Abstract:
A storage system includes a primary volume and a first controller that stores data in a primary volume and stores content at a prescribed point in time in the form of differential data. The first controller generates generational management information corresponding to the differential data of the prescribed point in time and manages the generational management information. The storage system also includes a secondary volume configured to store backup data and a second controller configured to receive data sent from the first controller, store data to the secondary volume, store the generational management information, receive a request for a certain generation of the generations and provide data of the primary volume corresponding to the requested generation based on the request. The first controller determines to send differential data and the generational management information to the second controller when an amount of differential data reaches a prescribed amount.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of U.S. application Ser. No. 10/852,218 filed May 25, 2004, and issued Sep. 18, 2007 as U.S. Pat. No. 7,272,693. Priority is claimed based on U.S. application Ser. No. 10/852,218 filed May 25, 2004, which claims the priority of Application No. JP 2004-102377 filed on Mar. 31, 2004, all of which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a storage system capable of generational management, and a back-up method for a storage system. 
     2. Description of the Related Art 
     For example, in various organizations, such as businesses, self-governing bodies, schools, or the like, large volumes of data of various different types are managed. These large volumes of data are managed by a storage system formed separately from a host computer. The storage system is constituted by comprising at least one or more storage device, such as a disk array device, for example. 
     A disk array device is constituted by arranging storage devices, such as hard disk drives, or semiconductor memory devices, for example, in an array configuration. The disk array device provides a storage region based on a RAID (Redundant Array of Independent Inexpensive Disks). 
     In the storage system, by managing the storage contents of a main volume, separately from the main volume, provision is made for cases in which data in the main volume is lost, or the like. For example, as one such method, a technique is known whereby the contents of the volume at a prescribed point in time are saved, by copying the storage contents of the main volume, exactly, to a secondary volume (Japanese Patent Laid-open No. (Hei)11-259348). In another such method, a technique is also known whereby an image (snapshot), which takes a static impression of the main volume at a prescribed point in time, is acquired, in such a manner that the data of the main volume can be managed in a plurality of generations (Japanese Patent Laid-open No. 2002-278819). 
     If the storage contents of the main volume are copied exactly to another volume, then a back-up volume of the same volume of the main volume is required, in order to store back-up data for a single generation. Consequently, if the back-up data is managed by means of a plurality of generations, then a large number of volumes are required and hence the back-up costs increase. Moreover, each time a new-generation volume is created, then it is necessary to execute an initial copy for transferring the whole volume, and hence the system load increases. 
     If the storage contents at a particular point in time are taken as a reference, and a differential snapshot is used to manage the differential only, then the required storage contents are relatively small compared to the aforementioned method, and back-up data can be managed in the form of a plurality of generations. However, the actual data used for managing generations on the basis of differential snapshots is not backed up, and there only exists one version of this data. Furthermore, the data for generational management cannot serve the purpose of data back-up on its own. The user is able to restore the data of a desired generation by referring to both the main volume forming the reference, and the data used for generational management. 
     Therefore, if a fault occurs in the main volume, it is not possible to restore data simply by means of the data for generational management in the differential snapshot. Furthermore, in the event that the data for generational management has been lost, it becomes impossible to revert to the storage contents of the main volume at a prescribed point of time in the past. 
     SUMMARY OF THE INVENTION 
     The present invention was devised with the foregoing problems in view, one object thereof being to provide a storage system and a back-up method for a storage system whereby it is possible also to back up generational management information from a copy source, to a copy destination. It is a further object of the present invention to provide a storage system whereby data management of a plurality of data generations is carried out respectively in both a copy source and a copy destination. 
     In order to achieve the aforementioned object, in the present invention, it is possible to manage data of a plurality of generations, respectively, in both a copy source and a copy destination, by also backing up the generational management information of the copy source disk array device to the copy destination disk array device. 
