Abstract:
A method and system of the present invention generates a representation of a new version of an original file system with reference to the original file system and the new version of the file system. Use of data from previous versions of the file system reduces the amount of data to be stored in the delta directory map file, delta modification data block file, and delta look up table generated for the data portions unique to the newest version of an original file system. The inventive process produces delta data block records that identify the location of data portions that may be used to generate the newest version of the file system. The data portions may be located in a file in the original file system, a delta modification data block file in a previous version of the original file system or a delta modification data block file for the newest version of the original file system. 
     The method of the present invention is performed by generating a basis index table identifying the data content of an original file system, generating a file of modification data blocks that may be used to modify the data content of the original file system, and generating a delta look up table that identifies the location of the data blocks used to represent the newest version of the original file system. The delta look up table and the file of modification data blocks may be stored for delivery to a computer on which a copy of the original file system is stored. The delta look up table and the file of modification data blocks are then used by the computer system on which a copy of the original file system is stored to provide the data content for a new version of the original file system. This is done in way that appears to provide a single file system containing the new version of the file system. Thus, the method of the present invention may be used to generate data for updating the content of a copy of the original file system without having to generate a copy of every file and data block for the new content of the original file system.

Description:
CROSS REFERENCE RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional application Ser. No. 60/091,291; which was filed on Jun. 30, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to file updating methods, and more particularly, to file updating methods for files stored on read-only media. 
     BACKGROUND OF THE INVENTION 
     Software programs and data are frequently distributed on large capacity storage media such as compact disc-read only memories (CD-ROM). These devices are preferably read-only devices to preserve the integrity of the data and program files stored on the device. Such storage devices contain multiple data and executable program files for an application program and typically include a program to install the program and data files on a user&#39;s computer. A common application for distributing a computer program and data files is to provide an interface program and data files for employees that use computers remotely from a central site. For example, a company may equip its sales force with CD-ROMs that contain an interface program that retrieves information from the data files. The retrieved data is used to respond to a customer&#39;s questions on product availability or product specifications. 
     Having timely information available to an organization&#39;s remote personnel or being able to provide program patches and other improvements to update software distributed to end users is important for customer service and product support. When the data or programs stored on read-only media are frequently updated or modified, the cost of providing the program or data updates on new storage media such as another CD-ROM can be prohibitively expensive. 
     Accordingly, what is needed is a way to update information stored on read-only media without having to produce new read-only media containing the program or data updates for distribution to a company&#39;s customers or remote personnel. 
     SUMMARY OF THE INVENTION 
     The above limitations of previously known program and data storage read-only storage devices are overcome by a system and method made in accordance with the principles of the present invention. The method of the present invention includes the steps of generating a basis index table identifying the data content of an original file system, generating a file of modification data blocks that may be used to modify the data content of the original file system, and generating a delta look up table for identifying the data blocks in the original file system and the data blocks in the file of modification data blocks that provide the data content for a new version of the original file system. The delta look up table and the file of modification data blocks may be stored for delivery to a computer on which a copy of the original file system is stored. The delta look up table and the file of modification data blocks are then used by the computer system on which a copy of the original file system is stored to provide the data content for a new version of the original file system in way that appears to provide a single file system containing the new version of the file system. Thus, the method of the present invention may be used to generate data for updating the content of a copy of the original file system without having to generate a copy of every file and data block for the new content of the original file system. 
     Preferably, the method generates the basis index table by building a basis directory entry meta-data table and a basis index data block table. The basis directory entry meta-data table organizes the meta-data for each entry in a directory enumeration of the original file system by entry name. Preferably, the entry name identifies the entry and its parent. The meta-data stored for each entry is known meta-data such as file attributes. The basis index data block table uniquely identifies each data block found within the original file system. For each unique data block identifier, a source file identifier that identifies the source file for the data block, the offset to the first data unit for the block within the source file, and the length of the data block are stored. These two tables may then be used to generate the files for generating a new version of the file system. 
