Abstract:
In a cluster-structured disk subsystem, when creating a volume for an online backup separately from a volume for normal I/O, such creation is desired to be achieved for any volume under the subsystem. Further, with the increase in capacity of the subsystem, it becomes more difficult for a user to determine where to place a volume to which data is copied. The present invention makes it possible to reference/renew snapshot control information in shared memory of other clusters and achieves a snapshot between clusters via an inter-cluster connecting mechanism. Control is performed inside/outside the cluster, and a volume to which data is copied is suggested to the user.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of replication of data in a computer system.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, there have been demands for shortening time which is required to make, in another storage, a copy of data stored in a storage owned by a corporation (hereafter, called a “backup”). This is because while the time required for backups is ever increasing in accordance with the increase in the amount of information owned by the corporation, an actual time assigned for such backups is shortened due to the longer service hour of the corporation.  
           [0005]    As a technique to backup data stored in a storage without interrupting daily operation of the corporation, a snapshot is proposed. The snapshot is a function to make a copy of a storage area in the storage at the particular moment without using a computer connected to the storage. With this function, the user uses an original storage area for business use, and uses data stored in the copied storage area for a backup.  
           [0006]    As a technique to enhance scalability of the storage connected to a network, a cluster-structured storage system can be used. The cluster-structured storage system is a storage system in which a conventional storage system such as a disk array device is regarded as one cluster, and a single storage system comprises a plurality of clusters.  
           [0007]    So far, there are no documents in which execution of a snapshot in the cluster-structured storage system is suggested. Further, a simple combination of the cluster-structured storage system and a conventional snapshot technique results in a technique of copying storage areas within one cluster.  
           [0008]    However, when copies of storage areas cannot be made between different clusters, there exist a storage area to which another storage area can be copied and a storage area to which another storage area cannot be copied in one cluster-structured storage system, ruining scalability, the primary object of the cluster-structured storage system.  
           [0009]    Further, if there are both the storage area from which data is copied and the storage area to which data is copied in the same cluster, performance of such a cluster will not be improved. For example, when a storage area to which data is copied is created in the same cluster, I/O of the operation is performed in the storage area from which data is copied and an online backup is made from the storage area to which data is copied, cache memory contained in one cluster is shared by the operation and the backup. Thus, the backup disturbs normal I/O, which results in the delay in the operation. Further, if loads are concentrated on a particular cluster, overall performance will also be deteriorated.  
         SUMMARY OF THE INVENTION  
         [0010]    The object of the present invention is to provide a cluster-structured storage system in which a copy of a storage area can be made freely without concern for clusters.  
           [0011]    Further, in the cluster-structured storage system in which a copy of a storage area can be made freely without concern for clusters, a user may be at a loss where to reserve a storage area to which data is copied.  
           [0012]    The second object of the present invention is to help the user determine a storage area to which data is copied.  
           [0013]    In order to solve the above problems, in one preferred embodiment of the present invention, there is provided a means to achieve a snapshot between clusters in the cluster-structures storage system. To be specific, the present invention makes it possible to reference information such as unassigned volumes of the cluster stored in memory of each cluster so that each cluster can acquire information such as unassigned volumes of other clusters.  
           [0014]    Further, when the user determines a volume to which a snapshot is copied, there is provided a means to suggest which volume to select by using a computer connected to the cluster-structured storage system.  
           [0015]    Also, in an alternative embodiment of the present invention, when the cluster-structured storage system executes a snapshot, the computer connected to the cluster-structured storage system recognizes in which cluster a volume from which data is copied exists. Further, a means is provided to suggest to the user that, of all the unassigned volumes in the disk subsystem, a volume in a cluster different from the cluster in which the volume from which data is copied exists should be used as a volume to which data is copied.  
           [0016]    Also, to avoid the biased backup load between clusters, a means is provided in which a computer connected to the cluster-structured storage system refers to the monitored result of the load in the disk subsystem and, being based on the result of the load, a volume to which data is copied is proposed.  