     The storage system according to the present invention comprises a first disk array device and a second disk array device mutually connected. (1) The first disk array device comprises: a first volume; a first withdrawal volume for storing data withdrawn from the first volume; a generational management control section for generating generational management information which records the storage contents of the first volume at a prescribed point in time, in the form of differential data, and managing generations of back-up data in the first volume; a first storing section for storing the generational management information generated by the generational management control section; a generational management information transferring section for transferring the generational management information stored in the first storing section to the second disk array device; and a volume transferring section for respectively transferring the storage contents of the first volume and the first withdrawal volume, to the second disk array device. (2) The second disk array device comprises: a second volume to which the storage contents of the first volume received from the volume transferring section are copied; a second withdrawal volume to which the storage contents of the first withdrawal volume received from the volume transferring section are copied; and a generational management information duplicating section which causes the generational management information received from the generational management information transferring section to be copied to a second storing section. 
     Here, the first disk array device and the second disk array device may be respectively constituted by comprising, for example, a channel adapter for controlling transmission and reception of data to and from an external device (a corresponding display device or host computer), a disk adapter for controlling transmission and reception of data to and from a storage device providing a storage volume, and a memory shared by the aforementioned channel adapter and disk adapter (a cache memory or control memory). When data is withdrawn from the first volume to the first withdrawal volume, for example, the disk adapter reads out the data to be withdrawn, temporarily, to the cache memory, and then writes the data to a prescribed position in the first withdrawal volume. Furthermore, if the first volume is copied to a second volume, or if the first withdrawal volume is copied to a second withdrawal volume, then the disk adapter reads out the data to be copied, to the cache memory. The channel adapter reads in the data from the cache memory, and transmits it to the copy destination disk array device, via a communications network. 
     The generational management information may be constituted by a set of a plurality of partial information elements, such as a differential bitmap table, or a withdrawal destination address management table, or the like, for example. A differential bitmap table contains information for managing whether or not data has been updated, with respect to each demarcated block of a prescribed size. The withdrawal destination address management table is used for managing the address to which the data prior to updating is withdrawn. 
     According to the composition described above, the main volume, the first withdrawal volume and the generational management information are each copied respectively to the second disk array device forming the copy destination. Therefore, the back-up data can be managed respectively in a plurality of generations, in both the first disk array device forming the copy source, and the second disk array device forming the copy destination. 
     Moreover, the generational management information records the storage contents of the first volume in the form of differential data. Therefore, in contrast to a method in which the contents of the volume at any given point in time are copied completely and exactly, only the difference with respect to the previous generation needs to be managed, for example, and therefore the amount of storage capacity required for back up can be reduced, and the data transfer time can also be shortened. 
     In the present embodiment, the volume transfer section is composed in such a manner that it can transmit the differential in the first volume and the differential in the first withdrawal volume, respectively to the second disk array device. Transmitting the differential of the volume means, for example, that only the portion that has changed in comparison to the contents in the case of the previous transmission (namely, the differential), is transmitted. By copying the differential only in this way, it is possible to reduce the amount of data transferred. 
     In the present embodiment, the transfer of the respective differentials of the first volume and the first withdrawal volume by means of the volume transferring section, and the transfer of the generational management information by means of the generational management information transferring section are respectively carried out simultaneously. Thereby, the management of the respective generations of the first disk array device (copy source) and the second disk array device (copy destination) can be synchronized. 
     In the present embodiment, prior to transferring the generational management information to the second disk array device, the generational management information transferring section reports the data size of the respective partial information elements constituting the generational management information, respectively, to the generational management information duplicating section, and the generational management information duplicating section manages the storage position of the generational management information on the basis of the data size of each partial information element. For example, the partial information, such as the differential bitmap table, the withdrawal destination address management table, and the like may be stored in a distributed fashion in a plurality of storage devices, such as the memory, disks, or the like, of the first disk array device. Alternatively, the respective partial information elements may be stored respectively in different locations of the same storage device. By previously reporting the data size of each partial information element, from the generational management information transferring section to the generational management information duplicating section, the generational management information duplicating section is able to reserve a storage area for storing the respective partial information elements, and to manage the storage positions of the respective partial information elements. 
     In the present embodiment, the generational management information transferring section is composed in such a manner that it can transfer the generational management information for any previously selected generation, to the second disk array device. 
