Abstract:
Provided is a computer system including: one first server; a plurality of second servers; and a plurality of storage subsystems, in which the computer system applies to each file stored in the storage subsystems one of a first file storage mode and a second file storage mode in a distributive manner, the first server integrates directory structures that are unique throughout the respective second servers to provide the integrated directory structures as a directory structure that is unique throughout the computer system to the client computer, and at least one of the first server and the second servers analyzes a utilization state of the file and switches a file storage mode that is currently applied to the file to the other file storage mode based on a change in the analyzed file utilization state.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application P2007-25387 filed on Feb. 5, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND 
       [0002]    This invention relates to a computer system having a plurality of servers and a plurality of storage subsystems, and more specifically, to a technique of storing files in storage subsystems. 
         [0003]    JP 2006-172217 A discloses a technique called “global name space” (GNS). In GNS, a plurality of network attached storage (NAS) elements are provided as a single NAS element to a client computer. 
         [0004]    There has also been known a distributed file system (cluster file system). In a cluster file system, a single file is distributed for storage among a plurality of NAS elements. 
       SUMMARY 
       [0005]    Usually, one file is managed by one file system of NAS. A file system that manages a large-sized file is therefore accessed intensively. 
         [0006]    In a cluster file system where a large-sized file is distributed for storage among a plurality of NAS elements, an access concentration on a specific file system is avoided. However, file distribution can cause overhead when distributed pieces of a file are small in size. 
         [0007]    This invention has been made in view of the above problem, and it is therefore an object of this invention to provide a computer system that stores files in storage subsystems suitably. 
         [0008]    According to an exemplary embodiment of this invention, there is provided a file storage method for a computer system having one first server which is accessed by a client computer, a plurality of second servers which are coupled to the first server via a network, and a plurality of storage subsystems which are coupled to the second servers, comprising: applying to each file stored in the storage subsystems one of a first file storage mode in which one file is stored in one of the storage subsystems and a second file storage mode in which one file is stored in two or more of the storage subsystems in a distributive manner; integrating, by the first server, directory structures that are unique throughout the respective second servers to provide the integrated directory structures as a directory structure that is unique throughout the computer system to the client computer; and analyzing, by at least one of the first server and the second servers, a utilization state of the file and switching a file storage mode that is currently applied to the file to the other file storage mode based on a change in the analyzed file utilization state. 
         [0009]    According to the representative aspect of this invention, files can be stored in storage subsystems suitably. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
           [0011]      FIG. 1  is a block diagram showing the configuration of a computer system according to the embodiment of this invention; 
           [0012]      FIG. 2  is a block diagram showing the configuration of the disk subsystem in the computer system according to the embodiment of this invention; 
           [0013]      FIG. 3  is a block diagram showing the configuration of the NAS server in the computer system according to the embodiment of this invention; 
           [0014]      FIG. 4  is a configuration diagram of the GNS definition table according to the embodiment of this invention; 
           [0015]      FIG. 5  is an explanatory diagram of GNS in the computer system according to the embodiment of this invention; 
           [0016]      FIG. 6  is an explanatory diagram of the distributed storage mode A according to the embodiment of this invention; 
           [0017]      FIG. 7  is an explanatory diagram of the distributed storage mode B according to the embodiment of this invention; 
           [0018]      FIG. 8  is a configuration diagram of the file division policy according to the embodiment of this invention; 
           [0019]      FIG. 9  is a configuration diagram of the file section management table according to the embodiment of this invention; 
           [0020]      FIG. 10  is an explanatory diagram of the distributed storage mode C according to the embodiment of this invention; 
           [0021]      FIG. 11  is an explanatory diagram of characteristics of the distributed storage modes A to C according to the embodiment of this invention; 
           [0022]      FIG. 12  is an explanatory diagram of the distributed storage mode selection policy according to the embodiment of this invention; 
           [0023]      FIG. 13  is a configuration diagram of the distributed storage mode management table according to the embodiment of this invention; 
           [0024]      FIG. 14  is a configuration diagram of the distributed storage mode change management table according to the embodiment of this invention; 
           [0025]      FIG. 15  is a configuration diagram of the distributed storage mode change history table according to the embodiment of this invention; 
           [0026]      FIG. 16  is a configuration diagram of the distributed storage mode application state history table according to the embodiment of this invention; 
           [0027]      FIG. 17  is a flow chart for read processing that is executed by the NAS server and the parent NAS server according to the embodiment of this invention; 
           [0028]      FIG. 18  is a flow chart for read processing that is according to the distributed storage mode B and executed by the parent NAS server according to the embodiment of this invention; 
           [0029]      FIG. 19  is a flow chart for write processing that is executed by the NAS server and the parent NAS server according to the embodiment of this invention; 
           [0030]      FIG. 20  is an explanatory diagram of the distributed storage mode change notification screen which is displayed by the management computer according to the embodiment of this invention; 
           [0031]      FIG. 21  is a flow chart for write processing that is according to the distributed storage mode B and executed by the parent NAS server according to the embodiment of this invention; 
           [0032]      FIG. 22  is an explanatory diagram of file migration according to the embodiment of this invention; 
           [0033]      FIG. 23  is an explanatory diagram of the GNS environment change notification screen which is displayed by the management computer according to the embodiment of this invention; 
           [0034]      FIG. 24  is an explanatory diagram of the GNS environment change request screen which is displayed by the management computer according to the embodiment of this invention; 
           [0035]      FIG. 25  is a flow chart for snapshot processing that is according to the distributed storage mode B and executed by the parent NAS server according to the embodiment of this invention; and 
           [0036]      FIG. 26  is an explanatory diagram of a modification example of the distributed storage mode B according to the embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0037]    An embodiment of this invention will be described below with reference to the accompanying drawings. 
         [0038]      FIG. 1  is a block diagram showing the configuration of a computer system according to the embodiment of this invention. 
         [0039]    The computer system has parent NAS, NAS, a client computer  4 , a management computer  5 , and a local area network (LAN)  7 . 
         [0040]    The parent NAS has a parent NAS server  9  and a disk subsystem  3 . The NAS has a NAS server  1 , a fibre channel (FC) switch  2 , and its own disk subsystem  3 . 
         [0041]    The LAN  7  interconnects the parent NAS server  9 , the NAS server  1 , the client computer  4 , and the management computer  5 . The FC switch  2  connects the NAS server  1  and the disk subsystem  3  to each other. 
         [0042]    The NAS server  1  is connected to one or more disk subsystems  3  via the FC switch  2 . The parent NAS server  9 , which, in  FIG. 1 , is connected to one disk subsystem  3 , may be connected to a plurality of disk subsystems  3 . 
         [0043]    Each disk subsystem  3  stores data requested by the client computer  4  to be written. Details of the disk subsystem  3  will be described with reference to  FIG. 2 . 
         [0044]    The computer system may have a semiconductor memory device in place of the disk subsystem  3 . The semiconductor memory device uses a flash memory or the like as a memory device. 