     The method of the present invention also includes the steps of generating a delta directory map file to identify the structure of the entries in the new version of the original file system, a delta look up table (LUT) file for identifying the location of the data blocks to generate the files in the new version of the original file system, and a delta modification data block file that contains the new data content for the new version of the original file system. The delta directory map file contains the name for the entries in the new version of the original file system, the modification status for the entries in the new version of the file system, the meta-data for each entry having a modification status of “modified,” “contents modified” or “new,” the first look up table record for each file entry, and the number of look up table records used to construct the file in the new version of the original file system. The delta look up table contains at least one LUT record for each file entry having a modification status of “contents modified” or “new.” An LUT record identifies the source file containing the data block, the location of the first data unit of the data block in the identified source file, the length of the data block, and the offset of the first data unit of the data block in the file being processed. The source file identifier either identifies a file of the original file system or the modification data block file for the new version. The LUT records for all of the files in the new version of the original file system are stored in an LUT file. The location of the first LUT record for a file is identified by a pointer stored in the meta-data of the delta directory map file for file entries having a modification status of “contents modified” or “new.” The directory map file may also be used in the computer having a copy of the original file system to generate information for display or use regarding the structure of the new version of the original file system and its new data content. Structure data includes data that is displayed in response to a directory enumeration command or the like. The delta modification data block file contains the data blocks having new data content for the file entries of the new version of the original file system. As the data blocks for the new data content of the new version of the original file system are stored in the delta modification data block file, a delta index data block table is generated. This table includes a unique identifier for each data block stored in the delta data block modification file that has unique data content, an identifier that indicates the version of the delta data block modification file that is the source file for the block, the offset to the first data unit for the data block and the length of the data block. The delta index data block table is appended to the basis index data block table. 
     The delta directory map file, the delta modification data block file and the delta look up table may be compressed and stored on storage media or downloaded to a computer having a copy of the original file system. The downloaded delta directory map file, the delta modification data block file and the delta look up table file are used to seamlessly regenerate a new version of the original file system. This regeneration of the new version of the original file system is done in a manner which gives the appearance that the contents of the device on which a copy of the original file system is stored have been modified, even if the device uses read-only media for storage of the original file system. Thus, the system and method of the present invention provide a mechanism for updating the contents of a file system without requiring the production of a complete file system corresponding to the new version of the program and/or data stored in the file system. 
     Preferably, the method and system of the present invention may be used to generate a representation of a new version of an original file system with reference to the original file system and to the delta modification data block files for previous versions of the original file system. This use of previous versions reduces the amount of data to be stored in the delta directory map file, delta modification data block file, and delta look up table for the latest version of an original file system. In this embodiment of the present invention, the process for generating the files for the new version of the original file system produces delta data block records that identify the source file for a data block as being either a file in the original file system, a delta modification data block file for a previous version of the original file system or the delta modification data block file for the new version of the original file system. The version of the delta modification data block table containing a data block is determined from the delta index data block tables appended to the basis index data block table. 
     These and other benefits and advantages of the present invention shall become apparent from the detailed description of the invention presented below in conjunction with the figures accompanying the description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a depiction of a screen shot of a file system hierarchy that may be evaluated by the system of the present invention; 
     FIG. 2 is a flowchart of an exemplary process that generates a representation of the original file system; 
     FIGS. 3A and 3B are a flowchart of an exemplary process that generates the look-up table file and modification data block file for an update to the representation of the original file system generated by the process shown in FIG. 2; 
     FIG. 4 is a flowchart of an exemplary process that generates a delta directory map file for the new version of the original file system from the delta directory entry meta-data table generated by the process shown in FIGS. 3A and 3B; and 
     FIG. 5 is a flowchart of an exemplary process that uses the files for an update generated by the process shown in FIGS. 3A,  3 B and  4  to generate a latest version of the original file system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a screen shot of a file system hierarchy. The hierarchy for the file system is comprised of a directory having a list of file entries and subdirectory entries. The subdirectory entries may include additional files for the file system. Each entry in the directory for the file system hierarchy also contains meta-data. For the file entries the meta-data includes known file meta-data such as the file name, file attributes, and other known file meta-data. 
     In order to generate modification data files for a file system hierarchy, the original version of the file system hierarchy is processed and information about the system is stored in a file system map file. This process is depicted in FIG.  2 . The process begins by processing the directory file for the highest level of the file system hierarchy to identify the entries for the subdirectories and files at the highest level in the file system (Block  50 ). For each entry, meta-data for the entry is stored in a basis directory meta-data table (Block  54 ). If the entry is a subdirectory (Block  56 ), the process determines whether another directory entry exists for processing (Block  90 ). If there is, it is processed (Block  54 ). Otherwise, the process terminates as the basis directory entry meta-data table and basis index data block table have been generated. 