     In the present embodiment, a generation restoring section is provided in the second disk array device for restoring the storage contents of a designated generation, by referring to the second volume and the second withdrawal volume, on the basis of the generational management information copied to the second storing section. 
     In the present embodiment, a restoring section capable of transferring the storage contents respectively stored in the second volume, the second withdrawal volume and the second storing section, to the first disk array device, is provided in the second disk array device. 
     The present invention may also be realized in the form of a computer program for causing the first disk array device to execute prescribed functions (generational management control functions, generational management information transfer functions, volume transfer functions), or a computer program for causing the second disk array device to execute prescribed functions (generational management information duplication functions, generation restoring functions, restoring functions). This program may, for example, be distributed by being recorded onto a storage medium of various types, such as a hard disk device, a semiconductor memory device, or the like. Alternatively, this program may be distributed by means of a communications network, for example. 
     Further objects of the present invention will become apparent from the following description of the embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative diagram showing an overview of an embodiment of the present invention; 
         FIG. 2  is an external view of a disk array device which can be used in the present invention; 
         FIG. 3  is a block diagram showing an example of the composition of a disk array device; 
         FIG. 4  is a block diagram showing the overall composition of a storage system in which all data, including generational management information in the copy source, is backed up to a disk array device forming a copy destination; 
         FIG. 5  is an illustrative diagram showing the composition of tables, wherein (a) shows a differential bitmap table, and (b) shows a withdrawal destination address management table; 
         FIG. 6  is an illustrative diagram showing a schematic view of the copying of generational management information; 
         FIG. 7  is a flowchart showing back-up processing; 
         FIG. 8  is a flowchart showing processing for creating a virtual volume; 
         FIG. 9  is a flowchart showing back-up processing relating to a second embodiment of the present invention; 
         FIG. 10  is a block diagram of a storage system relating to a third embodiment of the present invention; and 
         FIG. 11  is a flowchart showing processing for restoring a volume, or the like. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, embodiments of the present invention are described with respect to the drawings. In the present embodiment, as described hereinafter, a plurality of disk array devices, each constituted by a plurality of channel adapters, a plurality of disk adapters, a cache memory, a shared memory, and the like, are connected mutually. By also backing up the generational management information of the copy source disk array device onto the copy destination disk array device, it is possible to manage back-up data, for a plurality of generations, in the copy source and the copy destination, respectively. 
     The present embodiment discloses a back-up method for a storage system comprising: a step of transmitting the back-up data of a main volume, from a first disk array device to a second disk array device, and storing same in the second disk array device, and a step of transmitting generational management information used for generational management, from the first disk array device to the second disk array device, and storing same in the second disk array device. 
       FIG. 1  is an illustrative diagram showing an overview of the present embodiment. This storage system comprises a primary site  1  having a copy source disk array device, and a secondary site  2  having a copy destination disk array device. An example of the specific composition of a disk array device is described in more detail below. The primary site  1  holds, for example, a data group used by the current server (host computer). The secondary site  2  is used as a back-up site for the primary site  1 , for example. The primary site  1  and the secondary site  2  may be respectively provided inside frames which are physically separate, for example. Alternatively, for example, the primary site  1  and the secondary site  2  may be provided respectively within the same frame. 
     For example, a host computer  3  constituted as a server is able to access both sites  1  and  2 , respectively. By setting up access control, it is also possible to achieve a composition wherein only a particular host computer is able to access a particular site. The respective host computers  3  and the respective sites  1 ,  2  are connected by means of a communications network  4 . The respective sites  1 ,  2  are connected mutually by means of a separate communications network  5 . The communications network  4  may be constituted by a LAN (Local Area Network), or the like, for example. The communications network  5  may be constituted by a SAN (Storage Area Network), or the like, for example. 