         [0045]    The NAS server  1  provides a file sharing service to the client computer  4 . For instance, the NAS server  1  receives a file input/output request and converts the received file input/output request into a block input/output request. The NAS server  1  then sends the block input/output request obtained by the conversion to the disk subsystem  3 . Details of the NAS server  1  will be described with reference to  FIG. 3 . 
         [0046]    The parent server  9  has a global name space (GNS) function in addition to the function of the NAS server  1 . Details of GNS will be described with reference to  FIG. 5 . 
         [0047]    The client computer  4  receives a file sharing service from the parent NAS server  9  and the NAS server  1 . The client computer  4  sends a file input/output request to the parent NAS server  9  to receive the file sharing service. 
         [0048]    The management computer  5  handles the overall management of this computer system. 
         [0049]      FIG. 2  is a block diagram showing the configuration of the disk subsystem  3  in the computer system according to the embodiment of this invention. 
         [0050]    The disk subsystem  3  has a disk controller (DKC) and a physical disk. 
         [0051]    The physical disk stores data requested by the client computer  4  to be written. The disk controller provides the storage area of the physical disk to the NAS server  1  or the parent NAS server  9  as one or more logical volumes (LUs)  35 . 
         [0052]    The disk controller has an FC port  31 , a CPU  32 , and a memory  33 . The FC port  31  is an interface connected to the NAS server  1  or the parent NAS server  9  via the FC switch  2 . The CPU  32  performs various types of processing by executing a program stored in the memory  33 . 
         [0053]    The memory  33  stores a program executed by the CPU  32 , information necessary for the CPU  32 , and the like. For example, the memory  33  stores an IOP, which inputs and outputs data to and from the physical disk in accordance with a block input/output request received from the NAS server  1  or the parent NAS server  9 . 
         [0054]    Part of the memory  33  is used as a cache memory. The cache memory temporarily stores data to be written in the physical disk and data read out of the physical disk. 
         [0055]    The LU  35  of the disk subsystem  3  that is connected to the parent NAS server  9  stores a GNS definition table  91 , a file division policy  92 , a file section management table  93 , a distributed storage mode management table  94 , a distributed storage mode change management table  95 , distributed storage mode change history table  96 , a distributed storage mode application state history table  97 , and a distributed storage mode selection policy  138 . 
         [0056]    The GNS definition table  91  shows the association between a global path, which is used to identify a file system  134  uniquely throughout the computer system, and a local path, which is used to identify the file system  134  uniquely throughout the NAS server  1 . Details of the GNS definition table  91  will be described with reference to  FIG. 4 . 
         [0057]    The file division policy  92  shows how many sections a file is to be divided into. Details of the file division policy  92  will be described with reference to  FIG. 8 . 
         [0058]    The file section management table  93  is used to manage storage destinations of a file to which a distributed storage mode B is applied. Details of the file section management table  93  will be described with reference to  FIG. 9 . Details of the distributed storage mode B will be described with reference to  FIG. 7 . 
         [0059]    The distributed storage mode management table  94  shows a distributed storage mode that is currently adopted by the file system  134  in the NAS server  1 , and a distributed storage mode that the file system  134  can adopt. Details of the distributed storage mode management table  94  will be described with reference to  FIG. 13 . 
         [0060]    The distributed storage mode change management table  95  is used to manage a file for which a change should be made from one distributed storage mode to another. Details of the distributed storage mode change management table  95  will be described with reference to  FIG. 14 . 
         [0061]    The distributed storage mode change history table  96  is used to manage a history of switches of distributed storage modes. Details of the distributed storage mode change history table  96  will be described with reference to  FIG. 15 . 
         [0062]    The distributed storage mode application state history table  97  is used to manage, for each distributed storage mode, a history of the count of files to which the distributed storage mode is applied. Details of the distributed storage mode application state history table  97  will be described with reference to  FIG. 16 . 
         [0063]    The distributed storage mode selection policy  138  is information for judging whether or not a distributed storage mode that is currently applied to a file needs to be changed to another distributed storage mode. The distributed storage mode selection policy  138  is stored also in the NAS server  1 . Details of the distributed storage mode selection policy  138  will be described with reference to  FIG. 12 . 
         [0064]      FIG. 3  is a block diagram showing the configuration of the NAS server  1  in the computer system according to the embodiment of this invention. 
         [0065]    The NAS server  1  has an FC port  11 , a CPU  12 , a memory  13 , and a LAN port  14 . 
         [0066]    The FC port  11  is an interface connected to the disk subsystem  3  via the FC switch  2 . The LAN port  14  is an interface connected to the client computer  4  and the management computer  5  via the LAN  7 . 
         [0067]    The CPU  12  performs various types of processing by executing a program stored in the memory  13 . 
         [0068]    The memory  13  stores a program executed by the CPU  12 , information necessary for the CPU  12 , and the like. Specifically, the memory  13  stores a file sharing program  131  and an OS  137 . 
         [0069]    The file sharing program  131  provides a file sharing service to the client computer  4 . A file sharing protocol such as Network File System (NFS) or Common Internet File System (CIFS) is used between the NAS server  1  and the client computer  4 . The file sharing program  131  contains a read request receiving sub-program  1311  and a write request receiving sub-program  1312 . 
         [0070]    The read request receiving sub-program  1311  receives a read request issued by the client computer  4  and performs processing that fulfills the received read request. Read processing executed by the read request receiving subprogram  1311  will be described in detail with reference to  FIG. 17 . 
         [0071]    The write request receiving subprogram  1312  receives a write request issued by the client computer  4  and performs processing that fulfills the received write request. Write processing executed by the write request receiving subprogram  1312  will be described in detail with reference to  FIG. 19 . 
         [0072]    The OS  137  controls the overall processing of the NAS server  1 . The OS  137  contains the file system  134 , a logical volume manager (LVM)  135 , and a device driver  136 . 
         [0073]    The file system  134  provides data stored in the disk subsystem  3  to the client computer  4  and others as a file. For instance, the file system  134  converts a file input/output request into a block input/output request. 
         [0074]    The LVM  135  provides a plurality of LUs  35  that are provided by the disk subsystem  3  to the file system  134  as one LU. 
         [0075]    The device driver  136  sends a block input/output request to the disk subsystem  3 , thereby inputting and outputting data to and from the disk subsystem  3 . 
         [0076]    The parent NAS server  9  has the same configuration as that of the NAS server  1 , and a description on the configuration of the parent NAS server  9  will be omitted. 
         [0077]      FIG. 4  is a configuration diagram of the GNS definition table  91  according to the embodiment of this invention. 
         [0078]    The GNS definition table  91  contains a global path  911 , a NAS name  912 , and a local path  913 . 
         [0079]    The global path  911  indicates a path that is used to identify each file system  134  in the NAS server  1  uniquely throughout the computer system. 
         [0080]    The NAS name  912  indicates an identifier unique to the NAS server  1  that has the file system  134  identified by the global path  911  of the record entry in question. In the case where the distributed storage mode B is applied to the file system  134  identified by the global path  911  of the record in question, no value is stored as the NAS name  912 . 
