Patent Publication Number: US-2009228496-A1

Title: Method for accessing distributed file system

Description:
CROSS-REFERENCES 
     This is a continuation application of U.S. Ser. No. 12/073,286, filed Mar. 4, 2008, which is a continuation application of U.S. Ser. No. 11/488,685, filed Jul. 19, 2006 (now U.S. Pat. No. 7,363,352), which is a continuation application of U.S. Ser. No. 10/645,813, filed Aug. 20, 2003 (now U.S. Pat. No. 7,143,146). The entire disclosures of all of the above-identified applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a distributed file system (DFS), a distributed file system server, and a method for accessing the distributed file system. More specifically, the invention relates to the distributed file system that distributes a file to a plurality of servers on a network, for storage, thereby constituting a single file system, a server in the distributed file system, and the method for accessing the distributed file system. 
     As file systems using the network, the file systems such as a file system referred to as a common Internet file system (CIFS) and a network file system (NFS) constructed on a UNIX® operating system (OS) are known. These file systems are centralized file systems, in which a single file system is constituted by a single server. In these systems, file entities reside on a specific server. When accessing a file in each of these file systems, the client first accesses a server on which the targeted file resides, employing a protocol for the file system. At this point, in order to specify the file on the server, the client uses a directory structure. 
     In contrast therewith, in the DFS in an “OceanStore”, which is a utility infrastructure designed to span the globe and provide continuous access to persistent information, when accessing a file, the client employs an identifier for the file uniquely assigned by a global unique identifier (GUID) system, instead of specifying the server and a pathname for the file. In the DFS, the entity of the file resides on a plurality of DFS servers on the network. The entity of the file does not need to be held by a single DFS server, and may reside on other server as a copy. Alternatively, the file may be divided into some portions; one of the divided portions of the file, referred to as a fragment, may reside on a single DFS server, and remaining fragments may reside on other DFS server or servers. 
     When accessing a file in the DFS to refer to or perform writing to the file, the client specifies the GUID for the file for identification of the file, thereby accessing one of the servers on the network. 
     As a prior art associated with the present invention, a system equipped with a communication interface for connection to all kinds of user data from a storage server is disclosed in U.S. Pat. No. 6,446,141, for example. 
     The DFS has characteristics different from those of the network file systems such as the NFS and the CIF. Thus, in order to access a file in the DFS, or a DFS file, a protocol dedicated to the DFS must be employed. For this reason, a conventional client that uses only the protocol for the NFS or the CIFS cannot access the DFS file. Thus, it was necessary to make a modification on a client side, such as an improvement in a program used so far, to accommodate the DFS. In other words, in order to function as a DFS client in the DFS system, there was a need for the client in the centralized file system, which uses the conventional protocol, to modify the program. 
     SUMMARY OF THE INVENTION 
     In view of the problem described above, the present invention has been made. It is therefore an object of the present invention to provide a distributed file system that allows access to a DFS file using a conventional protocol without making a modification on a side of a client that uses the conventional protocol, a server in the distributed file system, and a method for accessing the distributed file system. 
     According to the present invention, the above mentioned object is achieved by a distributed file system that can be accessed using a protocol for accessing a centralized file system, including: 
     a plurality of DFS servers on a network for distributing a file, for storage; and 
     a gateway unit in at least one of the DFS servers, for converting a protocol for accessing a centralized file system into a protocol capable of accessing the distributed file to access the distributed file. In other words, the above-mentioned object is achieved by that at least one of the DFS servers in the distributed file system has a function of a server in the centralized file system. 
     The above-mentioned object is also achieved by providing the gateway unit for the distributed file system. In other words, the above-mentioned object is achieved by that at least one DFS client in the distributed file system has a function of a server in the centralized file system. 
     Further, the above-mentioned object is achieved by a method for accessing a distributed file system using a protocol for accessing a centralized file system comprising the step of: 
     converting the protocol for accessing the centralized file system into a protocol capable of accessing the distributed file system to access a distributed file. 
     Thus, according to the present invention, access to a file on the DFS can be made using a conventional protocol such as the one for the NFS or CIFS, without making a modification on the client&#39;s side. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a distributed file system according to an embodiment of the present invention; 
         FIG. 2  is a diagram explaining an example of a method for determining a GUID from a file group identifier and a generation number; 
         FIG. 3  is a diagram explaining a directory structure used by a conventional protocol client; 
         FIG. 4  is a block diagram showing a configuration of a DFS server; 
         FIG. 5  is a block diagram showing a configuration of a gateway unit; 
         FIG. 6  is a diagram explaining a structure of directory management information  50 ; 
         FIG. 7  is a flowchart that explains processing operations when a DFS file is accessed for reference using a conventional protocol; 