           [0017]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram showing an embodiment of a computer system of the present invention;  
         [0019]    [0019]FIG. 2 shows an example of a pair information table;  
         [0020]    [0020]FIG. 3 shows an example of a volume information table;  
         [0021]    [0021]FIG. 4 shows a listing table for pair information table storing addresses of other clusters;  
         [0022]    [0022]FIG. 5 is a flowchart showing a procedure of executing a snapshot;  
         [0023]    [0023]FIG. 6 is a processing flowchart of a snapshot in a cluster;  
         [0024]    [0024]FIG. 7 is a processing flowchart of a true volume cluster of a snapshot between clusters;  
         [0025]    [0025]FIG. 8 is a processing flowchart of a sub-volume cluster of a snapshot between clusters;  
         [0026]    [0026]FIG. 9 shows a block diagram of a program for suggesting a volume to which data is copied;  
         [0027]    [0027]FIG. 10 shows an example of a sub-volume selection-support screen; and  
         [0028]    [0028]FIG. 11 shows an example of a sub-volume selection-support screen. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    [0029]FIG. 1 is a block diagram showing an embodiment of a computer system of the present invention.  
         [0030]    The computer system comprises a cluster-structured storage system  1 , a computer (hereafter called a “server”) using data stored in the cluster-structured storage system  1 , and a computer (hereafter called a “user input device”)  4  for controlling the cluster-structured storage system  1 . Also, these devices are connected to each other via a network  2 .  
         [0031]    The cluster-structured storage system  1  comprises a plurality of clusters  11  and an inter-cluster connecting mechanism  12  connecting the clusters  11 . Further, the inter-cluster switching mechanism  12  may be a switch or a communication line.  
         [0032]    The cluster  11  comprises a control section  10 , a plurality of disk units  15 , and a network interface  18 . The cluster  11  is connected to the server  3 , which is its host computer, and the user input device  4  via the network  2  and the network interface  18 .  
         [0033]    The control section  10  comprises a memory unit  19 , CHA (CHannel Adapter)  13 , and DKA (DisK Adapter)  14 .  
         [0034]    The CHA  13  and the DKA  14  are processors mounted in the control section  10 . The CHA  13  conducts an analysis of commands inputted from the server  3  to the cluster  11  via the network interface  18 , and executes a program for transferring data between the server  3  and the cluster  11 . Also, it is possible for a single processor to serve as both of the CHA and DKA. In this case, designing will be easier compared to the case of a structure using a plurality of processors.  
         [0035]    When the cluster-structured storage system  1  corresponds to the server  3  in which an operating system of so called an open system is put into effect, as hardware and a program related to the interface  18  between the server  3  and the cluster-structured storage system  1 , the one corresponds to the operating system of the open system is adopted.  
         [0036]    The DKA  14  carries out control of a disk array such as generating parity, and executes a program to control transferring data between the disk unit  15  and the control section  10 . The disk unit  15  has a plurality of ports, and is connected to different DKAs  14  via a plurality of paths. Therefore, the disk unit  15  can be accessed from any DKA  14 .  
         [0037]    The memory unit  19  comprises shared memory  17  and cache memory  16 , which can be accessed from processors such as CHA and DKA. Those processors store data, which should be shared among them such as information necessary for job management and information for controlling cache memory, in the shared memory  17 .  
         [0038]    Stored in the shared memory  17  are volume pair information  172  to be used when executing a snapshot, a process request queue  173 , and other cluster information storing position  171  which is the information necessary during the interaction between clusters. A “pair” means a set of a storage area from which data is copied (hereafter called an “original volume”) and a storage area to which data is copied (hereafter called a “target volume”).  
         [0039]    The DKA  14  has, in its memory, snapshot execution  141  which is a program for executing a snapshot, inter-cluster information access  142  which is a program for accessing information between clusters, and inter-cluster communication  143  which is a program for communication between clusters.  
         [0040]    The user input device  4  is a device to be used by the user of the cluster-structured storage system  1  to indicate a target volume to the cluster-structured storage system  1  when the user makes the cluster-structured storage system  1  execute the snapshot. To be specific, it is a service terminal such as SVPs. The user input device  4  comprises an operation part, memory, an interface with a network, and a display screen  41 . The operation part of the user input device  4  executes a program stored in the memory, presents candidate target volumes to a user on the display screen  41  to visually support the user when the user selects the target volume. Also, the server  3  and the user input device  4  may be the same computer.  