         [0081]    The local path  913  indicates a path that is used for unique identification of the file system  134  identified by the global path  911  of the record in question throughout the NAS server  1  identified by the NAS name  912  of the record. 
         [0082]      FIG. 5  is an explanatory diagram of GNS in the computer system according to the embodiment of this invention. 
         [0083]    The parent NAS server  9  provides a plurality of file systems  134  in a plurality of NAS servers  1  as one tree view to the client computer  4 . 
         [0084]    As an example, a case will be described in which the GNS definition table  91  of  FIG. 4  is stored in the LU  35  of the disk subsystem  3  that is connected to the parent NAS server  9 . 
         [0085]    In this example, the NAS server  1  that is identified by “NAS-01” has the file system  134  that is identified by “FS1”. The NAS server  1  that is identified by “NAS-02” has the file system  134  that is identified by “FS2”. The NAS server  1  that is identified by “NAS-03” has the file system  134  that is identified by “FS3”. The NAS server  1  that is identified by “NAS-04” has the file system  134  that is identified by “FS4”. Each file system  134  is mounted under a local directory “/mnt” of its NAS server  1 . 
         [0086]    The client computer  4  accesses the parent NAS server  9  to refer to a tree view as the one shown in the balloon of  FIG. 5 . 
         [0087]    In the case where the client computer  4  is to access a file identified by “/GNS-Root/Dir-01/FS2/a.txt”, the client computer  4  sends an access request that contains “/GNS-Root/Dir-01/FS2/a.txt” as an access destination to the parent NAS server  9 . An access request is a write request, a read request, or the like. 
         [0088]    Receiving the access request, the parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a global path “/GNS-Root/Dir-01/FS2” contained in the received access request. The parent NAS server  9  extracts the NAS name  912  and the local path name  913  from the chosen record. 
         [0089]    The parent NAS server  9  thus recognizes the file identified by “/GNS-Root/Dir-01/FS2/a.txt” as a file provided by the file system  134  that is identified by the extracted local path  913 , “/mnt/FS2”. The file system  134  that is identified by the extracted local path  913 , “/mnt/FS2”, is located in the NAS server  1  that is identified by the extracted NAS name  912 , “NAS-02”. 
         [0090]    The parent NAS server  9  accordingly converts the destination “/GNS-Root/Dir-01/FS2/a.txt” contained in the received access request into “/mnt/FS2/a.txt”. The parent NAS server  9  sends the converted access request to the NAS server  1  that is identified by the extracted NAS name  912 , “NAS-02”. 
         [0091]    Alternatively, the parent NAS server  9  may perform the following processing. 
         [0092]    Upon reception of an access request, the parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a global path contained in the received access request. From the chosen record, the parent NAS server  9  extracts the NAS name  912  and the local path  913 . 
         [0093]    The parent NAS server  9  then sends the extracted NAS name  912  and local path  913  to the client computer  4 . The client computer  4  sends an access request that contains the received NAS name  912  and local path  913  as an access destination to the NAS server  1  that is identified by the received NAS name  912 . 
         [0094]    This way, the client computer  4  can access a file without the intervention of the parent NAS server  9 , and the load on the parent NAS server  9  is accordingly lessened. 
         [0095]    Described next are three different distributed storage modes that are used in the embodiment of this invention. One of a distributed storage mode A, the distributed storage mode B and a distributed storage mode C is applied to a file. 
         [0096]      FIG. 6  is an explanatory diagram of the distributed storage mode A according to the embodiment of this invention. 
         [0097]    In the distributed storage mode A, one file is stored in one disk subsystem  3 . In other words, according to the distributed storage mode A, a file is not divided. 
         [0098]      FIG. 7  is an explanatory diagram of the distributed storage mode B according to the embodiment of this invention. 
         [0099]    In the distributed storage mode B, one file is divided into a plurality of sections, which are stored in a plurality of NAS elements. In other words, according to the distributed storage mode B, one file is divided into sections and the file sections are stored in a plurality of disk subsystems  3  that are connected to different NAS servers  1  from one another. 
         [0100]    The parent NAS server  9  manages file division. The LU  35  of the disk subsystem  3  that is connected to the parent NAS server  9  therefore stores the file division policy  92  and the file section management table  93 . 
         [0101]      FIG. 8  is a configuration diagram of the file division policy  92  according to the embodiment of this invention. 
         [0102]    The file division policy  92  contains a size  921  and a section count  922 . The size  921  indicates the size of a file to which the distributed storage mode B is applied. The section count  922  indicates the count of sections into which a file that meets the size  921  of the record entry in question should be divided. 
         [0103]      FIG. 9  is a configuration diagram of the file section management table  93  according to the embodiment of this invention. 
         [0104]    The file section management table  93  contains a local path  931 , a NAS name  932 , and a file section path  933 . 
         [0105]    The local path  931  indicates a path that is used to identify a file to which the distributed storage mode B is applied. The NAS name  932  indicates an identifier unique to the NAS server  1  connected to the disk subsystem  3  that stores a section of the file identified by the local path  931  of the record entry in question. The file section path  933  indicates a path that is used for unique identification of the file section identified by the local path  931  of the record in question throughout the NAS server  1  identified by the NAS name  932  of the record. 
         [0106]      FIG. 10  is an explanatory diagram of the distributed storage mode C according to the embodiment of this invention. 
         [0107]    In the distributed storage mode C, one file is divided into sections and the file sections are stored in a plurality of disk subsystems  3  that are connected to the same NAS server  1 . 
         [0108]    The LVM  135  in the NAS server  1  provides a plurality of LUs  35  provided by a plurality of disk subsystems  3  to the file system  134  as one LU. In this manner, one file is divided and stored in a plurality of disk subsystems  3  that are connected to this NAS server  1 . 
         [0109]      FIG. 11  is an explanatory diagram of characteristics of the distributed storage modes A to C according to the embodiment of this invention. 
         [0110]    The distributed storage mode A, which stores one file in one disk subsystem  3 , is not suitable for storage of large-sized files since it allows access concentration on NAS where a large-sized file is stored. 
         [0111]    On the other hand, the distributed storage mode A is suitable for storage of small-sized files since file distribution causes overhead when a small-sized file is distributed for storage among a plurality of disk subsystems  3 . 
         [0112]    The distributed storage mode B, which divides one file into sections to store the file sections in a plurality of NAS elements, is suitable for storage of large-sized files since access concentration on one NAS server  1  can be avoided. 
         [0113]    On the other hand, the distributed storage mode B is not suitable for storage of small-sized files since file distribution causes overhead when a small-sized file is distributed for storage among a plurality of NAS elements. 
         [0114]    In the distributed storage mode B, the granularity of file distribution can be changed. The distributed storage mode B is therefore suitable for storage of files that are frequently changed in size. 
         [0115]    The distributed storage mode C, which divides one file into sections to store the file sections in a plurality of disk subsystems  3  that are connected to the same NAS server  1 , is suitable for storage of large-sized files since access concentration on one disk subsystem  3  can be avoided. 
         [0116]    In the distributed storage mode C, the granularity of file distribution cannot be changed. The distributed storage mode C is therefore not suitable for storage of files that are frequently changed in size. 