     For a file entry being processed, the file is segmented into blocks of one or more fixed lengths (Block  60 ). For each block, an iterative checksum (Block  62 ) and then a safe checksum (Block  64 ) is generated. The iterative checksum is a value that is computed from the data values for each byte within a block beginning at the first byte of the block to the last byte in the block. It possesses the property that an iterative checksum for a data block comprised of the first N data units in a data string may be used to generate the iterative checksum for the next data block comprised of the N data units beginning at the second data byte. This is done by performing the inverse iterative checksum operation on the iterative checksum using the data content of the first data unit of the first block to remove its contribution to the iterative checksum and then performing the iterative checksum operation on the resulting value using the N+1 data unit that forms the last data unit for the next data block. Thus, two data operations may be used to generate the iterative checksum for the next block in a data string in which the successive data blocks are formed by using a sliding data window in the data string. For example, an addition operation may be used to generate an iterative checksum having the property noted above. A safe checksum is generated by a process that is less likely to produce the same checksum for two blocks having different data contents than the storage media is likely to return an inaccurate data value. A safe checksum generation method well known within the data communication art is the MD 5  checksum. The iterative and safe checksum pair for a data block form a checksum identifier that is used to identify the data block. The iterative checksum is not as computationally complex as the safe checksum so the iterative checksum is a relatively computational resource efficient method for a determining that two data blocks may be the same. The safe checksum may then be used to verify that the data content of the blocks are the same and reduce the likelihood of a false positive identification. If the checksum identifier is the same as the checksum identifier for a data block previously stored in the index data block table (Block  68 ) then the data content of the data block is not unique. Thus, the data block record in the index data block table for the corresponding checksum identifier adequately defines the data block being processed so the checksum identifier is not stored in the index data block table and the process determines whether another data block is to be processed (Block  82 ). 
     If the checksum identifier indicates the data block content is unique, the iterative checksum is stored as the primary key in the index data block table and the safe checksum is stored in the index data block table as a qualified key (Block  70 ). Associated with the checksum identifier for the block is an identifier for the file from which the data block came (Block  74 ), the offset from the first byte within the file to the first byte in the data block (Block  76 ), and the length of the data block (Block  78 ). The source file identifier may be the name of the file in which the data block is stored, but preferably, it is a pointer to the meta-data in the basis directory entry meta-data table for the source file. This process of identifying and storing information about each data block in the index data block table continues (Block  82 ) until all of the blocks for a file entry have been processed. A safe checksum for the entire data content of the file is then generated and stored in the basis directory entry meta-data table (Block  84 ). The process continues (Block  90 ) until all entries for the entire directory structure for the original file system have been processed. The basis directory entry meta-data table and basis index data block table file system map file representing the meta-data and data content for each entry within the file system hierarchy is then stored on storage media (Block  96 ). This data forms the baseline for generating modification data files for updating the original file system. 
     Whenever a new version of a file system hierarchy is generated, either by changing, deleting or adding data to a file or its meta-data or by adding or deleting data files to the file system, a delta modification data block file and delta look up table may be generated to provide the update information for the differences between the original file system hierarchy and the new version of the file system hierarchy. The process for generating the delta modification data block file and the delta look up table is shown in FIG.  3 . That process begins by reading the directory file for the new file system hierarchy and identifying the entries for the subdirectories and files in the file system hierarchy (Block  100 ). Each entry is then processed by storing the meta-data for the entry in a delta directory entry meta-data table (Block  104 ). The status of the entry is then determined by searching the basis directory entry meta-data table for an entry having the same name under the same parent (Block  108 ). If no corresponding entry is located in the basis directory entry meta-data table (Block  110 ), then the modification status for the entry in the new file system hierarchy is set to “new” (Block  112 ). If a corresponding entry is located in the basis directory entry meta-data table then the meta-data for the corresponding entry is compared to the meta-data for the entry in the delta directory entry meta-data table (Block  114 ) and, if the meta-data is the same for both entries, the modification status is set to “unmodified” (Block  116 ). If the meta-data for the entries do not correspond and the entries are not files (Block  120 ), the modification status is set to “modified” (Block  122 ). If the meta-data for the entries do not correspond and the entries are files, a safe checksum is generated for the data contents of the file entry in the new file system (Block  126 ). This safe checksum is compared to the safe checksum for the entire data content of the file stored in the basis directory entry meta-data table (Block  128 ) and if they are not equal, the modification status is set to “contents modified” (Block  130 ). Otherwise, the modification status is set to “modified” (Block  134 ). The modification status is stored in the delta directory entry meta-data table. This process continues until all of the entries in the new version of the original file system have been processed (Block  136 ). 