         [0041]    [0041]FIGS. 2 and 3 show the configuration of volume pair information  172  to be stored in the shared memory  17 . The volume pair information  172  comprises a pair information table  21  and a volume information table  31 .  
         [0042]    Hereafter, an original volume is called a “primary volume” and a target volume is called a “secondary volume.” 
         [0043]    The pair information table  21  contains a pair number entry  22 , a primary volume number entry  23 , a secondary volume volume number entry  24 , a secondary volume cluster number entry  25 , a pair state entry  26 , and a copy pointer entry  27 .  
         [0044]    Information to show a pair of a primary volume and a secondary volume, namely, a pair number is registered in the pair number entry  22 . Information to show a primary volume constituting a corresponding pair is registered in the primary volume number entry  23 . Information to show numbers of a cluster to control the secondary volume and the volumes are registered in the secondary volume volume number entry  24  and the secondary volume cluster number entry  25 .  
         [0045]    Registered in the pair state entry  26  is information to show whether the corresponding pair is being copied or has been copied. Registered in the copy pointer entry  27  is information to show how far the primary volume of the corresponding pair has been copied.  
         [0046]    In the volume information table  31 , information to be used for a snapshot is registered. The volume information table  31  contains a volume number entry  32 , a primary/secondary entry  33 , a secondary volume volume number entry  34 , a secondary volume cluster number entry  35 , and a volume attributes entry  36 .  
         [0047]    Registered in the volume number entry  31  is information to specify a storage area (hereafter called a “volume”) contained in the cluster-structured storage system  1 . Registered in the primary/secondary entry  33  is information to show whether a corresponding volume is a primary volume or a secondary volume. When the corresponding volume is the primary volume, information on the secondary volume constituting the pair is registered in the secondary volume volume number entry  34  and the secondary volume cluster number entry  35 . Registered in the volume attributes entry  36  is information to show whether the corresponding volume can be set as a target volume of the snapshot.  
         [0048]    As an example, FIG. 3 shows a case in which three pairs are produced with respect to a volume whose volume number is “0.” The counterparts of the volume whose volume number is “0” are defined as volume Nos. 20, 158, and 426 of the cluster No. 1.  
         [0049]    In order to achieve a snapshot between clusters, every cluster  11  has a pair information table  21  and a volume information table  31 . The volume information table  31  of a cluster  11  has the information of all the volumes contained in the cluster  11 . On the other hand, in the pair information table  21  of a cluster  11 , only the information of a pair related to the cluster  11  is registered. Therefore, the information of pairs that the whole cluster-structured storage system  1  includes is distributed to and held by each cluster. For example, in a cluster  11  in which a primary volume of a snapshot is physically stored, pair information corresponding to the stored primary volume is stored.  
         [0050]    [0050]FIG. 4 shows contents of other cluster information-storing position table  171  stored in the shared memory  17 .  
         [0051]    In the present invention, memory space of the shared memory  17  contained in each cluster  11  is treated virtually as one memory space. In this case, the memory space of each cluster  11  may be sequentially allocated to single virtual memory space, or may be allocated independently.  
         [0052]    The cluster information-storing position table  171  contains a cluster number entry  82 , a top address entry  83  of the pair information table, and a top address entry  84  of the volume information table. In the entry  82 , cluster numbers to specify clusters are registered.  
         [0053]    Registered in the entry  83  is a top address to indicate a storing position of the pair information table in memory space of the virtual shared memory. Registered in the entry  84  is a top address which contains a storing position of the volume information table in memory space of the virtual shared memory.  
         [0054]    When executing a snapshot between clusters, the DKA  14  has to reference/renew snapshot information (the information stored in the pair information table  21  and the volume information table  31 ) of other clusters. In this case, according to a volume number indicating a volume to be accessed and a cluster number indicating a cluster  11  controlling the volume, the DKA  14  searches the other cluster information-storing position table  171 , and calculates a position in which information on the corresponding volume is registered.  
         [0055]    For example, volume information on the volume of cluster No. 1 and volume No. 4 is stored in the shared memory  17  of a cluster  11 , which corresponds to an address in virtual shared memory space calculated by the following expression.  