         [0117]    One of the distributed storage modes A, B and C is applied to a file to take advantage of their respective characteristics. 
         [0118]      FIG. 12  is an explanatory diagram of the distributed storage mode selection policy  138  according to the embodiment of this invention. 
         [0119]    The NAS server  1  receives a write request from the parent NAS server  9  and requests the disk subsystem  3  to write data. At this point, the NAS server  1  judges based on the distributed storage mode selection policy  138  whether or not a distributed storage mode that is currently applied to a file in which data is requested to be written needs to be changed to another distributed storage mode. 
         [0120]    Judging that a switch of distributed storage modes is necessary, the NAS server  1  sends a distributed storage mode change request to the parent NAS server  9 . A distributed storage mode change request designates a distributed storage mode that is to be applied as a result of the switch. 
         [0121]    The distributed storage mode selection policy  138  is set such that the respective characteristics of the distributed storage modes A to C are exploited. For instance, the distributed storage mode policy  138  dictates that the distributed storage mode A is applied to a file that has a file size less than a first threshold whereas the distributed storage mode B is applied to a file that has a file size equal to or more than the first threshold and a write frequency equal to or more than a second threshold, and the distributed storage mode C is applied to a file that has a file size equal to or more than the first threshold and a write frequency less than the second threshold. 
         [0122]    The NAS server  1  therefore measures the write frequency of each file and stores a history of the measured write frequency. 
         [0123]      FIG. 13  is a configuration diagram of the distributed storage mode management table  94  according to the embodiment of this invention. 
         [0124]    The distributed storage mode management table  94  contains a NAS name  941 , a currently adopted distributed storage mode  942 , and an adoptable distributed storage mode  943 . 
         [0125]    The NAS name  941  indicates an identifier unique to each NAS server  1 . The currently adopted distributed storage mode  942  indicates an identifier assigned to a distributed storage mode that is applied to a file currently managed by the file system  134  in the NAS server  1  that is identified by the NAS name  941  of the record entry in question. The adoptable distributed storage mode  943  indicates an identifier assigned to a distributed storage mode that is applied to a file manageable by the file system  134  in the NAS server  1  that is identified by the NAS name  941  of the record in question. 
         [0126]      FIG. 14  is a configuration diagram of the distributed storage mode change management table  95  according to the embodiment of this invention. 
         [0127]    The distributed storage mode change management table  95  contains a global path  951 , a file name  952 , and a next-applied distributed storage mode  953 . 
         [0128]    The global path  951  indicates a path that is used to identify the file system  134  in the NAS server  1  uniquely throughout the computer system. The file name  952  indicates an identifier unique to a file managed by the file system  134  that is identified by the global path  951  of the record entry in question. 
         [0129]    The next-applied distributed storage mode  953  indicates an identifier unique to a distributed storage mode that is to be applied next to a file identified by the global path  951  and file name  952  of the record in question. 
         [0130]      FIG. 15  is a configuration diagram of the distributed storage mode change history table  96  according to the embodiment of this invention. 
         [0131]    The distributed storage mode change history table  96  contains a date/time  961 , a file path  962 , a pre-switch distributed storage mode  963 , and a post-switch distributed storage mode  964 . 
         [0132]    The date/time  961  indicates a date and time when a switch is made for a file from one distributed storage mode to another. The file path  962  indicates a path that is used to identify, uniquely throughout the computer system, a file for which a switch of distributed storage modes is made at the date/time  961  of the record entry in question. 
         [0133]    The pre-switch distributed storage mode  963  indicates an identifier assigned to a distributed storage mode that is applied before a switch of distributed storage modes is made to a file identified by the file path  962  of the record in question. The post-switch distributed storage mode  964  indicates an identifier assigned to a distributed storage mode that is applied after a switch of distributed storage modes is made to a file identified by the file path  962  of the record in question. 
         [0134]      FIG. 16  is a configuration diagram of the distributed storage mode application state history table  97  according to the embodiment of this invention. 
         [0135]    The distributed storage mode application state history table  97  contains a date/time  971 , a distributed storage mode A application count  972 , a distributed storage mode B application count  973 , and a distributed storage mode C application count  974 . 
         [0136]    The date/time  971  indicates a date and time when a switch is made for a file from one distributed storage mode to another. The distributed storage mode A application count  972  indicates the count of files to which the distributed storage mode A is applied at the date/time  971  of the record entry in question. The distributed storage mode B application count  973  indicates the count of files to which the distributed storage mode B is applied at the date/time  971  of the record entry in question. The distributed storage mode C application count  974  indicates the count of files to which the distributed storage mode C is applied at the date/time  971  of the record entry in question. 
         [0137]      FIG. 17  is a flow chart for read processing that is executed by the NAS server  1  and the parent NAS server  9  according to the embodiment of this invention. 
         [0138]    The NAS server  1  or the parent NAS server  9  executes this read processing upon receiving a read request. First, the NAS server  1  or the parent NAS server  9  extracts a source IP address from the received read request (S 601 ). Next, the NAS server  1  or the parent NAS server  9  judges from the extracted source IP address whether itself is the parent NAS server  9  or not (S 602 ). 
         [0139]    The parent NAS server  9  receives a read request that contains, as a source IP address, an IP address that is not assigned to any of the parent NAS server  9  and the NAS servers  1 . Each NAS server  1  receives a read request that contains, as a source IP address, an IP address that is assigned to the parent NAS server  9 . 
         [0140]    Judging itself as the parent NAS server  9 , the server proceeds to Step S 603 . In short, the parent NAS server  9  proceeds to Step S 603 . Judging that the parent NAS server  9  is one of other servers than itself, the server proceeds to Step S 607 . In short, the NAS server  1  proceeds to Step S 607 . 
         [0141]    The parent NAS server  9  extracts an access destination global path from the received read request. The parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a portion of the extracted global path. From the chosen record, the parent NAS server  9  extracts the NAS name  912  and the local path  913 . 
         [0142]    The parent NAS server  9  judges from the extracted NAS name  912  whether or not the distributed storage mode B is currently applied to a file that is requested by the received read request to be read (a read target file) (S 603 ). When there is no identifier stored as the extracted NAS name  912 , it means that the distributed storage mode B is currently applied to the read target file. When there is an identifier stored as the extracted NAS name  912 , it means that the distributed storage mode B is not applied to the read target file at present. 
         [0143]    In the case where the distributed storage mode B is currently applied to the read target file, the parent NAS server  9  executes read processing that is according to the distributed storage mode B (S 606 ). The read processing according to the distributed storage mode B will be described in detail with reference to  FIG. 18 . 
         [0144]    In the case where the distributed storage mode B is not applied to the read target file at present, the parent NAS server  9  converts a portion of the global path in the received read request into the extracted local path  913 . The parent NAS server  9  sends the converted read request to the NAS server  1  that is identified by the extracted NAS name  912  (S 604 ). 