     The primary site  1  may comprise, for example, a main volume  1 A, a main pool  1 B, generational management information  1 C, and a generational management information transferring section  1 D. The main volume  1 A is a logical storage region for storing data used by the host computer  3 . The main pool  1 B is a logical storage region used for withdrawing the original data, before the data in the main volume  1 A is updated. The generational management information  1 C acquires a static image of the main volume as a prescribed point in time, on the basis of a snapshot acquisition request from the user. This snapshot is acquired in the form of a differential snapshot which obtains the differential created by the change in the data. The generational management information transferring section  1 D transfers the generational management information  1 C to the secondary site  2 . 
     The secondary site  2  may comprise, for example, a secondary volume  2 A, a secondary pool  2 B, generational management information  2 C, and a generational management information duplicating section  2 D. The secondary volume  2 A forms a copy pair with the main volume  1 A. The main volume  1 A and the secondary volume  2 A are synchronized. The secondary pool  2 B forms a copy pair with the main pool  1 B, and it stores the storage contents of the main pool  1 B. The main pool  1 B and the secondary pool  2 B are synchronized. The generational management information  2 C is generated by copying the generational management information  1 C to the secondary site  2 . The generational management information duplicating section  2 D receives the generational management information  1 C from the generational management information transferring section  1 D and copies the information. 
     This storage system executes operations of the following kind. Firstly, the storage contents of the main volume  1 A are copied to the secondary volume  2 A (S 1 ). Thereupon, the storage contents of the main pool  1 B are copied to the secondary pool  2 B (S 2 ). Here, when the respective volumes are synchronized, the storage contents of the copy source volumes  1 A,  1 B at a particular point in time are respectively copied, exactly, to the copy destination volumes  2 A,  2 B (initial copy), and thereafter, only the difference generated after completion of the initial copy is copied (differential copy). 
     A host computer  3  accesses the main volume  1 A and updates the data therein. Accordingly, the storage contents of the main volume  1 A change occasionally, from time to time. In the primary site  1 , the storage contents of the main volume  1 A are managed for a plurality of generations. In other words, each time a snapshot acquisition request is issued by the user, the storage contents of the main volume  1 A at that respective point in time (generation) are managed. 
     After creating a copy of the main volume  1 A and the main pool  1 B in the secondary site  2  (S 1 , S 2 ), generational management information  1 C is transmitted from the primary site  1  to the secondary site  2  (S 3 ). By this means, generational management information  2 C is created and stored in the secondary site  2 . 
     Thereby, it is possible to manage back-up data for a plurality of generations of the main volume  1 , in both the primary site  1  and the secondary site  2 . 
     1. First Embodiment 
     Firstly, an example of a disk array device provided respectively in the primary site and the secondary site is described, whereupon the unique composition according to the present invention is described. The disk array device in the primary site and the disk array device in the secondary site may also have different compositions. 
       FIG. 2  is a general oblique view showing the external composition of a disk array device  10 . The disk array device  10  may be constituted, for example, by a base frame body  11  and a plurality of add-on frame bodies  12 . 
     The base frame body  11  is the smallest compositional unit of the disk array device  10 , and it is provided with both storage functions and control functions. The add-on frame bodies  12  are optional items of the disk array device  10 , and are controlled by means of the control functions provided in the base frame body  11 . For example, it is possible to connect a maximum of four add-on frame bodies  12  to the base frame body  11 . 
     The base frame body  11  comprises a plurality of control packages  13 , a plurality of power supply units  14 , a plurality of battery units  15 , and a plurality of disk drives  26 , provided respectively in a detachable fashion. A plurality of disk drives  26 , a plurality of power supply units  14  and a plurality of battery units  15  are provided detachably in the add-on frame bodies  12 . Moreover, a plurality of cooling fans  16  are also provided respectively in the base frame body  11  and the respective add-on frame bodies  12 . 
     The control packages  13  are modules for respectively realizing the channel adapters (hereinafter, CHA)  21 , disk adapters (hereinafter, DKA)  22  and cache memory  23 , and the like, described hereinafter. More specifically, a plurality of CHA packages, a plurality of DKA packages, and one or more memory package are provided in a detachable fashion in the base frame body  11 , in such a manner that they can be exchanged in package units. 