         [0056]    Top address for storing volume information table of cluster No. 1+Amount of information per volume×4  
         [0057]    As described above, by treating all the shared memory  17  contained in the cluster-structured storage system  1  as virtually one memory space, it becomes possible for the DKA  14  to reference/renew snapshot information stored in the shared memory  17  of all the clusters  11  of the cluster-structured storage system  1 .  
         [0058]    [0058]FIG. 5 is a flowchart showing a procedure of executing a snapshot in the cluster-structured storage system  1 .  
         [0059]    First, with the user input device  4 , a user selects an original volume and a target volume for the snapshot, regards the selected volumes as a copy pair, and specifies a corresponding pair number. The procedure in detail of the selection of the target volume will be described later (step  4001 ).  
         [0060]    When the pair is specified, in order to allow the cluster-structured storage system  1  to execute the copying with respect to the pair, the server  3  gives the cluster-structured storage system  1  a command for requesting to start the snapshot. This command is issued to a cluster  11  having a true volume of the determined pair (step  4002 ).  
         [0061]    Upon receipt of the command, the CHA  13  of the cluster  11  analyzes the command, creates a job that can be processed by the DKA  14 , and registers it in a process request queue  173  of the shared memory  17  as a process request queue. The process request queue may be the same as that of normal I/O or a queue exclusively used for a snapshot (step  4003 ).  
         [0062]    The DKA  14  which is not executing a job examines contents of the process request queue  173  at determined time intervals. When a queue to indicate the unexecuted process is registered in the process request queue  173 , the DKA  14  acquires the queue indicating the process and executes a snapshot corresponding to that queue. Further, when the specified true volume is already processing another snapshot, the DKA  14  waits until the processing is over or notifies a user of the situation. The DKA  14  examines contents of a pair state entry  26  corresponding to the specified primary volume, and checks if another snapshot is being processed (step  4004 ).  
         [0063]    Now, the snapshot processing in step  4004  will be described.  
         [0064]    [0064]FIG. 6 is a flowchart to show a procedure of creating a target volume in the cluster  11  where the original volume exists. In this case, one cache memory  16  contained in the cluster  11  is used for the snapshot processing.  
         [0065]    Hereafter, storage capacity of the whole original volume is indicated as “n,” and the amount of data to be transferred from the original volume to the target volume in a single process is indicated as “m” data blocks. Further, the amount of data of one block is typically 512 bytes.  
         [0066]    The DKA  14  of the cluster  11  checks to see, among data stored in the primary volume, whether the first m data blocks to be copied exist in the cache memory  16  (step  7001 ). When the data blocks do exist, the DKA  14  further checks how many of m data blocks remain in the cache memory  16  (step  7002 ).  
         [0067]    When even one data block of m data blocks is missing in the cache memory  16 , the DKA  14  reserves an area in the cache memory  16  to read the rest of the data blocks from the disk unit  15  and record it to the cache memory  16  (step  7004 ). Then, the DKA  14  reads out the rest of the data blocks from the disk unit  15  to the reserved area in the cache memory  16  (step  7005 ).  
         [0068]    Upon completing the readout of the data block of the primary volume, DKA reserves a storage area to which the data block is copied in the area corresponding to the secondary volume of the cache memory  16  (step  7006 ). When the storage area is reserved, the DKA  14  transfers m data blocks from the area of the cache memory  16  corresponding to primary volume to the area of the cache memory  16  corresponding to the secondary volume. Also, when copying data, identifiers have to be given to the data independently. Since different identifiers are given to data from different volumes, data are transferred while their identifiers being converted (step  7007 ).  
         [0069]    On the other hand, when it is determined in step  7002  that m data blocks are all ready in the cache memory  16 , DKA carries out steps  7006  and  7007  without any adjustment.  
         [0070]    When it is determined that data blocks to be copied do not exist at all in the cache memory  16  in step  7001 , the DKA  14  reserves a storage area as much as the amount of data to be copied to the secondary volume side, namely, a storage area for m data blocks, in an area corresponding to the secondary volume of the cache memory  16  (step  7008 ).  
         [0071]    The DKA  14  then reads m data from the disk unit  15  on the primary volume side, and transfers the data to an area of the cache memory  16  on the secondary volume side while converting identifiers (step  7008 ).  