         FIG. 8  is a flowchart that explains processing operations when a DFS file is accessed for updating using the conventional protocol; 
         FIG. 9  is a diagram explaining a structure of a file monitoring table; 
         FIG. 10  is a flowchart that explains processing operations of a gateway unit  25  at the time of completion of creation of a new generation file; and 
         FIG. 11  is a block diagram showing a configuration of a distributed file system according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of a distributed file system (DFS) and a DFS server according to the present invention will be described in detail below with reference to the appended drawings. 
       FIG. 1  is a block diagram showing a configuration of a distributed file system according to an embodiment of the present invention. Referring to  FIG. 1 , reference numeral  1  denotes a network, numeral  2  denotes DFS servers, numeral  3  denotes DFS clients, numeral  4  denotes NFS clients, and numeral  5  denotes a CIFS client. 
     The distributed file system (DFS) according to the embodiment of the present invention, shown in  FIG. 1  is constituted from the network  1 , DFS servers  2  connected to the network  1 , DFS clients  3  for accessing the file system on the network  1  using a DFS protocol, NFS clients  4  and CIFS client  5  for making access though a conventional protocol for accessing a centralized file system such as an NFS or a CIFS basically implemented with a single file server on the network  1 . Hereinafter, the NFS clients  4  and the CIFS client  5  may be referred to as conventional protocol clients. 
     In the DFS shown in  FIG. 1 , a DFS client  3  on the network  1  accesses a DFS server  2  using the dedicated DFS protocol to refer to or perform writing to a file. Identifiers referred to as GUIDs, which are numbers that can uniquely identify respective files on the system, are attached to the files on the DFS. The GUID is used to identify a file when the DFS client  3  refers to the file. The GUID for a file is uniquely assigned by a DFS server  2  when writing to the file is performed, and is informed to a DFS client  3  as a response to the writing. 
     More specifically, when referring to a file, the DFS client  3  specifies to one of the DFS servers  2  on the network the GUID for the file to identify the file, thereby making access to the DFS server  2 . When an entity of the file does not reside on the accessed DFS server  2 , the DFS server  2  inquires of other DFS server or servers  2  on which the entity of the file resides, and collects data to construct the entity of the file on itself, thereby allowing access from the DFS client  3 . 
     When writing to a file, the DFS client  3  transmits write data to a DFS server  2 , and receives the GUID for the file from the DFS server  2  as a response to the writing. Access to the file to which writing has been performed is made, using the GUID received from the DFS server  2 . 
     In the DFS, the entity of an identical file sometimes resides on a plurality of DFS servers. When the DFS client has performed update writing to modify contents of once-written data in this situation, it is difficult to ensure consistency of file data among the respective DFS servers. Thus, among the DFSs, there is a file system in which writing to a file specified by a GUID can be performed once and after the writing, only referencing is allowed. In this system, update writing is not allowed. The file system with this characteristic is referred to a write-once, read-many file system. 
     In the write-once read-many file system, modification to the contents of a file, corresponding to update writing, is equivalent to writing to a new generation file. The writing to the new generation file is performed after a new GUID has been assigned to the file. A series of generations of files are managed as file groups. 
     The DFS shown in  FIG. 1  is the write-once read-many file system described above. A once-written file to which its GUID has been assigned cannot be updated. Instead of update writing, there are generations of files in the DFS in  FIG. 1 ; update writing to a file corresponds to creation of a new generation file. Different GUIDs are assigned to respective generations. A collection of files of different generations is managed as a file group. A file group identifier is uniquely assigned to each file group. By specifying a file group identifier and a generation number indicating the generation of a file, a GUID identifying the file can be obtained. 
       FIG. 2  is a diagram that explains an example of a method of determining the GUID from the file group identifier and the generation number. As shown in  FIG. 2 , high-order bits in the GUID are used as a file group identifier  61 , while remaining low-order bits are used as a generation number  62 . The method shown in  FIG. 2  is just one example. The GUID may also be determined by using other method. 
     An NFS client  4  and a CIFS client  5 , which are the conventional protocol clients, make access to a file using the conventional protocol for the centralized network file system other than the DFS protocol. When making access to the file, these conventional protocol clients usually use the directory structure to specify the file, as described before. 
       FIG. 3  is a diagram that explains the directory structure used by the conventional protocol clients. Referring to  FIG. 3 , the directory structure will be described. In  FIG. 3 , a directory is indicated by a solid block, while a file is indicated by a dotted-line block. 
     All directories are defined in terms of tree structures that belong to a route directory  41  beginning with “/”. All the files belong to any one or more of the directories in these tree structures. In the respective directories of the tree structures, a unique file name can be assigned to a file in the directory. For this reason, when specifying a file, a pathname indicating the directory to which the file belongs and its file name should be specified. A file  45  shown in  FIG. 3 , for example, can be specified in the form of/dira/dira2/dira22/file0001. Accordingly, when making access to a file for referencing and updating, the client specifies to the server on which the file resides the pathname and the file name of the file, thereby identifying the file. Then, the client makes a request to reference or update the file. 