         [0145]    Next, the parent NAS server  9  stands by until a file is received from the NAS server  1 . Receiving the file from the NAS server  1 , the parent NAS server  9  sends the received file to the client computer  4  that is the sender of the read request (S 605 ). The parent NAS server  9  then ends this read processing. 
         [0146]    The NAS server  1 , on the other hand, obtains from the disk subsystem  3  a file that is identified by a local path contained in the received read request (S 607 ). The NAS server  1  sends the obtained file to the parent NAS server  9  (S 608 ). The NAS server  1  then ends this read processing. 
         [0147]      FIG. 18  is a flow chart for read processing that is according to the distributed storage mode B and executed by the parent NAS server  9  according to the embodiment of this invention. 
         [0148]    The read processing according to the distributed storage mode B is executed in Step S 606  of the read processing that is shown in  FIG. 17 . 
         [0149]    First, the parent NAS server  9  judges whether or not the file section management table  93  has a record entry whose local path  931  matches the local path  913  extracted in Step S 603  of the read processing that is shown in  FIG. 17 . The parent NAS server  9  thus judges whether or not the read target file is divided into sections (S 611 ). 
         [0150]    In the case where the file section management table  93  does not have a record whose local path matches the extracted local path, it means that the read target file is not divided. In the case where the file section management table  93  has a record whose local path matches the extracted local path, it means that the read target file is divided. 
         [0151]    When the read target file is not divided, the parent NAS server  9  converts a portion of the global path in the received read request into the extracted local path  913 . The parent NAS server  9  sends the converted read request to the NAS server  1  (S 612 ). 
         [0152]    Next, the parent NAS server  9  stands by until a file is received from the NAS server  1 . Receiving the file from the NAS server  1 , the parent NAS server  9  sends the received file to the client computer  4  that is the sender of the read request (S 613 ). The parent NAS server  9  then ends this read processing according to the distributed storage mode B. 
         [0153]    When the read target file is divided, on the other hand, the parent NAS server  9  selects from the file section management table  93  every record entry whose local path  931  matches the local path  913  extracted in Step S 603  of the read processing that is shown in  FIG. 17 . 
         [0154]    The parent NAS server  9  identifies the selected records in turn. From the identified record, the parent NAS server  9  extracts the NAS name  932  and the file section path  933 . Next, the parent NAS server  9  converts the global path contained in the received read request into the extracted file section path  933 . The parent NAS server  9  sends the converted read request to the NAS server  1  that is identified by the extracted NAS name  932 . 
         [0155]    The parent NAS server  9  repeats the processing until every record selected from the file section management table  93  is identified. In this manner, the parent NAS server  9  sends the read request to every NAS that stores a section of the read target file (file section). 
         [0156]    Next, the parent NAS server  9  stands by until a file section is received from every NAS server  1  to which the read request has been sent. Receiving a file section from every NAS server  1  to which the read request has been sent, the parent NAS server  9  combines the received file sections to thereby create the read target file. The parent NAS server  9  sends the created file to the client computer  4  that is the sender of the read request (S 615 ). The parent NAS server  9  then ends this read processing according to the distributed storage mode B. 
         [0157]      FIG. 19  is a flow chart for write processing that is executed by the NAS server  1  and the parent NAS server  9  according to the embodiment of this invention. 
         [0158]    The NAS server  1  or the parent NAS server  9  executes this write processing upon receiving a write request. First, the NAS server  1  or the parent NAS server  9  extracts a source IP address from the received write request (S 621 ). Next, the NAS server  1  or the parent NAS server  9  judges from the extracted source IP address whether itself is the parent NAS server  9  or not (S 622 ). 
         [0159]    The parent NAS server  9  receives a write request that contains, as a source IP address, an IP address that is not assigned to any of the parent NAS server  9  and the NAS servers  1 . Each NAS server  1  receives a write request that contains, as a source IP address, an IP address that is assigned to the parent NAS server  9 . 
         [0160]    Judging itself as the parent NAS server  9 , the server proceeds to Step S 623 . In short, the parent NAS server  9  proceeds to Step S 623 . Judging that the parent NAS server  9  is one of other servers than itself, the server proceeds to Step S 641 . In short, the NAS server  1  proceeds to Step S 641 . 
         [0161]    The parent NAS server  9  extracts an access destination global path from the received write request. The parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a portion of the extracted global path. From the chosen record, the parent NAS server  9  extracts the NAS name  912  and the local path  913 . 
         [0162]    The parent NAS server  9  judges from the extracted NAS name  912  whether or not the distributed storage mode B is currently applied to a file that is requested by the received write request to be written (a write target file) (S 623 ). When there is no identifier stored as the extracted NAS name  912 , it means that the distributed storage mode B is currently applied to the write target file. When there is an identifier stored as the extracted NAS name  912 , it means that the distributed storage mode B is not applied to the write target file at present. 
         [0163]    In the case where the distributed storage mode B is currently applied to the write target file, the parent NAS server  9  executes write processing according to the distributed storage mode B (S 624 ). The write processing according to the distributed storage mode B will be described in detail with reference to  FIG. 21 . 
         [0164]    In the case where the distributed storage mode B is not applied to the write target file at present, the parent NAS server  9  judges whether or not the distributed storage mode change management table  95  has a record entry whose combination of the global path  951  and the file name  952  matches the extracted global path (S 625  and S 626 ). 
         [0165]    When such a record is not found in the distributed storage mode change management table  95 , it means that a distributed storage mode that is currently applied to the write target file does not need to be replaced. Then the parent NAS server  9  converts a portion of the global path in the received write request into the local path  913  extracted in Step S 623 . The parent NAS server  9  sends the converted write request to the NAS server  1  that is identified by the NAS name  912  extracted in Step S 623  (S 627 ). 
         [0166]    Next, the parent NAS server  9  stands by until a write completion notification is received from the NAS server  1 . Receiving a write completion notification from the NAS server  1 , the parent NAS server  9  sends a response to the received write request to the client computer  4 . 
         [0167]    The parent NAS server  9  next judges whether or not the received write completion notification contains a distributed storage mode change request (S 628 ). A distributed storage mode change request designates a distributed storage mode that is to be applied as a result of a switch of distributed storage modes. 
         [0168]    In the case where the write completion notification does not contain a distributed storage mode change request, it means that the NAS server  1  is not requesting the parent NAS server  9  to change the distributed storage mode that is currently applied to the write target file to another distributed storage mode. The parent NAS server  9  accordingly ends this write processing at this point. 
         [0169]    In the case where the write completion notification contains a storage mode change request, it means that the NAS server  1  is requesting the parent NAS server  9  to change the distributed storage mode that is currently applied to the write target file to another distributed storage mode. 
         [0170]    Then the parent NAS server  9  updates the distributed storage mode change management table  95  (S 629 ). Specifically, the parent NAS server  9  creates a new record entry in the distributed storage mode change management table  95 . The parent NAS server  9  stores a portion of the global path extracted from the received write request other than the file name as the global path  951  of the new record. As the file name  952  of the new record, the parent NAS server  9  stores the file name in the global path extracted from the received write request. As the next-applied distributed storage mode  953  of the new record, the parent NAS server  9  stores the identifier of the distributed storage mode that is designated in the received storage mode change request. 
