420   a  is created to link a portion of replica file  418  which is not associated with independent allocated block  419   a  and therefore dependent on preceding replica file  414  with corresponding portions of that replica file. Similarly a block  415   a , used to store data associated with the described write operation, and extent  416   a  are created or allocated within replica file  414 . 
       FIG. 5   a  illustrates a first portion of a process for forming one or more replica files of a source file using a chain structure according to an embodiment of the present invention. In the illustrated process, a comparison of the data contents of a first file and a second file is first performed (process block  501 ) to determine whether the first and second files are, in fact, copies of one another. A determination is then made whether or not the comparison completed successfully (e.g., if the comparison was actually performed and the first file and second file were determined to be copies of one another) (process block  502 ). The threshold and technique for establishing that the first and second files are in fact copies may be varied using any of a number of known techniques and does not require absolute identity between the two files in all cases. According to an alternative embodiment of the present invention, a threshold of similarity between two files can be established (e.g., statically or dynamically) and used to determine for which groups of files a source-replica relationship is to be established and how such a relationship (e.g., which file is to be designated the source and which the replica) will be set up. According to yet another embodiment, individual files may be subdivided into smaller component parts (e.g., groups of blocks or extents) with source-replica relationships being established for at least a portion of the identified components. If the comparison failed to complete successfully, any resource locks associated with the files are released and a source-replica relation failure is indicated (process block  503 ) as shown. 
     If the comparison was determined to be successful however, another determination is made whether the first and second files are already actually part of the same source-replica chain (process block  504 ). If so, an indication is made that the source-replica relationship was successfully established (process block  514 ). If it is determined that the first and second files are not already actually part of the same source-replica chain, a determination is next made whether the first file is part of a source-replica chain while the second file is not (process block  505 ). If so, data blocks are deallocated from the second file as appropriate (process block  506 ) and the second file is added just after the first file in its source-replica chain (process block  507 ) before success of the operation is indicated (process block  514 ). If it is not determined that the first file is part of a source-replica chain while the second file is not, a determination is made whether the second file is part of a source-replica chain while the first file is not (process block  508 ). If so, data blocks are deallocated from the first file as appropriate (process block  509 ) and the first file is added just after the second file in its source-replica chain (process block  510 ) before success of the operation is indicated (process block  514 ). 
     Subsequently, a determination is made whether the first and second files are each part of separate source-replica chains (process block  511 ). If the first and second files are each part of separate source-replica chains, a sub-process illustrated by  FIG. 5   b  is performed which returns and indicates success as shown, otherwise a prospective successor file is selected from the first file and the second file and data blocks are deallocated from the prospective successor file as appropriate (process block  512 ). According to one embodiment of the present invention, the selection of the prospective successor file is arbitrary or random. Thereafter the first file and the second file are appropriately linked to form a source-replica chain (process block  513 ) as shown (e.g., the prospective successor file is added to the other file just behind it in the source-replica chain). Success is then indicated and the illustrated process terminates as shown. 
       FIG. 5   b  illustrates a second portion of a process for forming one or more replica files of a source file using a chain structure according to an embodiment of the present invention. Entering the depicted process from the process illustrated by  FIG. 5   a , a determination is first made which of the first and second file differs most from the predecessor file in their respective source-replica chain (process block  515 ). If the first file exhibits the greatest difference, the first successor file within the first file&#39;s source-replica chain which is not identical to the first file is identified (process block  516 ). Next, data is copied from allocated blocks of the first file to the identified successor file (process block  517 ). According to one embodiment of the present invention, such data is only copied into previously unallocated blocks of the successor file. In other words, existing data within the successor file is not overwritten. Once this has been accomplished, blocks are deallocated from the first file as appropriate (process block  518 ). 