     The distributed file system in this embodiment allows the conventional protocol clients such as the NFS and CIFS clients to access a DFS file while eliminating the need for incorporating new software into a client side. Next, a configuration of the DFS server for achieving this effect will be described. 
       FIG. 4  is a block diagram showing the configuration of a DFS server, and  FIG. 5  is a block diagram showing a configuration of a gateway unit shown in  FIG. 4 . Referring to  FIGS. 4 and 5 , reference numeral  21  denotes a DFS control unit, numeral  22  denote disk drives, numeral  23  denotes a main memory, numeral  24  denotes an OS, numeral  25  denotes the gateway unit, numeral  26  denotes a DFS processing unit, numeral  27  denotes a disk drive processing unit, numeral  28  denotes a CPU, numeral  29  denotes an HDD, reference numeral  31  denotes a conventional protocol processing unit, numeral  32  denotes a DFS accessing unit, and numeral  33  denotes a directory managing unit. 
     As shown in  FIG. 4 , the DFS server  2  is constituted from the CPU  28  for executing overall processing of the server, main memory  23 , disk drives  22  equipped with disks storing files, and HDD  29  storing the OS loaded into the main memory  23  for use and applications. The main memory  23  includes the OS  24  and the DFS control unit  21 . The DFS control unit  21  is constituted from the gateway unit  25 , DFS processing unit  26 , and disk drive processing unit  27 . 
     The DFS control unit  21  functions as the DFS server by running of a program providing a DFS function on the CPU  28 . The DFS control unit  21  performs processing in response to requests from clients including the conventional protocol clients such as the NFS client  4 , CIFS client  5  and the DFS client  3  and other DFS servers  2  over the network  1  through processing of the gateway unit  25 , DFS processing unit  26 , and disk drive processing unit  27 . 
     The DFS processing unit  26  receives a request through the DFS protocol and according to this request, makes a request for processing to the disk drive processing unit  27  or other DFS server  2 . Then, the DFS processing unit  26  prepares a response using a result of the processing and returns the response to the client which made the request. 
     The gateway unit  25  receives a request identical to the one for the network file system such as the NFS or CIFS using the conventional protocol, from the NFS or CIFS client. Then, the gateway unit  25  performs processing according to the request by making a request for the processing to other processing unit as necessary, and then returns the result of the processing to the NFS or CIFS client as a response. 
     As described above, like the file server that uses the conventional protocol, the DFS servers  2  can receive access in response to the request from the conventional protocol client and can make a response. More specifically, due to the presence of the gateway unit  25 , the DFS server  2  shown in  FIG. 4  functions as a single NFS or CIFS server for the NFS or CIFS client. 
     The gateway units  25  do not need to be present on all of the DFS servers  2 : they should be present on the DFS servers  2  that can become access points from the conventional protocol clients. In other words, the gateway units  25  are required only for the DFS servers  2  to be accessed by the conventional protocol clients. For this reason, when the usual DFS client  3  makes access, the gateway unit  25  is unnecessary. 
     As shown in  FIG. 5 , the gateway unit  25  is constituted from the conventional protocol processing unit  31 , DFS accessing unit  32 , and directory managing unit  33 . 
     The conventional protocol processing unit  31  is the processing unit for receiving the conventional protocol from the NFS or CIFS client and returning a response in conformity with the conventional protocol. The conventional protocol processing unit  31  is present for each of the protocols received, judges the contents of the associated protocol, and makes a request for processing to other processing unit as necessary. 
     The DFS accessing unit  32  receives the request from the conventional protocol processing unit  31  to act as a bridge between the gateway unit  25  and the DFS processing unit  26  in the DFS control unit  21 . More specifically, the DFS accessing unit  32  is the processing unit for creating a request for the DFS according to the request from the conventional protocol processing unit  31  and accessing a DFS file via the DFS processing unit  26 . Incidentally, the conventional protocol processing unit  31  may create the request for the DFS in conformity with the conventional protocol. In this case, the DFS accessing unit  32  is unnecessary. 
     The directory managing unit  33  is the processing unit for managing correspondence between the files in the directory structures of the file systems that uses the conventional protocols and the GUIDs, which are the identifiers for DFS files. Specification of a file through the conventional protocol is performed by the pathname indicating the directory in which the file resides and the file name uniquely assigned in the directory. In contrast therewith, specification of a DFS file is performed by the GUID uniquely assigned within the system. If the DFS has structures corresponding to directories in the conventional protocol file system, the GUIDs can be associated with the files in the conventional file systems through the use of the structures. Among the distributed file systems, however, there is also the system that does not have the structures corresponding to the directories. In other words, the DFS that does not have the structures corresponding to the directories can uniquely identify the files in the entire system by the GUIDs alone. Thus, it does not need a structure corresponding to the pathnames for the files. 
     In order to allow access using the conventional protocol, it is necessary to provide for the DFS that does not have the directory structure a scheme for associating the GUIDs with the files specified by their pathnames in the directory structure. The directory managing unit  33 , which manages correspondence between the GUIDs and the files in the directory structure, provides the scheme for associating the GUIDs with the files specified by their pathnames in the directory structure. 