       FIG. 3  is a block diagram showing a general overview of a disk array device  10 . The disk array device  10  can be connected respectively to a plurality of host computers  30 , in a mutually communicable fashion, via a communications network CN 1 . 
     The communications network CN 1  is, for example, a LAN, SAN, the Internet or a dedicated circuit, or the like. If a LAN is used, then the data transfer between the host computer  30  and the disk array device  10  is conducted in accordance with a TCP/IP protocol. If a SAN is used, data transfer is conducted between the host computer  30  and the disk array device  10  in accordance with a fiber channel protocol. 
     Furthermore, if the host computer  30  is a mainframe computer, then data transfer is conducted in accordance with a communications protocol, such as FICON (Fibre Connection: registered trademark), ESCON (Enterprise System Connection: registered trademark), ACONARC (Advanced Connection Architecture: registered trademark), FIBARC (Fibre Connection Architecture: registered trademark), or the like. 
     Each of the host computers  30  is constituted, for example, by a server, personal computer, workstation, mainframe computer, or the like. For example, the respective host computers  30  are connected via a separate communications network to a plurality of client terminals, which are situated outside the range of the drawing. The respective host computers  30  provide services to the respective client terminals, by reading or writing data, from or to the disk array device  10 , in response to requests from the respetive client terminals, for example. 
     Each of the CHAs  21  controls data transfer with the respective host computers  30 , and is provided with a communications port  21 A. 32 CHAs  21 , for example, can be provided in the disk array device  10 . A CHA  21  is prepared, for example, in accordance with the type of host computer  30 , such as an open CHA, a main frame CHA, or the like, for example. 
     Each CHA  21  receives commands and data requesting data read out, or writing, from the host computer  30  connected respectively thereto, and operates in accordance with the commands received from the host computer  30 . 
     To describe the operation of the CHA  21  and that of the DKA  22 , in advance, when the CHA  21  receives a read command from the host computer  30 , this read command is stored in the shared memory  24 . The DKA  22  refers to the shared memory  24  occasionally, and if it discovers an unprocessed read command, then it reads out the data from the disk drive  26 , and stores this data in the cache memory  23 . The CHA  21  reads out the data transferred to the cache memory  23 , and then transmits the data to the host computer  30 . 
     On the other hand, if the CHA  21  receives a write command from the host computer  30 , then it stores this write command in the shared memory  24 . Moreover, the CHA  21  stores the received data (user data) to the cache memory  23 . When the CHA  21  has stored the data in the cache memory  23 , it then reports completion of writing to the host computer  30 . The DKA  22  reads out the data stored in the cache memory  23 , in accordance with the write command stored in the shared memory  24 , and stores this data in the prescribed disk drive  26 . 
     Each of the DKAs  22  may be provided in a plural fashion, for instance, comprising 4 or 8 adapters, in the disk array device  10 . Each DKA  22  respectively controls data communications with a particular disk drive  26 . The respective DKAs  22  and the respective and the respective disk drives  26  are connected by means of a communications network CN 4 , such as a SAN, for example, and perform data transfer in block units, in accordance with a fiber channel protocol. Each DKA  22  monitors the state of the corresponding disk drive  26  occasionally, and the result of this monitoring operation is transmitted via the internal network CN 3 , to the SVP  28 . 
     The respective CHAs  21  and the respective DKAs  22  are provided respectively with a printed circuit board on which a processor, memory, and the like, are mounted, and a control program stored in the memory, for example, (neither of these elements being depicted in the drawings), and they respectively achieve prescribed functions by means of combined operation of these hardware and software elements. 
     The cache memory  23  stores user data, and the like, for example. The cache memory  23  is constituted by a non-volatile memory, for example. When a volume copy, or a differential copy, or the like, is performed, the data to be copied is read out to the cache memory  23 , and it is then transferred from the cache memory  23  to the copy destination, by means of either the CHA  21  or DKA  22 , or alternatively, by means of both the CHA  21  and the DKA  22 . 