         [0072]    When the data is stored in the area corresponding to the secondary volume of the cache memory  16 , the DKA  14  starts the process of writing data on the disk unit  15  corresponding to the secondary volume. First, the DKA  14  checks to see, at predetermined time intervals, if the amount of data not written into the disk unit  15  in the cache memory  16  exceeds a predetermined boundary value (steps  7010  and  7017 ).  
         [0073]    When the amount of data is less than the boundary value, the DKA  14  checks that the predetermined time has passed, and checks again if the amount of data has exceeded the boundary value (step  7011 ).  
         [0074]    When the amount of data is equal to or over the boundary value, in order to generate parity of the data on the secondary volume side, the DKA  14  reserves a storage area of the cache memory  16  for the amount of data corresponding to the parity (step  7012 ). Then, the DKA  14  carries out generation of parity (step  7013 ).  
         [0075]    The DKA  14  then checks if the data stored in the cache memory  16  with its parity generated is as much as the amount of data of the unit to be written to the disk unit  15  (step  7014 ). If the predetermined amount of data has been achieved, the DKA  14  writes the data of the cache memory  16  to the disk unit  15  (step  7015 ). If the predetermined amount of data has not been achieved, the DKA  14  repeats the process of step  7014 .  
         [0076]    Then, the DKA  14  checks to see if the copied amount of data has reached “n” (step  7016 ). If the amount of data has not reached “n”, the DKA  14  repeats the process of step  7001  and succeeding steps until the copied amount of data reaches “n,” and completes the process when the amount of data reaches “n.” Further, in step  7016 , pair state  26  of the volume pair information  21  and a value of the copy pointer  27  are renewed.  
         [0077]    The amount of data “n” is determined optionally. However, in general, it corresponds to the amount of a logical volume provided to the external device by the cluster-structured storage system. In the present embodiment, one cache memory  16  is divided to be used on the primary volume side and on the secondary volume side. However, the cache memory  16  may be used without being divided.  
         [0078]    Now, the process of a snapshot between clusters creating a target volume in a cluster  11  which is different from an original volume will be described.  
         [0079]    [0079]FIG. 7 is a flowchart showing steps of processing the cluster  11  of the primary volume in the snapshot between clusters.  
         [0080]    The process from step  5001  to step  5009  is the same as the process from step  7001  to step  7009 . However, the process from step  5006  to step  5009  extends over the cluster  11 . In this case, the DKA  14  transfers data via an inter-cluster connecting mechanism  12 . Further, the cluster  11  of the primary volume can use, as cache memory  16  to which data is transferred, either cache memory  16  contained in the cluster  11  on the primary volume side or cache memory  16  contained in the cluster  11  on the secondary volume side. The following example will be discussed on the assumption that the cache memory  16  contained in the cluster on the secondary volume side is used.  
         [0081]    When the DKA  14  of the cluster  11  of the primary volume reserves a storage area of the cache memory  16  in the cluster  11  of the secondary volume or transfers data to the cache memory  16  in the cluster  11  of the secondary volume, the DKA  14  of the cluster  11  of the primary volume directly gains access to shared memory  17  in the cluster  11  of the secondary volume via the inter-cluster connecting mechanism  12 . Where to access in the shared memory  17  is determined according to the previously described method and by calculating a virtual address of the shared memory  17  to be accessed.  
         [0082]    Upon completing the transfer of m data blocks to the cache memory  16  of the cluster  11  of the secondary volume, the DKA  14  of the cluster  11  of the primary volume checks to see if there is next data (step  5010 ). If there is, the DKA  14  returns to step  5001  and repeats the transfer. If there isn&#39;t, the DKA  14  completes the process.  
         [0083]    [0083]FIG. 8 is a flowchart of a snapshot in the cluster  11  of the secondary volume.  
         [0084]    In the cluster-structured storage system  1 , processing between clusters  11  is carried out independently. Therefore, the cluster  11  having a storage area to be a secondary volume cannot determine whether the snapshot has started in the cluster  11  of the primary volume, in particular, whether data has been transferred to its own cache memory  16 . Therefore, according to the present embodiment, the DKA  14  of the cluster  11  of the primary volume notifies the DKA  14  of the cluster  11  of the secondary volume of timing for executing the snapshot.  