     The directory managing unit  33  performs processing on requests for directory manipulation and directory information reading through the conventional protocol. For this reason, the directory managing unit  33  has directory management information comparable to the directory management information in the conventional protocol file system. 
       FIG. 6  is a diagram that explains the structure of directory management information  50 . The directory management information  50  is constituted from a plurality of entries  51 . The entries  51  are respectively present for each directory described with reference to  FIG. 3  or for each file. 
     Each entry  51  includes a p_parent  52 , a p_subdir  53 , a p_child  54 , and an f_name  55 . The p_parent  52  is a pointer to the entry of a parent directory to which a directory or a file belongs. The route directory has no parent directory. When the entry  51  indicates a directory, the p_subdir  53  is a pointer to a subordinate directory or a file which belongs to the directory. The p_child  54  is a pointer to an entry at the same level in the directory, and the f_name  55  stores the name of a file or the directory. When the entry  51  indicates a file, the file group identifier for identifying the file on the DFS is stored in an f_id  56 . Each entry  51  of the directory management information  50  is structured to accommodate the directory structure by three kinds of the pointers of the p_parent  52 , p_subdir  53 , and p_child  54 . For this reason, by sequentially tracking the pointers to the entry of a directory specified by its pathname, it is possible to reach the entry  51  of a targeted file. Since the entry  51  stores its file group identifier, the file group identifier for the DFS can be determined from a file and the pathname of the directory to which the file belongs, specified by the conventional protocol file system. 
     If the entry  50  in a top portion of  FIG. 6  is a route entry for the route directory  41  in  FIG. 3 , the entries in a middle portion of  FIG. 6  correspond to directories dira, dirb, and filel in  FIG. 3 . The p_subdir  53  of the entry  50  in the top portion of  FIG. 6  points to the entry on a left side of the middle portion of  FIG. 6 , corresponding to the directory dira in  FIG. 3 , while the p_child  54  of the entry  50  on the left side of the middle portion of  FIG. 6  points to the entry  50  on a right side of the middle portion of  FIG. 6 , corresponding to the directory dirb in  FIG. 3 . Likewise, the p_subdir  53  of the entry  50  on the left side of the middle portion of  FIG. 6  corresponding to the directory dira in  FIG. 3  points to the entry  50  in a bottom portion of  FIG. 6 , corresponding to the directory dira  1  in  FIG. 3 . 
     Assume that the pathnames and the file name of a file from the conventional client have been specified as /dira/dira2/dira22/file0001/, for example, as described in  FIG. 3 . Then, by sequentially tracking the entries  51  according to the directories specified by the pathnames and the file name, the entry that has the specified file name in the f_name  55  that stores the directory or file name therein can be searched. The file group identifier f_id  56  for the file can be thereby obtained. 
     Registration of a file in the directory management information  50  as described above is performed in following cases: One is the case where the file is created in the directory management information  50 , in response to a request for file registration from the centralized file system that uses the conventional protocol. In this case, the file is created by specification of the pathname and file name of the file. Thus, the conventional protocol processing unit  31  should create the entry for the file and adds the entry at a location in the directory management information  50 , corresponding to the pathname. The other is the case where the DFS creates the file using the DFS protocol. Since the directory management information  50  is not created by using DFS files alone in this case, the DFS file created through the DFS protocol cannot be accessed, using the conventional protocol. For this reason, the DFS processing unit  26  makes a request to register the DFS file in the directory management information  50  in the directory managing unit  33  to the gateway unit  25  so that the DFS file can be accessed through the conventional protocol. The DFS processing unit  26  specifies the pathname, file name, and the file group identifier of the file to be registered and makes a request for the registration to the gateway unit  25 . The gateway unit  25  then performs the registration. After the file has been registered in the directory management information  50  in this manner, the file can be accessed by the centralized file system using the conventional protocol, for referencing. 
     The directory management information  50  may be shared between the DFS servers  2  through communication between the gateway units  25  in the DFS servers  2  within the system, and can be thereby regarded as an identical file system. With this arrangement, from respective access points, access can be made to the identical file system. Alternatively, the respective gateway units  25  of the DFS servers  2  may include different directory management information  50 . With this arrangement, the centralized file system can access different file systems that differ depending on the respective access points. 
       FIG. 7  is a flowchart that explains processing operations when a DFS file is referred to using the conventional protocol. Next, the processing operations of the flowchart will be described. 
     (1) When the file in a certain directory is referenced through the conventional protocol, the conventional protocol client  4  or  5  specifies the pathname indicating the location of the directory to which the file belongs and the file name to identify the file. Then, the client accesses the DFS server  2  that holds the directory management information  50  so as to refer to the file.
 