     The shared memory (or the control memory)  24  is constituted by a non-volatile memory, for example. Control information, management information, and the like, is stored in the shared memory  24 , for example. The shared memory  24  and cache memory  23  may respectively be provided in a plural fashion. Furthermore, it is also possible to provide both a cache memory  23  and a shared memory  24  on the same memory board. Alternatively, one portion of the memory may be used as a cache region and another portion thereof may be used as a control region. 
     The switching section  25  respectively connects together the respective CHAs  21 , the respective DKAs  22 , the cache memory  23  and the shared memory  24 . Thereby, all of the CHAs  21  and the DKAs  22  may respectively access the cache memory  23  and the shared memory  24 . The switching section  25  may be constituted as an ultra-high-speed cross-bar switch, or the like, for example. 
     A plurality of disk drives  26  may be installed in the disk array device  10 . Each of the disk drives  26  can be realized in the form of a hard disk drive (HDD), a semiconductor memory device, or the like, for example. 
     A disk drive  26  is a physical storage device. Although the situation varies depending on the RAID composition, or the like, a RAID group  27  which is a virtual logical region is constructed on a physical storage region provided by one group of four disk drives  26 , for example. Moreover, one or more virtual logical devices (LU: Logical Unit) can be established in a RAID group  27 . 
     The storage resources used by the disk array device  10  do not all have to be provided inside the disk array device  10 . The disk array device  10  is able to incorporate and use storage resources existing externally to the disk array device  10 , exactly as if there were its own storage resources. 
     The service processor (SVP)  28  is connected respectively to each of the CHAs  21  and the DKAs  22 , by means of an internal network CN 3 , such as a LAN. Furthermore, the SVP  28  may be connected to a plurality of management terminals  31 , by means of a communications network CN 2 , such as a LAN. The SVP  28  accumulates the respective states inside the disk array device  10 , and provides this information to the management terminal  31 . 
       FIG. 4  is a block diagram showing the main composition of a storage system for carrying out back up including generational management information. The storage system is constituted by connecting a copy source disk array device  100  provided in the primary site and a copy destination disk array device  200  provided in the secondary site. The respective disk array devices  100 ,  200  may each be provided with the composition described above with reference to  FIG. 2  and  FIG. 3 , for example. 
     The respective disk array devices  100  and  200  are connected together by means of a communications network CN 11 , such as a SAN, or the like, for example. Furthermore, the respective disk array devices  100 ,  200  and the host computers (hereinafter, “hosts”)  30 A,  30 B are connected by means of a communications network CN 12 , such as a LAN, SAN, or the like, for example. The host  30 A accesses the disk array device  100  of the primary site. The host  30 B accesses the disk array device  200  of the secondary site. Where there is no need to distinguish between the primary host  30 A and the secondary host  30 B, these devices are referred to simply as “host  30 ”. 
     The disk array device  100  comprises a main volume  110 , a main pool  120 , a differential transfer section  130 , a snapshot control section  140 , a generational management information storing section  150 , and a generational management information transferring section  160 . 
     The main volume  110  is a volume for storing a group of data used by the host  30 A. The main pool  120  is a volume for saving data withdrawn from the main volume  110 . The differential transfer section  130  serves to transfer the respective differential data in the storage contents of the main volume  110  and the main pool  120 , to the disk array device  200  of the secondary site. The differential transfer section  130  may be realized, for example, by means of a processor provided in the CHA  21  executing micro code for differential transfer, for example. Here, for example, it is possible to transfer a plurality of differential data elements, together, once a prescribed amount of differential data has been accumulated, or when a prescribed time has been reached, or the like. 
     The snapshot control section  140  manages data by acquiring a snapshot of the main volume  110 , on the basis of an instruction (user instruction) from the host  30 A. A snapshot may be a volume snapshot in which the whole of the volume is copied, exactly, or it may be a differential snapshot in which only the differential from the time at which the previous snapshot was created, is controlled. The snapshot control section  140  creates a differential snapshot. 