     The basis directory entry meta-data table is now searched to determine whether a corresponding entry exists in the delta directory entry meta-data table. Specifically, a directory entry in the basis directory entry meta-data table is selected (Block  140 ) and the delta directory entry meta-data table is searched for a corresponding entry (Block  142 ). If no corresponding entry is located, an identifier for the entry and a modification status of “deleted” is generated and stored in the delta directory entry meta-data table (Block  144 ). The process continues until all entries in the basis index directory entry meta-data table have been checked (Block  146 ). 
     The process now selects an entry in the delta directory entry meta-data table (Block  150 ) and determines whether it has a modification status of “new” or “contents modified” (Block  152 ). For these entries, look up table (LUT) records are generated and data blocks stored in the delta modification data block file, if necessary. If an entry is identified as being a “new” or “contents modified” entry, a sliding window of N data units, such as 256 bytes, is used to define data blocks (Block  156 ). As noted before, the number N must be one of the block sizes used to segment files in the original file system for constructing the basis index data block table. An iterative checksum is computed for the first data block formed by the sliding window being placed at the first data unit of the data contents of the “new” or “contents modified” file (Block  158 ). This iterative checksum is compared to the iterative checksums of the checksum identifiers stored in the basis index data block table to determine whether a corresponding entry may exist (Block  160 ). If no corresponding iterative checksum is found, the checksum identifier for the data block being processed cannot be the same as one in the index basis data block table so the first data unit of the data block in the sliding window is stored in a delta modification data block file (Block  162 ). The sliding window is then moved to remove the first data unit from the data block in the file being processed and to add the next data unit (Block  156 ). The iterative checksum for the data block in the sliding window is computed (Block  158 ) and compared to the iterative checksums of the checksum identifiers in the basis index data block table(Block  160 ). Because the iterative checksum has the property discussed above, the iterative checksum for each successive data block only requires calculations to remove the contribution of the data units removed from the block by moving the sliding window and to add the contributions of the data units added by moving the sliding window. Moving the sliding window, generating the next iterative checksum and comparing the generated iterative checksum to those for the checksum identifiers in the basis index data block table continues until a corresponding iterative checksum for one of the checksum identifiers is located or the number of data units stored to the delta modification data block file corresponds to the number of data units for a data block (Block  172 ). When a data block of modification data has been stored to the delta modification data block file, the iterative and safe checksums for the block are generated to form a checksum identifier for the block (Block  174 ). The iterative checksum and safe checksum for the block of modification data are then stored as the primary key and qualified key, respectively, in a delta index data block table associated with the new version of the original file system. An identifier of the delta modification data block file in which the data block is stored, the offset into that file that defines the location of the first data unit for the data block being processed, and the length of the data block being processed are also stored in the delta index data block table in association with the iterative and safe checksums (Block  176 ). 
     Once an iterative checksum for a data block within the sliding window corresponds to one or more iterative checksums in the checksum identifiers stored in the basis index data block table, the process computes the safe checksum for the block within the sliding window and compares it to the safe checksums of the checksum identifiers selected from the basis index data block table (Block  178 ). Only one, if any, safe checksum of the checksum identifiers should be the same as the safe checksum computed for the data block. If a corresponding safe checksum is identified, the data blocks are the same. The process determines whether the previous data block checksum identifier comparison indicated a corresponding checksum identifier in the basis index data block table was located (Block  180 ). If the previous checksum identifier comparison did not find a corresponding checksum identifier, a look up table (LUT) record is generated for the data units stored in the delta modification data block file since the last corresponding checksum identifier was detected (Block  182 ). That is, all of the data following the identification of the last data block that is also in the basis index data block table is stored in the delta data modification file and the LUT record for that data indicates that the data is a contiguous block of data. The LUT record is comprised of a delta modification data block file identifier, the offset from the first data unit in the modification data file to the contiguous data block stored in the modification data file, the number of data units in the contiguous data block stored in the modification data file, and the offset of the data block in the file currently being processed. The first three data elements in the LUT to identify the source file for the data block in the new version of the original file system and its location in that file while the fourth data element defines the location of the data block in the file of the new version of the original file system. As discussed below, this permits the application program that controls access to the new version of the original file system to not only know from where it can retrieve the data block but where it goes in the new version of the file. 