         [0171]    The parent NAS server  9  thus updates the distributed storage mode change management table  95 . This enables the parent NAS server  9  to change the distributed storage mode that is currently applied to the file for which a distributed storage mode change has been requested to another distributed storage mode when a write request for this file is received next time. The parent NAS server  9  then ends this write processing. 
         [0172]    In the case where a record entry that meets the search criterion is found in the distributed storage mode change management table  95  in Step S 626 , the parent NAS server  9  needs to change the distributed storage mode that is currently applied to the write target file to another distributed storage mode. 
         [0173]    The parent NAS server  9  in this case picks up the found record from the distributed storage mode change management table  95 . From the picked up record, the parent NAS server  9  extracts the next-applied distributed storage mode  953 . The parent NAS server  9  next chooses from the distributed storage mode management table  94  a record entry whose adoptable distributed storage mode  943  matches the extracted next-applied distributed storage mode  953 . In the case where a plurality of records in the distributed storage mode management table  94  meet this condition, the parent NAS server  9  chooses one out of these records by any appropriate method. 
         [0174]    From the chosen record, the parent NAS server  9  extracts the NAS name  941 . The parent NAS server  9  next chooses a record entry of the GNS definition table  91  that has the NAS name  912  matching the extracted NAS name  941 . From the chosen record, the parent NAS server  9  extracts the local path  913  (S 630 ). 
         [0175]    The parent NAS server  9  then judges whether or not “B” is stored as the extracted next-applied distributed storage mode  953 . 
         [0176]    In the case where “B” is not stored as the next-applied distributed storage mode  953 , the parent NAS server  9  converts the portion of the global path in the received write request into the extracted local path  913 . The parent NAS server  9  sends the converted write request to the NAS server  1  that is identified by the extracted NAS name  941  (S 631 ). 
         [0177]    Next, the parent NAS server  9  stands by until a write completion notification is received from the NAS server  1 . Receiving a write completion notification from the NAS server  1 , the parent NAS server  9  sends a response to the received write request to the client computer  4 . 
         [0178]    In the case where “B” is stored as the next-applied distributed storage mode  953 , on the other hand, the parent NAS server  9  chooses from the file division policy  92  a record entry whose size  921  matches the size of the write target file. From the chosen record, the parent NAS server  9  extracts the section count  922 . 
         [0179]    The parent NAS server  9  then divides the write target file into as many sections as indicated by the extracted section count  922 . The parent NAS server  9  thus divides one write target file into a plurality of file sections. 
         [0180]    Next, the parent NAS server  9  sets a file section path for identifying a storage area in which a file section is stored. 
         [0181]    The parent NAS server  9  sends to the NAS server  1  a write request that requests to store a file section in the storage area identified by the set file section path (S 631 ). The parent NAS server  9  also updates the file section management table  93 . 
         [0182]    Specifically, the parent NAS server  9  creates a new record entry in the file section management table  93 . The parent NAS server  9  stores the local path  913  extracted in Step S 623  of the write processing as the local path  931  of the new record. As the NAS name  932  of the new record, the parent NAS server  9  stores the identifier of the NAS server  1  that manages a file section identified by the set file section path. As the file section path  933  of the new record, the parent NAS server  9  stores the set file section path. 
         [0183]    After sending the write request, the parent NAS server  9  stands by until a write completion notification is received from every NAS server  1  to which the write request has been sent. Receiving a write completion notification from every NAS server  1  to which the write request has been sent, the parent NAS server  9  sends a response to the received write request to the client computer  4 . 
         [0184]    The parent NAS server  9  next updates the GNS definition table  91  (S 632 ). Specifically, the parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a portion of the global path extracted from the received write request other than the file name. The parent NAS server  9  stores the extracted NAS name  941  as the NAS name  912  of the chosen record. However, in the case where “B” is stored as the next-applied distributed storage mode  953 , the parent NAS server  9  stores no value as the NAS name  912  of the chosen record. As the local path  913  of the chosen record, the parent NAS server  9  stores the extracted local path  913 . 
         [0185]    The parent NAS server  9  thus finishes changing the distributed storage mode that is currently applied to the write target file to another distributed storage mode. 
         [0186]    Next, the parent NAS server  9  updates the distributed storage mode change history table  96 . Specifically, the parent NAS server  9  adds a new record entry to the distributed storage mode change history table  96 . 
         [0187]    The parent NAS server  9  stores the current date and time as the date/time  961  of the new record. As the file path  962  of the new record, the parent NAS server  9  stores the global path extracted from the received write request. As the pre-switch distributed storage mode  963  of the new record, the parent NAS server  9  stores the identifier of the distributed storage mode that has been applied to a file that is identified by the global path extracted from the received write request. As the post-switch distributed storage mode  964  of the new record, the parent NAS server  9  stores the next-applied distributed storage mode  953  extracted. 
         [0188]    The parent NAS server  9  updates the distributed storage mode change history table  96  in the manner described above. 
         [0189]    The parent NAS server  9  next updates the distributed storage mode application state history table  97 . Specifically, the parent NAS server  9  obtains the count of files to which the distributed storage mode A is currently applied, the count of files to which the distributed storage mode B is currently applied, and the count of files to which the distributed storage mode C is currently applied. 
         [0190]    The parent NAS server  9  then adds a new record entry to the distributed storage mode application state history table  97 . The parent NAS server  9  stores the current date and time as the date/time  971  of the new record. 
         [0191]    As the distributed storage mode A application count  972  of the new record, the parent NAS server  9  stores the obtained count of files to which the distributed storage mode A is currently applied. As the distributed storage mode B application count  973  of the new record, the parent NAS server  9  stores the obtained count of files to which the distributed storage mode B is currently applied. As the distributed storage mode C application count  974  of the new record, the parent NAS server  9  stores the obtained count of files to which the distributed storage mode C is currently applied. 
         [0192]    The parent NAS server  9  updates the distributed storage mode application state history table  97  in the manner described above. 
         [0193]    The parent NAS server  9  may send a distributed storage mode change notification to the management computer  5 . Receiving the distributed storage mode change notification, the management computer  5  displays a distributed storage mode change notification screen shown in  FIG. 20 . 
         [0194]    The parent NAS server  9  then ends this write processing. 
         [0195]      FIG. 20  is an explanatory diagram of the distributed storage mode change notification screen which is displayed by the management computer  5  according to the embodiment of this invention. 
         [0196]    The distributed storage mode change notification screen informs that a distributed storage mode applied to a file has been switched to another distributed storage mode. 
         [0197]    The description returns to  FIG. 19 . 
         [0198]    Processing that is performed when the server judges in Step S 622  that one of other servers than itself is the parent NAS server  9  will now be described. The NAS server  1  stores, in the disk subsystem  3 , a file that is contained in the received write request (S 641 ). The NAS server  1  next judges whether or not the distributed storage mode B is currently applied to the stored file (S 642 ). 
         [0199]    In the case where the distributed storage mode B is currently applied to the stored file, the NAS server  1  sends a write completion notification to the parent NAS server  9 , and then ends this write processing. 