(2) When the conventional protocol processing unit  31  has received a reference request specifying the file by the pathname as described before, makes an inquiry to the directory managing unit  33  at step  81  to obtain the file group identifier for the file. The directory managing unit  33  sequentially tracks the pointers of the entries in the directory management information  50  in the directory managing unit  33 , thereby reaching the entry of the specified file, as in hierarchical directory tracking in the conventional file system. Then, the directory managing unit  33  obtains the file group identifier for the file with its name specified by the entry, and returns the file group identifier to the conventional protocol processing unit  31 .
 
(3) The conventional protocol processing unit  31  inquires of the DFS accessing unit  32  a latest generation number of the file group identifier so as to obtain the GUID for the file, at step  82 . The DFS accessing unit  32  uses a method such as the one for inquiring of other DFS server  2  that manages file group identifiers through the DFS protocol, thereby determining the latest generation number, and returns the latest generation number to the conventional protocol processing unit  31 .
 
(4) The conventional protocol processing unit  31  determines the GUID from the file group identifier and the generation number obtained at steps  81  and  82 , at step  83 .
 
(5) The conventional protocol processing unit  31  uses the GUID determined at step  83  to issue a request to read data associated with the GUID to the DFS accessing unit  32 , at step  84 . The DFS accessing unit  32 , upon reception of this request, issues a request to the DFS to obtain the data for the read request.
 