         [0085]    For example, the DKA  14  of the cluster  11  of the primary volume stores a message to start the DKA  14  of the cluster  11  of the secondary volume in an exclusive queue for massages (not shown) provided in the shared memory  17 . The DKA  14  of the cluster  11  of the secondary volume periodically checks if a message is registered in the exclusive queue for massages. If there is a message, the DKA  14  of the cluster  11  of the secondary volume acquires the message, and starts a job corresponding to the snapshot processing.  
         [0086]    Upon acknowledging the message from the cluster  11  of the primary volume, the DKA  14  of the cluster  11  of the secondary volume references control information of cache memory in the shared memory  17  of the cluster  11  and checks if data to be copied is accumulated in its own cache memory  16 . If the data has not been accumulated, the DKA  14  of the cluster  11  of the secondary volume waits a predetermined period of time, and checks again (steps  6001  and  6002 ).  
         [0087]    In the case where data is accumulated in the cache memory  16 , in order to generate parity corresponding to the data stored in the cache memory  16 , the DKA  14  of the cluster  11  of the secondary volume reserves a storage area of the cache memory  16  for the parity (step  6003 ).  
         [0088]    Then, the DKA  14  of the cluster  11  of the secondary volume generates parity for the data stored in the cache memory  16  (step  6004 ). When data is stored up to a predetermined unit in the cache memory  16 , the DKA  14  of the cluster  11  of the secondary volume stores the data in the disk unit  15  (step  6005  and  6006 ).  
         [0089]    In the present embodiment, when the DKA  14  of the cluster  11  gains access to information stored in the shared memory  17  of the other cluster  11 , an inter-cluster connecting mechanism  12  is used. However, when the inter-cluster connecting mechanism  12  has a fault and so on, the DKA  14  of the cluster  11  gains access to the shared memory  17  of the other cluster  11  via a network  2 . In that case, it is necessary to define in advance which one of communication paths to the network  2  to use for transfer.  
         [0090]    When a snapshot is carried out, the cluster  11  on the primary volume side references dirty amount in the cluster  11  of the secondary volume, and executes the snapshot while watching that the dirty amount does not exceed the threshold value. Among data in cache memory, “dirty data” is the one which is not reflected in the disk unit  15 . Also, “dirty amount” is the total amount of the dirty data in all the cache memory in the cluster.  
         [0091]    When executing the snapshot in the cluster-structured storage system  1 , first, a user has to select a secondary volume to which data is copied. Now, a method to support the selection of a secondary volume in the cluster-structured storage system  1  will be described.  
         [0092]    A user input device  4  provides the user with information on a volume to which the snapshot is taken by executing a program  42 , shown in FIG. 9, for presenting a target volume. The user input device  4  is, for example, a personal computer or a notebook-sized personal computer.  
         [0093]    The user input device  4  recognizes the cluster-structured storage system  1  via the network  2 . Also, the user input device  4  may be directly connected with each cluster  11  contained in the cluster-structured storage system  1  using a dedicated line. In such a case, exchange of data between the user and the cluster-structured storage system  1  does not affect the traffic load of the network  2 .  
         [0094]    A target volume presenting program  42  comprises a program  191  for acquiring data in the cluster which acquires constituent information (such as unassigned volume numbers) of each cluster  11  of the cluster-structure storage system  1 , a suggested volume control program  192  selecting a volume to be suggested to the user from the acquired constituent information of the cluster  11  and various conditions (such as a request to select a target volume inside/outside the cluster of the original volume), and a subprogram of a display program  193  for displaying the suggested volume on the display screen  41 .  
         [0095]    The data acquisition program  191  is executed when acquiring unassigned volume information stored in the target cluster-structured storage system  1 . Further, unassigned volume information is acquired from all the clusters  11 . Upon acquiring of data, an exclusively command is used, and volume state information, which indicates whether the volume stored in the shared memory  17  via the CHA  13  is available, is acquired.  
         [0096]    Also, there is another method to select an unassigned volume in the cluster  11  and give information on the unassigned volume alone to the user input device  4 . This method takes some of the load off the user input device  4 .  
         [0097]    Based on acquired data of the cluster structure, the suggested volume control program  192  is executed to control candidate volumes according to each cluster  11 .  