     At this point in the process, the checksum identifier for the data block within the sliding window has been identified as being the same as a checksum identifier in the basis index data block table. As this block already exists in a file in the original version of the file system, a different LUT record is generated for the data block within the sliding window (Block  198 ). The LUT record for the data block that corresponds to the checksum identifier stored in the basis index data block table is comprised of the same source file identifier as the one in the basis index data block table, the same offset from the start of the source file, the same data block length stored in the basis index data block table, and the offset of the data block in the file currently being processed. The process then continues by determining whether the previous LUT record for the file being processed has a source file identifier that is the same as the one for the LUT record generated for the data block within the sliding window (Block  200 ). If it does and the LUT just generated is for a data block that is contiguous with the data block identified by the previous LUT record, the process increases the length stored in the previous LUT record by the length of the data block in the LUT record generated for the data block just processed and discards the new LUT record (Block  202 ). This corresponds to the situation where contiguous blocks of the data in a file of the new version of the original file system are the same as a group of contiguous blocks in a file of the original file system. Thus, one LUT record can identify a source for the contiguous group of blocks. If the data block for the new LUT record is not contiguous with the data block of the previous LUT record or is not from the same source file, then the LUT record is appended to the previous LUT record (Block  206 ). If the safe checksum does not correspond to the safe checksum for a data block having the corresponding iterative checksum, the process determines whether a data block of modification data has been defined (Block  172 ). The process continues until it determines whether all data units in the file have been processed (Block  210 ). If more data units exist, the sliding window is moved by its length to capture a new data block (Block  212 ). If the number of remaining data units do not fill the sliding window (Block  214 ), the remaining data units are stored in the delta modification data block file (Block  218 ) and a corresponding LUT record is generated (Block  220 ). The LUT records generated for the file being processed are then appended to the LUT records for other files previously stored in an LUT file for the new version of the original file system (Block  222 ) and the LUT records for the file are stored in the LUT file (Block  224 ). The offset for the first LUT for the file being processed and the number of LUT records for this file are then stored in the meta-data of the delta directory entry meta-data table for the file being processed (Block  228 ). The process then checks for more entries in the delta directory entry meta-data table to process (Block  230 ). If there are more entries the process continues (Block  150 ). If all of the delta directory entries have been processed, the delta directory entry meta-data table for the entries in the new version of the original file system is then searched for any entries having a modification status of “unmodified.” These entries and their meta-data are removed unless they have a descendant having a modification status other than “unmodified” (Block  238 ). 
     In an embodiment of the present invention that utilizes previous updates provided for the original file system, the above process is modified to evaluate the delta index data block tables for previous versions of the original file system. Specifically, the process searches the basis index data block tables and the delta index data block tables files for update versions to locate data blocks having corresponding iterative and safe checksums for corresponding “new” or “contents modified” files in the latest version. Additionally, the source of data blocks may also include delta modification data files for previous update versions of the original file system as well as the files of the original file system and the delta modification data block file for the latest version. 
     The delta directory entry meta-data table for the new version of the original file system generated by the process in FIG. 3 is then used by the process shown in FIG. 4 to generate a delta directory map file. An entry is selected from the delta directory entry meta-data table (Block  250 ) and an entry in the delta directory map file system is generated. The entry at least includes the name of the entry (Block  254 ) and its modification status (Block  256 ). If the modification status is “new”, “modified” or “contents modified” (Block  260 ), the new meta-data is also stored in the delta directory map file for the entry (Block  264 ). If the modification status is “new” or “contents modified,” (Block  266 ), the offset to the first LUT record for the file in the LUT file and the number of LUT records for the file in the LUT file are stored in the delta directory map file (Block  268 ). The process continues until all entries in the delta directory entry meta-data table have been processed (Block  270 ). The name of the new file system hierarchy, its version identifier, directory map file, LUT file, and modification data files may now be compressed for delivery to a system having a copy of the original file system. 