(6) The conventional protocol processing unit  31  returns the data read through the DFS accessing unit  32  after step  84  to the client that has made the reference request, as a response, at step  85 .
 
       FIG. 8  is a flowchart that explains processing operations when a file is updated by accessing the DFS server  2  that holds the directory management information  50  from the centralized file system using the conventional protocol. Next, the processing operations of this flowchart will be described. 
     (1) When the file in a certain directory is updated using the conventional protocol, the conventional protocol client  4  or  5  specifies the pathname indicating the location of the directory to which the file belongs and the file name to identify the file, as in the case of file referencing. Then, the client accesses the DFS server  2  so as to update the file.
 
(2) When the conventional protocol processing unit  31  has received an update request made by identifying the file as described before, the conventional protocol processing unit  31  determines whether the received update request is a second or subsequent request for updating of data resulting from division of data in the identified file or not. Division of update data and division of the update request will be described later. More specifically, it is determined whether the request has been made to update the data in a file being already updated or not, at step  91 .
 
(3) If it has been determined at step  91  that the request to update data in the file being already updated is made, the conventional protocol processing unit  31  inquires of the directory managing unit  33  to obtain the file group identifier for the file at step  92 . For this reason, the conventional protocol processing unit  31 , as in the case of processing for file referencing, inquires of the directory managing unit  33  the file group identifier. When it has been determined that the request to update data in the already updated file is made, it is the writing to the file that is already existent, so that the entry for the file is present in the directory management information  50 . By determining the entry as in the case of file referencing, the file group identifier for the file can be obtained, at step  92 . If it has been determined that the request is made to create a new file rather than to perform updating, the entry for the new file is not present in the directory management information  50 . In this case, when the request to create the new file has been made, it is necessary to create and register the entry for the new file in the directory management information  50  and also issue a request for creation of the new file to the DFS processing unit  26  to obtain the file group identifier of the new file from the DFS processing unit  26 . Then, data writing should be performed on the newly created file by performing processing described below after step  95 , as in the case of updating of the file.
 
(4) After the file group identifier has been obtained, it is necessary to create a new generation file in the file group indicated by the file group identifier so as to perform update writing of the contents of the file. For this purpose, the conventional protocol processing unit  31  asks the DFS accessing unit  32  to issue a request to create the new generation file, at step  93 .
 
(5) The DFS accessing unit  32  registers the new generation file for the DFS processing unit  26 , and returns an obtained generation number to the conventional protocol processing unit  31  as a response. The conventional protocol processing unit  31  can obtain the GUID from the file group identifier and the generation number returned as the response, at step  94 .
 
(6) According to the conventional protocol, a series of updating processes on a file does not always be performed through a single processing request. In the NFS, for example, a size of a file to be updated can be changed by setting. However, an update request is usually fulfilled as an update process on the file of the defined size of 8 KB, for example. If updating of data exceeding 8 KB is performed, a request for the updating is divided into a plurality of update requests and fulfilled. If updating of 24 KB data is performed, a request for the updating is divided into three update requests each for updating 8 KB data, and they are fulfilled. In order to generate a new generation number for each of the divided update requests, once updating of a file has been started, it is necessary to memorize until completion of the updating that the file is being updated. In order to achieve this purpose, the conventional protocol processing unit  31  registers a file of which updating has been started in a file monitoring table  70 , which will be described later, at step  95 . The file monitoring table  70  is held in the directory managing unit  33 , for example.
 
(7) If it has been determined at step  91  that updating is to be performed on the data in the file already being updated, the conventional protocol processing unit  31  obtains the GUID for the file from the file monitoring table  70 , which will be described later, at step  96 .
 
(8) After registration of the file in the file monitoring table  70  at step  95  or obtaining the GUID for the file from the file monitoring table  70  at step  96 , the conventional protocol processing unit  31  specifies the GUID for the file to issue to the DFS accessing unit  32  a request to write data in the file to which the GUID is assigned, at step  97 . The DFS accessing unit  32 , upon reception of this request, issues a request for data writing to the DFS processing unit  26 .
 
(9) After the data writing, the conventional protocol processing unit  31  receives a response from the DFS processing unit  26  via the DFS accessing unit  32 , and returns a response indicating completion of the update request to the conventional protocol client at step  98 .
 