     Once the appropriate blocks, if any, have been deallocated from the first file, the portion of the first file&#39;s source-replica chain which includes the first file and all identical successor files (process block  519 ) is removed from the first file&#39;s replica chain. This removed portion of the first file&#39;s source-replica chain is then added to the second file&#39;s source-replica chain immediately after the second file (process block  520 ) as shown. In an alternative embodiment, files which are identical to the first file are disregarded and only the first file is moved to the second file&#39;s source-replica chain. While this described embodiment is simpler to implement, the resultant storage space savings are less than obtainable using the technique which is depicted in  FIG. 5   b  and described herein. Thereafter, the process illustrated by  FIG. 5   a  is re-entered as previously described herein. If a determination is made that the second file differs the most from its predecessor file the process elements (process blocks  516 - 520 ) as previously described with respect to the first file are performed substituting the first and second files. 
       FIG. 5   c  illustrates a process for maintaining data coherency within a fileset including a source file and a number of replica files formed using a chain structure according to an embodiment of the present invention. In the illustrated process, a determination is first made whether a requested write operation is an “extending” write (process block  526 ). According to one embodiment, an extending write is a write beginning at the boundary of a file which consequently adds to or “grows” a file. If the requested write is an extending write, no changes are required to the source-replica relationship or any files of the chain. Rather, the requested write operation is simply performed on the target file (process block  531 ). If it is determined that the requested write does more than merely extend the target file, another determination is made whether the target file includes, within the range of the write, references (e.g., overlay extents) corresponding to data blocks within a predecessor file (process block  527 ). If such references are discovered, data from the referenced data blocks is copied to the successor file as appropriate from the nearest predecessor file(s) (process block  528 ) and data blocks corresponding to the references are allocated within the target file (process block  529 ). Thereafter, or if no such references are discovered within the target file, data block data, if any, is copied from the target file to the successor file as appropriate (process block  530 ). The requested write operation may then be performed on the target file (process block  531 ) as shown. 
       FIG. 5   d  illustrates a process for retrieving data stored within a fileset including a source file and a number of replica files formed using a chain structure according to an embodiment of the present invention. In the illustrated embodiment, a determination is first made whether or not the target file includes within the range of a read operation, references to data blocks within a predecessor file (process block  532 ). If so, data block data corresponding to the referenced data is read from the nearest predecessor file(s) (process block  533 ). Thereafter or if a determination is made that no such references exist, data block data is read from the target file as appropriate (process block  534 ) as shown. 
       FIGS. 6   a - 6   d  illustrate a fileset including a source file and a number of replica files arranged within a tree structure according to an embodiment of the present invention. Initially, fileset  400  includes a source file  402  and a number of replica file copies of the source file (e.g., replica files  406 ,  410 ,  414  and  418 ) arranged in a chain structure as depicted previously in  FIG. 4   a . In an alternative embodiment, source file  402  and replica files  406 ,  410 ,  414 , and  414  of fileset  400  are initially arranged in a tree structure in which each replica file depends or “branches” from source file  402 . The particular initial designation or graphical relationship between replica files (e.g., whether replica file  414  is a copy of source file  402  or of replica file  410 ) should not however be construed as limiting to the invention embodiments described herein. 
     Source file  402  of the described embodiment includes a number of associated data blocks  403   a - 403   e  which are referenced directly or indirectly by replica files  406 ,  410 ,  414  and  418 . In the illustrated embodiment, each replica file includes a file index (e.g., one or more i-nodes) with a pointer or reference to the “parent” or “root” file of the tree from which that replica file immediately depends. In the fileset embodiment illustrated by  FIG. 6   a , a write operation to a portion of replica file  406  has been depicted with the associated result to the dependency chain structure shown. 
     More specifically, replica file  406  is removed from the described initial chain of replica files such that replica file  410  depends directly from source file  402  (e.g., using a single overlay extent) and a block  407   c , used to store data associated with the described write operation, is allocated within replica file  406  along with extents  408   a  and  408   b  to link portions of replica file  406  not associated with independent allocated blocks with preceding source file  402 . 
     In the fileset embodiment illustrated by  FIG. 6   b , a write operation to a portion of source file  402  has been depicted with the associated changes to the dependency tree structure shown. More specifically, a block  611   c  is allocated within replica file  410  and used to store original data pushed/pulled from block  403   c  of source file  402 . Consequently, extents  612   a  and  612   b  are created to link portions of replica file  410  not associated with independent allocated block  611   c  and therefore dependent on preceding source file  402  with corresponding portions of that source file. The write operation may then be performed to allocated block  403   c  with no additional changes to the dependency or allocated blocks of replica files  406 ,  414  or  418 . 