     Once the compressed representation of the new version of the original file system is transferred to a computer on which a copy of the original file system hierarchy is stored, it may be used to update the original file system. An application program may be provided as part of that representation to perform the process depicted in FIG.  5 . Alternatively, the application program may be part of the interface program provided for accessing the content of the original file system hierarchy such as an extension to the file system program of the recipient computer. The program decompresses the representation of the new file system hierarchy and stores the delta directory map file, LUT file, and delta modification data block file in storage accessible to the computer. The process then determines whether a reference to the directory containing a delta modification data block file for a previous version of the original file system hierarchy is associated with a reference to the directory or drive containing the original file system hierarchy (Block  300 ). If there is an association between a directory or drive containing the original file system hierarchy and a directory containing a delta modification data block file, that association is merged with a reference to the directory or drive where the decompressed files for the new file system hierarchy are stored (Block  302 ). The merge replaces the existing associated delta directory map file and LUT file with the new delta directory map file and LUT file, but leaves any existing delta modification data block files referenced in the new LUT file. Otherwise, an association is generated between the drive or directory where the original file system hierarchy is stored and the directory where the downloaded decompressed files for the new version of the original file system hierarchy is stored (Block  308 ). The application program may be coupled to the operating system of the computer in which a copy of the original file system hierarchy and the decompressed files for the new version of the file system hierarchy are stored. In a known manner, the operating system is modified to detect any attempted access to the drive or directory containing the original file system hierarchy or the files for the new version of the file system hierarchy. In response to an attempted operation to change the physical media for the original file system hierarchy (Block  310 ), the application program stores a media change indicator (Block  314 ) and verifies the identity of the physical media when a subsequent attempt is made to access the original file system hierarchy (Block  318 ). If the physical media has changed, the application change program checks the media change indicator and determines whether the original file system media is available. If it is not, the program indicates that the original file system hierarchy is not available for access by the user. Otherwise, the access is processed. Attempts to write data to the drive or directory containing the original file system hierarchy or the files for the new version of the original file system detected by the application program (Block  320 ) are not processed (Block  324 ). 
     For commands attempting to interrogate the structure of the original file system hierarchy, the application program responds by building data in two passes and presenting that data to the user. A command to interrogate the structure of the original file system hierarchy is one such as a directory enumeration command. In response to a structure inquiry (Block  328 ), the application program first retrieves the requested structure data from the original file system and deletes the entries for which the modification status in the delta directory map file is “deleted,” “modified”, “new” or “contents modified.” The data for these entries is obtained from the delta directory map file and used to modify the structure data responsive to the structure query (Block  330 ). That is, the application program obtains the data to be displayed for the original file system hierarchy, deletes those files corresponding to delta directory map file entries having a modification status of “deleted”, adding structure data for those entries in the directory map file having a status of “new”, and modifying the structure data for those entries in the directory map file having a status of “modified” or “contents modified.” This data is then provided to the operating system for display to the user. 
     For file system operations that open a file in the new version of the original file system hierarchy (Block  340 ), the application program determines whether the modification status of the file is “unmodified.” If it is, the operation is processed using the contents of the original file system only. Otherwise, the application program constructs and returns an open file handle that identifies the file (Block  344 ). The open file handle identifies the file for subsequent file operation commands but does not open any underlying file. For any file system operation command that interrogates the properties of a file for which an open file handle exists, the application program returns data from the delta directory map file entries that correspond to the file identified by the open file handle. 
     In response to an I/O operation command that reads data from a file identified by an open file handle (Block  350 ), the application program constructs a response to the query by identifying the LUT record in the LUT file that corresponds to the start of the requested data (Block  352 ). If the underlying file referenced in the LUT record is not opened, the application program opens the underlying file and associates it with the open file handle. The program then reads from the LUT record whether the data for the requested data block is to be read from the original file system hierarchy or one of the delta modification data block files. After the source file is identified, the offset data and data block length are used to locate the first byte to be transferred from the identified source file and the number of bytes to be transferred, respectively. The corresponding number of bytes are transferred from the source file to a response being built (Block  356 ). If additional data is required for the response (Block  360 ), the next LUT record is used to extract data for the response (Block  364 ). This process continues until the data transferred for an LUT record provides all of the data requested or until the last entry for the file is reached. The response built from the transfer of data from the source files identified by the LUT records is then provided to the operating system for delivery to the requesting program (Block  368 ). In this manner, a response is provided to a file system operation that appears to be the result of a single contiguous read operation. In response to a file system operation that closes a data file (Block  370 ), the application program closes all corresponding files in the original file system hierarchy and the data files for the new file system hierarchy (Block  372 ). 
     The method and system presented above are only exemplary of the many ways in which the invention may be embodied. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or from practice of the invention disclosed herein. It is intended that the scope and spirit of the present invention be indicated by the following claims.