         [0200]    In the case where the distributed storage mode B is not applied to the stored file at present, the NAS server  1  identifies from the distributed storage mode selection policy  138  which distributed storage mode is to be applied to the stored file (S 643 ). The NAS server  1  compares the identified distributed storage mode against a distributed storage mode that is currently applied to this file, to thereby judge whether or not the distributed storage mode currently applied to the file needs to be changed to another distributed storage mode. 
         [0201]    When a switch of distributed storage modes is not necessary, the NAS server  1  sends a write completion notification to the parent NAS server  9 , and then ends this write processing. 
         [0202]    When a switch of distributed storage modes is necessary, the NAS server  1  sends a write completion notification that contains a storage mode change request to the parent NAS server  9  (S 644 ). A storage mode change request designates a distributed storage mode that is to be applied after a switch of distributed storage modes. The NAS server  1  then ends this write processing. 
         [0203]      FIG. 21  is a flow chart for write processing that is according to the distributed storage mode B and executed by the parent NAS server  9  according to the embodiment of this invention. 
         [0204]    The write processing according to the distributed storage mode B is executed in Step S 624  of the write processing that is shown in  FIG. 19 . 
         [0205]    First, the parent NAS server  9  judges whether or not the distributed storage mode change management table  95  has a record entry whose combination of the global path  951  and the file name  952  matches the global path extracted in Step S 623  (S 651  and S 652 ). 
         [0206]    When such a record is not found in the distributed storage mode change management table  95 , it means that a distributed storage mode that is currently applied to the write target file does not need to be replaced. Then the parent NAS server  9  chooses from the file division policy  92  a record entry whose size  921  matches the size of the write target file. From the chosen record, the parent NAS server  9  extracts the section count  922 . 
         [0207]    The parent NAS server  9  then divides the write target file into as many sections as indicated by the extracted section count  922  (S 653 ). The parent NAS server  9  thus divides one write target file into a plurality of file sections. 
         [0208]    Next, the parent NAS server  9  selects from the file section management table  93  every record entry whose local path  931  matches the local path  913  extracted in Step S 623  of the write processing that is shown in  FIG. 19 . 
         [0209]    The parent NAS server  9  identifies the selected records one at a time. From the identified record, the parent NAS server  9  extracts the NAS name  932  and the file section path  933 . Next, the parent NAS server  9  sends, to the NAS server  1  that is identified by the extracted NAS name  932 , a write request that requests to store a file section in a storage area that is identified by the extracted file section path  933  (S 654 ). 
         [0210]    The parent NAS server  9  repeats the processing until every record selected from the file section management table  93  is identified. In this manner, the parent NAS server  9  sends write requests each containing one file section to different NAS elements. 
         [0211]    In the case where the file section management table  93  does not have a record entry whose local path  931  matches the extracted local path  913 , the parent NAS server  9  sets a file section path for identifying a storage area in which a file section is stored. 
         [0212]    The parent NAS server  9  sends to the NAS server  1  a write request that requests to store a file section in the storage area identified by the set file section path. 
         [0213]    The parent NAS server  9  also updates the file section management table  93 . 
         [0214]    Specifically, the parent NAS server  9  creates a new record entry in the file section management table  93 . The parent NAS server  9  stores the local path  913  extracted in Step S 623  of the write processing as the local path  931  of the new record. As the NAS name  932  of the new record, the parent NAS server  9  stores the identifier of the NAS server  1  that manages a file section identified by the set file section path. As the file section path  933  of the new record, the parent NAS server  9  stores the set file section path. 
         [0215]    After sending the write request, the parent NAS server  9  stands by until a write completion notification is received from every NAS server  1  to which the write request has been sent. Receiving a write completion notification from every NAS server  1  to which the write request has been sent, the parent NAS server  9  sends a response to the received write request to the client computer  4 . 
         [0216]    Next, the parent NAS server  9  identifies from the distributed storage mode selection policy  138  which distributed storage mode is to be applied to the write target file. The parent NAS server  9  compares the identified distributed storage mode against a distributed storage mode that is currently applied to this file, to thereby judge whether or not the distributed storage mode currently applied to the file needs to be changed to another distributed storage mode (S 655 ). 
         [0217]    When a switch of distributed storage modes is not necessary, the parent NAS server  9  ends this write processing at this point. 
         [0218]    When a switch of distributed storage modes is necessary, the parent NAS server  9  updates the distributed storage mode management table  95  (S 656 ). 
         [0219]    Specifically, the parent NAS server  9  creates a new record entry in the distributed storage mode change management table  95 . The parent NAS server  9  stores a portion of the global path extracted from the received write request other than the file name as the global path  951  of the new record. As the file name  952  of the new record, the parent NAS server  9  stores the file name in the global path extracted from the received write request. As the next-applied distributed storage mode  953  of the new record, the parent NAS server  9  stores the identifier of the identified distributed storage mode. 
         [0220]    The parent NAS server  9  thus updates the distributed storage mode change management table  95 . This enables the parent NAS server  9  to change the distributed storage mode that is currently applied to the file for which a distributed storage mode change is necessary to another distributed storage mode when a write request for this file is received next time. The parent NAS server  9  then ends this write processing. 
         [0221]    In the case where a record entry that meets the search criterion is found in the distributed storage mode change management table  95  in Step S 652 , the parent NAS server  9  needs to change the distributed storage mode that is currently applied to the write target file to another distributed storage mode. In this example, the parent NAS server  9  makes a switch from the distributed storage mode B to the distributed storage mode A or the distributed storage mode C. 
         [0222]    The parent NAS server  9  in this case picks up the found record from the distributed storage mode change management table  95 . From the picked up record, the parent NAS server  9  extracts the next-applied distributed storage mode  953 . The parent NAS server  9  next chooses from the distributed storage mode management table  94  a record entry whose adoptable distributed storage mode  943  matches the extracted next-applied distributed storage mode  953 . In the case where a plurality of records in the distributed storage mode management table  94  meet this condition, the parent NAS server  9  chooses one out of these records by any appropriate method. 
         [0223]    From the chosen record, the parent NAS server  9  extracts the NAS name  941 . The parent NAS server  9  next chooses a record entry of the GNS definition table  91  that has the NAS name  912  matching the extracted NAS name  941 . From the chosen record, the parent NAS server  9  extracts the local path  913  (S 657 ). 
         [0224]    The parent NAS server  9  converts a portion of the global path in the received write request into the extracted local path  913 . The parent NAS server  9  sends the converted write request to the NAS server  1  that is identified by the extracted NAS name  941  (S 658 ). 
         [0225]    Next, the parent NAS server  9  stands by until a write completion notification is received from the NAS server  1 . Receiving a write completion notification from the NAS server  1 , the parent NAS server  9  sends a response to the received write request to the client computer  4 . 