       FIG. 9  is a diagram showing a structure of the file monitoring table  70 . Each entry in the file monitoring table  70  corresponds to a file being updated. The contents of each entry consist of information  72  for identifying the file being updated and a generation number  73  indicating the generation of the file being updated. In  FIG. 9 , p_dentry, which is a pointer to the entry for directory information is employed as the information  72  for identifying the file. 
     By checking information registered in the file monitoring table  70 , it can be found whether a received request is the second or subsequent request of divided requests. If the received request is the second or subsequent request, the conventional protocol processing unit  31  will not generate a new generation file. For this purpose, in the processing described with reference to  FIG. 8 , upon reception of the update request, the conventional protocol processing unit  31  examines the file monitoring table  70  in processing at step  91  to check whether the request is the update request for the file being already updated. Then, when it has been determined that the request is not the update request for the file being already updated, the operation proceeds to step  92 , as described above, a new generation file is generated, and then data writing is performed. If it has been found that the request is the update request for the file being already updated, the conventional protocol processing unit  31  obtains the GUID for the file from the file monitoring table  70  at step  96 , and makes a request to write data in the file to which the GUID is assigned, at step  97 . 
     The gateway unit  25  can process the request to update a file, as described above. However, in order to finish registration of the new generation file created, the gateway unit  25  needs to inform the DFS processing unit  26  of completion of data writing (update) to the new generation file. For this purpose, the gateway unit  25  needs to monitor completion of a series of request for updating transmitted from the conventional protocol client through the conventional protocol. 
     A trigger indicating completion of the series of request for updating differs according to the protocol of each network file system. The CIFS, for example, possesses the protocol that has file operation states in which a file is opened when an operation is performed on the file and is closed when the operation is finished. In this case, a close request at the time of completion of the operation corresponds to the trigger indicating completion of the update request. In contrast therewith, under the NFS protocol, operations such as opening and closing of a file do not exist, so that the NFS protocol does not have the state indicating whether the file is being operated or not. In such a protocol, it is necessary to determine whether the operation has been completed or not, by using other trigger. 
     In this case, a plurality of triggers can be employed for this purpose. One is a time interval between update requests. The conventional protocol client divides a series of requests for updating into a plurality of requests and transmits them to the gateway unit  25 . Usually, upon completion of one divided update request, the conventional protocol client issues the subsequent update request. Thus, if the gateway unit  25  monitors intervals of time when update requests have arrived and a certain time or longer has passed since the last update request arrived at the gateway unit  25 , it can be considered that the series of requests for updating has been completed. As another trigger, a request to issue a commit command can be used. In network file systems of NFS version 3 or later, a commit command is prepared as a method of reflecting on a disk the contents of an updated file. Though the time of issuance of the commit command is not specifically defined, the commit command is usually issued when a certain series of significant updating has been finished. For this reason, the gateway unit  25  can consider that a series of requests for updating had been completed when the request to issue the commit command arrived. The commit command, however, is not always issued when updating has been completed. Thus, if the commit command alone is employed for the trigger, it sometimes occurs that completion of updating cannot be recognized. Thus, it is necessary to combine with time interval monitoring described before, for use as the trigger. 
     If the trigger as described above is monitored to determine completion of creation of a new generation file, creation of the new generation file might be inadvertently finished before a series of requests for updating has been actually executed. In a method of employing a certain time interval as the trigger, for example, if arrival of a subsequent update request has been delayed due to a condition of the network  1  or the like, it is sometimes determined that updating, which actually should be still kept on, had been finished. In this case, though an additional new generation file is created, there is no serious problem if the updated contents of the file are not lost. If an update request has been received after creation of a new generation file because of inadvertent determination as to completion of a series of requests for updating, another new generation file should be created and data writing should be performed to the created file. 
       FIG. 10  is a flowchart that explains processing operations of the gateway unit  25  at the time of completion of creation of a new generation file. Next, the processing operations of this flowchart will be described. 