         [0098]    The display program  193  is a program to be executed when suggesting a volume to the user visually with a display screen  41 .  
         [0099]    The user input device  4  accepts input such as a change in display mode from the user via an interface for user input. With respect to contents displayed on the display screen  41 , when receiving input from the user, the user input device  4  executes the suggested volume control program  192  to process the input from the user and displays the result again on the display screen  41 .  
         [0100]    For example, when the user wishes candidates of the secondary volume to be suggested within the same cluster  11 , the user inputs accordingly. Then, the user input device  4  executes a candidate selection program  1921  according to the cluster in the suggested volume control program  192  to select the candidate volume and displays it on the display screen  41 .  
         [0101]    Further, the user input device  4  provides, when a particular volume of the volumes suggested above is selected and copied, information predicting how much the operation is delayed, selects the candidate volume according to such prediction, and displays it on the screen. For example, the user input device  4  acquires from history monitor information of the cluster-structured storage system  1  the number of I/O to and from the volume contained in the cluster-structured storage system  1  per last one hour (or specified time period) and the amount of data transferred.  
         [0102]    Assuming that the cluster-structured storage system  1  is in a state based on the acquired information, the user input device  4  simulates how much load is given in this state when a process to create a copy volume in the volume specified by the user is added to the cluster-structured storage system  1 , and how much the I/O is usually delayed. Then, according to the result, the user input device  4  puts candidate volumes in order of priority and suggests it to the user.  
         [0103]    [0103]FIG. 10 shows an example in which, when input data  92  inputted by the user are a cluster number and a volume number of the original volume, candidates for a target volume are narrowed and displayed on the display screen  41  by the user input device  4  so that the user can easily select the target volume.  
         [0104]    In FIG. 10, unassigned volume numbers in the cluster  11  where the original volume exists are listed and displayed. According to a display mode  93 , the user can select either a list mode (FIG. 10) or a view mode (FIG. 11).  
         [0105]    In the display of the view mode (FIG. 11), physical positional relationship of volumes is displayed diagrammatically according to each cluster  11 . Therefore, the user can recognize unassigned volumes, etc. at a glance. Further, in the display of the view mode (FIG. 11), the user can easily decide whether to make a copy in the cluster  11  where the original volume exists or to make a copy in a different cluster  11 .  
         [0106]    The user input device  4  may suggest to the user all the volumes which can be candidates of secondary volumes, or may narrow the candidates of secondary volumes and suggest the fewer volumes to the user. Further, the user input device  4  may have a function to select one volume from the previously described candidates of secondary volumes.  
         [0107]    When the user wishes to decide a use for each cluster and store a volume, it is possible for the user input device  4  to select the volume according to the decision of the user. For example, when the user wishes to store data of Company A in a cluster  1  and data of Company B in a cluster  2 , the user input device  4  acquires in advance information on a correlation between the clusters and the companies from the user. When the user newly creates a volume, according to a company name specified by the user, the user input device  4  recognizes a proper cluster and suggests an unassigned volume in the cluster.  
         [0108]    As an alternative embodiment, it is possible for the user to specify the usage of storing a primary volume in the cluster  1  and storing a secondary volume in the cluster  2 . In this case, the user can use each cluster for a different purpose such as the cluster  1  accepting normal I/O and the sub-cluster being used for test I/O, etc. With this usage, performance of the normal I/O does not deteriorate. Further, even when a cluster on the test side has a fault, the user can close the cluster without affecting the cluster of the normal I/O at all. Further, the cluster  2  serves as a backup of the cluster  1 . Thus, by making a backup in another cluster, the availability of the system as a whole is enhanced.  
         [0109]    Further, when a particular cluster is under heavy load conditions, the user input device  4  may suggest that the user select a secondary volume in a different cluster so that the load is distributed all over the cluster-structured storage system  1 .  
         [0110]    According to the present invention, without concern for clusters, the user can copy data by a snapshot function to any volume of any cluster in the cluster-structured storage system. Thus, scalability of the cluster-structured storage system is enhanced.  
         [0111]    The foregoing invention has been described in terms of preferred embodiments. However, those skilled, in the art will recognize that many variations of such embodiments exist. Such variations are intended to be within the scope of the present invention and the appended claims.