     In the fileset embodiment illustrated by  FIG. 6   c , a write operation to a portion of replica file  414  has been depicted with the associated changes to the dependency tree structure shown. More specifically, replica file  414  is removed from the described initial chain of replica files such that replica file  418  depends directly from replica file  410  (e.g., using a single overlay extent) and a block  615   a , used to store data associated with the described write operation, is allocated within replica file  414  along with an extent  616   a  to link the remainder of replica file  414  not associated with independent allocated blocks with preceding replica file  410 . 
     In the fileset embodiment illustrated by  FIG. 6   d , a write operation to a portion of replica file  410  has been depicted with the associated changes to the dependency tree structure shown. More specifically, replica file  410  is removed from the described initial chain of replica files such that the replica file chain is abolished and replica files  414  and  418  depend directly from source file  402  (e.g., using a single overlay extent) as shown. Moreover, data blocks  615   c  and  619   c  are allocated within replica files  414  and  418 , respectively, to store copies of the data previously stored within data block  611   c  as a result of this dependency change causing the creation or modification of extents  616   a ,  616   b ,  620   a , and  620   b  as shown. Following the abolition of the replica file chain, write operations to any new portion of source file  402  will cause the displaced original data of the source file to be copied to each of the replica files following any necessary data block allocations before such write operations can complete. 
       FIG. 7   a  illustrates a first portion of a process for forming one or more replica files of a source file using a tree structure according to an embodiment of the present invention. In the illustrated process, a comparison of the data contents of a first file and a second file is first performed (process block  701 ) to determine whether the first and second files are copies of one another. A determination is then made whether or not the comparison completed successfully (e.g., if the comparison was actually performed and the first file and second file were determined to be copies of one another) (process block  702 ). If the comparison failed to complete successfully, any resource locks associated with the files are released and a source-replica relation failure is indicated (process block  703 ) as shown. 
     If the comparison was determined to be successful however, another determination is made whether the first and second files are already actually part of the same source-replica tree (process block  704 ). If so, an indication is made that the source-replica relationship was successfully established (process block  714 ). If it is determined that the first and second files are not already part of the same source-replica tree, a determination is next made whether the first file is part of a source-replica tree while the second file is not (process block  705 ). If so, data blocks are deallocated from the second file as appropriate (process block  706 ) and the second file is added as a dependent of the first file in its source-replica tree (process block  707 ) before success of the operation is indicated (process block  714 ). If it is not determined that the first file is part of a source-replica tree while the second file is not, a determination is made whether the second file is part of a source-replica tree while the first file is not (process block  708 ). If so, data blocks are deallocated from the first file as appropriate (process block  709 ) and the first file is added as a dependent of the second file in its source-replica tree (process block  710 ) before success of the operation is indicated (process block  714 ). 
     Subsequently, a determination is made whether the first and second files are each part of separate source-replica trees (process block  711 ). If so, a sub-process illustrated by  FIG. 7   b  is performed which indicates success upon return as shown, otherwise a prospective branch file is selected from the first file and the second file and data blocks are deallocated from the prospective branch file as appropriate (process block  712 ). According to one embodiment of the present invention, the selection of the prospective successor file is arbitrary or random. Thereafter the first file and the second file are appropriately linked to form a source-replica tree (process block  713 ) as shown (e.g., the prospective branch or “child” file is made dependent of the other file in its source-replica tree). Success is then indicated and the illustrated process terminates as shown. 