     At step  101 , the gateway unit  25  waits for occurrence of the trigger indicating completion of updating as described above, and starts processing upon occurrence of the trigger. Upon occurrence of the trigger, the conventional protocol processing unit  31  refers to the file monitoring table  70  to obtain the GUID for the new generation file being created. At step  103 , the DFS accessing unit  32  uses the GUID to issue to the DFS processing unit  26  a request for completion of the updating. After completion of update processing at the DFS processing unit  26 , the conventional protocol processing unit  31  deletes from the file monitoring table  70  the entry for the file to which the update processing has been performed, and releases the state where the file is being updated. 
     As described above, the gateway unit  25  enables access to a DFS file from the conventional protocol clients  4  and  5 . 
     Each of the processing in this embodiment, described above can be constituted as a processing program. This program can be stored in a recording medium such as a hard disk (HD), a digital audio tape (DAT), a floppy disk (FD), a magneto-optical disk (MO), a digital versatile disc read-only memory (DVD-ROM), or a compact disc read-only memory (CD-ROM), and can be provided. 
     A foregoing description about the embodiment was given, assuming that the DFS is the write-once read-many file system. The present invention, however, can also be applied to the DFS other than the writeonce read-many file system. In the DFS other than the write-once read-many file system, there is no concept of the generation of a file; even if a file has been updated, the value of the GUID for the file is not changed. For this reason, the f_id  56  in the directory management information  50  may directly include the GUID for a file instead of the file group identifier for the file. In the flowchart in  FIG. 7  when a file is referred to, the GUID for the file can be obtained at step  81 , so that steps  82  and  83  become unnecessary. Likewise, in the flowchart in  FIG. 8  when a file is updated, the GUID for the file can be obtained at step  92 , and step  93  for creation of a new generation file and step  94  for obtaining the GUID is unnecessary. 
     Though the foregoing description was given, assuming that the gateway unit  25  is included in the DFS server  2 , the present invention is not limited to this configuration. The gateway unit  25  may be implemented as a gateway server on the network  1 , for performing processing of the gateway unit  25 , or incorporated into the conventional protocol client  4  or  5 , instead of being mounted in the DFS server  2 . 
       FIG. 11  is a block diagram showing a configuration of a DFS according to another embodiment of the present invention. Referring to  FIG. 11 , reference numeral  110  denotes a gateway server. Same reference numerals are assigned to other components that are the same as those in  FIG. 1 . 
       FIG. 11  is an example where the gateway unit  25  described above is provided as the gateway server  110  on the network  1 . In this case, the conventional protocol client  4  or  5  makes access to the gateway server  110 . Then, the gateway server  110  accesses a DFS server  2  using the DFS protocol. The gateway server  110  performs the same processing as the gateway unit  25 . 
     The DFS accessing unit  32  of the gateway unit  25  described above in the foregoing embodiment performs processing in conjunction with the DFS processing unit  26 . In the example shown in  FIG. 11 , a function of the gateway unit is implemented on the server different from the DFS server  2 . Thus, it becomes impossible to perform the processing in conjunction with the DFS processing unit  26 . Instead of this, in the system shown in  FIG. 11 , the DFS accessing unit  32  provided in the gateway server  110  becomes a DFS client, which uses the DFS protocol to make a request to the DFS server  2 , thereby enabling the same processing to be performed. In this case, this DFS client  3  can be said to have the function of a server in the centralized file system. 
     As the method of incorporating the gateway unit  25  into the conventional protocol client, insertion of the function of the gateway unit  25  between the NFS client  4  and the network  1 , for example, can be conceived. In this case, programs that use the protocol for a host NFS are processed through the NFS protocol. Through the function of the gateway unit  25 , the NFS protocol is converted into the DFS protocol and a DFS server is accessed through the converted DFS protocol, on the network. In this case, each NFS client needs the gateway unit. 
     According to the embodiments of the present invention, the directory structure which resides in the network file system that uses the conventional protocol, such as the NFS and CIFS is emulated. Further, the method of accessing a file using the pathname and the file name indicating the location of the file in the directory structure is converted to the method of accessing a file using the GUID, which is the identifier for the file in the DFS. Access to a DFS file through the conventional protocol can be thereby made. 
     According to the embodiments of the present invention, if the DFS is the write-once read-many file system, update processing using the conventional protocol is converted into processing for creation of a new generation file. With this arrangement, when file referencing is performed, a latest generation file can be determined from generation-managed file groups, so that access to data in the latest generation file becomes possible. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.