         [0226]    The parent NAS server  9  subsequently updates the GNS definition table  91  (S 659 ). Specifically, the parent NAS server  9  chooses from the GNS definition table  91  a record entry whose global path  911  matches a portion of the global path extracted from the received write request other than the file name. The parent NAS server  9  stores the extracted NAS name  941  as the NAS name  912  of the chosen record. As the local path  913  of the chosen record, the parent NAS server  9  stores the extracted local path  913 . 
         [0227]    The parent NAS server  9  thus finishes changing the distributed storage mode that is currently applied to the write target file to another distributed storage mode. 
         [0228]    Next, the parent NAS server  9  updates the distributed storage mode change history table  96 . The parent NAS server  9  also updates the distributed storage mode application state history table  97 . 
         [0229]    The parent NAS server  9  may notify the management computer  5  of a switch between distributed storage modes. Notified of a switch between distributed storage modes, the management computer  5  displays the distributed storage mode change notification screen shown in  FIG. 20 . 
         [0230]    The parent NAS server  9  then ends this write processing. 
         [0231]      FIG. 22  is an explanatory diagram of file migration according to the embodiment of this invention. 
         [0232]    In this explanatory diagram, the flow of a file in read processing is indicated by a dotted line whereas the flow of a file in write processing is indicated by a solid line. 
         [0233]    The parent NAS server  9  receives a read request from the client computer  4 . The parent NAS server  9  transfers the read request to NAS that stores a read target file. The parent NAS server  9  thus receives the read target file from the NAS, and transfers the received file to the client computer  4 . 
         [0234]    The parent NAS server  9  receives a write request from the client computer  4 , and judges whether or not a distributed storage mode that is currently applied to a write target file needs to be changed to another distributed storage mode. 
         [0235]    When a switch of distributed storage modes is not necessary, the parent NAS server  9  transfers the write request to the NAS that stores the write target file. 
         [0236]    When a switch of distributed storage modes is necessary, the parent NAS server  9  sends a write request containing a part of or the entirety of the write target file to NAS where a post-switch distributed storage mode is adopted. The parent NAS server  9  thus changes the location at which a file is stored. In other words, the parent NAS server  9  can make the write target file migrate without copying. The parent NAS server  9  is also capable of changing the distributed storage mode to be applied to a file. 
         [0237]    Described next is addition of a file system or the NAS server  1  that adopts a specific distributed storage mode. 
         [0238]    The parent NAS server  9  automatically adds a file system that adopts a necessary distributed storage mode when prompted by a given trigger event. 
         [0239]    For instance, when the distributed storage mode management table  94  does not have a record entry whose adoptable distributed storage mode  943  matches the next-applied distributed storage mode  953  in Step S 630  of the write processing, the parent NAS server  9  automatically adds a file system that adopts a necessary distributed storage mode. 
         [0240]    To give another example, when the rate of increase in number of files to which a specific distributed storage mode is applied exceeds a given value, the parent NAS server  9  automatically adds a file system that adopts this distributed storage mode. The parent NAS server  9  calculates the rate of increase in number of files to which a specific distributed storage mode is applied based on the distributed storage mode application state history table  97 . 
         [0241]    After automatic addition of a file system, the parent NAS server  9  sends a GNS environment change notification to the management computer  5 . Receiving the GNS environment change notification, the management computer  5  displays a GNS environment change notification screen shown in  FIG. 23 . 
         [0242]      FIG. 23  is an explanatory diagram of the GNS environment change notification screen which is displayed by the management computer  5  according to the embodiment of this invention. 
         [0243]    The GNS environment change notification screen informs that a file system that adopts a specific distributed storage mode has been added. 
         [0244]    Instead of automatically adding a file system, the parent NAS server  9  may request an administrator to add a file system or NAS that adopts a necessary distributed storage mode. The parent NAS server  9  in this case sends a GNS environment change request to the management computer  5 . Receiving the GNS environment change request, the management computer  5  displays a GNS environment change request screen shown in  FIG. 24 . 
         [0245]      FIG. 24  is an explanatory diagram of the GNS environment change request screen which is displayed by the management computer  5  according to the embodiment of this invention. 
         [0246]    The GNS environment change request screen requests the administrator to add a file system that adopts a specific distributed storage mode. The administrator specifies a size and then instructs the management computer  5  to add this file system. The management computer  5  sends a file system addition request that contains the specified size to the parent NAS server  9 . Receiving the file system addition request, the parent NAS server  9  adds the requested file system that adopts a specific distributed storage mode. 
         [0247]    Described next is how to take a snapshot of a file to which the distributed storage mode B is applied. The usual method cannot be employed in taking a snapshot of a file to which the distributed storage mode B is applied, since employing the usual way of taking a snapshot for this file only yields a snapshot in which the file is partially updated by a write request. 
         [0248]    To avoid this, the parent NAS server  9  takes a snapshot of a file to which the distributed storage mode B is applied by executing snapshot processing that is according to the distributed storage mode B. 
         [0249]      FIG. 25  is a flow chart for snapshot processing that is according to the distributed storage mode B and executed by the parent NAS server  9  according to the embodiment of this invention. 
         [0250]    First, the parent NAS server  9  receives a snapshot obtaining request from the management computer  5  or others (S 661 ). The parent NAS server  9  stops transferring write requests to the NAS servers  1  altogether, in other words, temporarily holds received write requests (S 662 ). 
         [0251]    Next, the parent NAS server  9  makes an inquiry to every NAS server  1  that manages a file to which the distributed storage mode B is applied in order to find out whether the NAS server  1  is performing write processing or not (S 663 ). 
         [0252]    When even one NAS server  1  is performing write processing, it means that data update by a write request is not finished. Then the parent NAS server  9  waits until no NAS server  1  is performing write processing. 
         [0253]    When no NAS server  1  is performing write processing, it means that data update by a write request is finished. Then the parent NAS server  9  sends a snapshot obtaining request to every NAS server  1  (S 664 ). The parent NAS server  9  thus obtains a snapshot from the NAS server  1  (S 665 ). 
         [0254]    The parent NAS server  9  sends the obtained snapshot to the management computer  5  or other components that have sent the snapshot obtaining request. The parent NAS server  9  then ends the snapshot processing according to the distributed storage mode B. 
         [0255]    A modification example of the distributed storage mode B will be described next. 
         [0256]      FIG. 26  is an explanatory diagram of a modification example of the distributed storage mode B according to the embodiment of this invention. 
         [0257]    In the distributed storage mode B where one file is divided into a plurality of sections to store the file sections in a plurality of NAS elements, the client computer  4  cannot access the file when a failure occurs in even one of the NAS servers  1  that manage the file sections. 
         [0258]    The modification example of the distributed storage mode B solves this problem by applying a cluster file system to the distributed storage mode B. In the cluster file system, a plurality of NAS servers  1  share the disk subsystem  3 . Four NAS servers  1  share three disk subsystems  3  in this explanatory diagram. 
         [0259]    When a failure occurs in one of the NAS servers  1  that manage sections of a file, the cluster file system enables the rest of the NAS servers  1  which are not suffering from a failure to obtain file sections from the disk subsystem  3 . The client computer  4  can therefore access the file despite a failure in the NAS server  1 . 
         [0260]    While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.