       FIG. 7   b  illustrates a second portion of a process for forming one or more replica files of a source file using a tree structure according to an embodiment of the present invention. Entering the depicted process from the process illustrated by  FIG. 7   a , a determination is first made which of the first and second file differs most from the file from which it depends within their respective source-replica tree (process block  715 ). If the first file exhibits the greatest difference, the subtree having the first file as its root is identified and removed (process block  716 ). Next, the removed subtree is added at the second file within the second file&#39;s source-replica tree (process block  717 ). Once this has been accomplished, blocks are deallocated from the first file as appropriate (process block  718 ). Thereafter, the process illustrated by  FIG. 7   a  is re-entered as previously described herein. If a determination is made that the second file differs the most from the file from which it depends within its respective source-replica tree the process elements (process blocks  716 - 718 ) as previously described with respect to the first file are performed substituting the first and second files (process blocks  719 - 721 ). 
       FIG. 7   c  illustrates a process for maintaining data coherency within a fileset including a source file and a number of replica files formed using a tree structure according to an embodiment of the present invention. In the illustrated process, a determination is first made whether or not the target file of the write operation has at least one dependent file within its source-replica tree (process block  722 ). If no dependent files are discovered within the target file&#39;s source-replica tree, data blocks are allocated within the target file as appropriate (process block  725 ) and the writer operation is then performed on the target file (process block  726 ). If at least one dependent file is located however, data is copied from the target file to its dependents (e.g., to the first or “root” dependent file in each subtree branching off of the target file) as appropriate (process block  723 ). Thereafter, the dependency of the files is changes such that any files depending from the target file (i.e., children of the target file) are made to depend from the file from which the target file depends (i.e., the parent of the target file) (process block  724 ) before data blocks are allocated within the target file as previously discussed (process block  725 ). 
       FIG. 7   d  illustrates a process for retrieving data stored within a fileset including a source file and a number of replica files formed using a tree structure according to an embodiment of the present invention. In the illustrated embodiment, a determination is first made whether or not the target file includes within the range of a read operation, references to data blocks within an ancestor (e.g., parent, grandparent, etc.) file (process block  727 . If so, data block data corresponding to the referenced data is read from the nearest ancestor file(s) (process block  728 ). Thereafter, or if a determination is made that no such references exist, data block data is read from the target file as appropriate (process block  729 ) as shown. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims. 
     The present invention is well adapted to attain the advantages mentioned as well as others inherent therein. While the present invention has been depicted, described, and is defined by reference to particular embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described embodiments are examples only, and are not exhaustive of the scope of the invention. 
     The foregoing detailed description has set forth various embodiments of the present invention via the use of block diagrams, flowcharts, and examples. It will be understood by those within the art that each block diagram component, flowchart step, operation and/or component illustrated by the use of examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or any combination thereof. 
     The present invention has been described in the context of fully functional data processing system or computer systems; however, those skilled in the art will appreciate that the present invention is capable of being distributed as a program product in a variety of forms, and that the present invention applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of such signal bearing media include recordable media such as floppy disks and CD-ROM, transmission type media such as digital and analog communications links, as well as media storage and distribution systems developed in the future. 
     The above-discussed embodiments may be implemented using software modules which perform certain tasks. The software modules discussed herein may include script, batch, or other executable files. The software modules may be stored on a machine-readable or computer-readable storage medium such as a disk drive. Storage devices used for storing software modules in accordance with an embodiment of the invention may be magnetic floppy disks, hard disks, or optical discs such as CD-ROMs or CD-Rs, for example. A storage device used for storing firmware or hardware modules in accordance with an embodiment of the invention may also include a semiconductor-based memory, which may be permanently, removably or remotely coupled to a microprocessor/memory system. Thus, the modules may be stored within a computer system memory to configure the computer system to perform the functions of the module. Other new and various types of computer-readable storage media may be used to store the modules discussed herein. 
     The above description is intended to be illustrative of the invention and should not be taken to be limiting. Other embodiments within the scope of the present invention are possible. Those skilled in the art will readily implement the steps necessary to provide the structures and the methods disclosed herein, and will understand that the process parameters and sequence of steps are given by way of example only and can be varied to achieve the desired structure as well as modifications that are within the scope of the invention. Variations and modifications of the embodiments disclosed herein can be made based on the description set forth herein, without departing from the scope of the invention. 
     Consequently, the invention is intended to be limited only by the scope of the appended claims, giving full cognizance to equivalents in all respects.