     The storage contents of the main volume  110  at a prescribed point in time can be managed by means of a differential bitmap table  151  and a withdrawal destination address management table  152  stored in the generational management information storing section  150 , for example. As shown in  FIG. 5 , for example, it is possible to constitute the generational management information  153  by means of the differential bitmap table  151  and the withdrawal destination address management table  152 . The generational management information  153  is respectively created for each generation of data. The generational management information  153  does not have to be stored in the same storage device, and it may also be stored in a distributed fashion in different storage devices. 
     The differential bitmap table  151  can be understood as a table which associates flag information indicating an updated status or a non-updated status, respectively, to each of a plurality of blocks achieved by dividing the main volume  110  into blocks of a prescribed size, for example. 
     The withdrawal destination address management table  152  can be constituted by associating each block of the main volume  110 , with a withdrawal address indicating whereabouts in the main pool  120  the data stored in that block is to be withdrawn to, for example. The composition of the generational management information  153  illustrated in  FIG. 5  is simply an example, and differential snapshots of the main volume  110  can be managed by means of various methods. 
     The generational management information transferring section  160  transmits the generational management information  153  stored in the generational management information transferring section storing section  150 , to the disk array device  200  of the secondary site. The generational management information transferring section  160  transmits the generational management information  153  at approximately the same timing as that at which the differential transfer section  130  transmits the respective differential data of the main volume  110  and the main pool  120 . 
     The secondary disk array device  200  may comprise a secondary volume  210 , a secondary pool  220 , a virtual volume creating section  230 , a virtual volume  240 , a generational management information storing section  250 , and a generational management information duplicating section  260 . 
     The secondary volume  210  forms a copy pair with the main volume  110 . The storage contents of the main volume  110  are copied to the secondary volume  210 . The secondary pool  220  forms a copy pair with the main pool  120 . The storage contents of the main pool  120  are copied to the secondary pool  220 . 
     The virtual volume creating section  230  generates a virtual volume  240  for the designated generation, on the basis of an instruction from the host  30 . The virtual volume creating section  230  refers to the secondary volume  210  and the secondary pool  220 , on the basis of the generational management information  253  stored in the generational management information storing section  250 , and creates a virtual volume  240  which reproduces the storage contents of the designated generation, in a virtual manner. 
     The generational management information storing section  250  stores the generational management information  253 . This generational management information  253  is a copy of the generational management information  153  of the primary site. Therefore, the generational management information  253  comprises, for example, a differential bitmap table  251 , which is a copy of the differential bitmap table  151 , and a withdrawal destination address management table  252  which is a copy of the withdrawal destination address management table  152 . 
     The generational management information duplicating section  260  stores the generational management information  253  in the generational management information storing section  250 , on the basis of data received from the generational management information transferring section  160  of the primary site. As described hereinafter, the generational management information duplicating section  260  establishes and manages a storage destination address for the generational management information  253 , on the basis of the data size of the generational management information  153  reported by the generational management information transferring section  160 . The storage destination of the generational management information  253  is recorded in the generational management information storage destination address information  261 . 
       FIG. 6  is an illustrative diagram showing a schematic view of the process of storing generational management information  253  in the generational management information storing section  250 . Firstly, the respective partial information elements (differential bitmap table  151 , withdrawal destination address management table  152 ) constituting the generational management information  153  are stored respectively in a distributed fashion, in the disk array device  100  of the primary site. 
     For example, the differential bitmap table  151  is stored in the cache memory of the disk array device  100 . Moreover, for example, the withdrawal destination address management table  152  is stored in a prescribed disk. 
     The generational management information transferring section  160  acquires the respective data sizes of the differential bitmap table  151  and the withdrawal destination address management table  152  constituting the generational management information  153 , when transferring the generational management information  153 . The generational management information transferring section  160  previously reports the data sizes of the respective tables  151 ,  152 , to the generational management information duplicating section  260 , before transferring the generational management information  153 . 
     Upon receiving the data sizes of the respective tables  151 ,  152 , the generational management information duplicating section  260  reserves the storage region required for copying the generational management information  153 , in the generational management information storing section  250 . The generational management information duplicating section  260  stores a differential bitmap table  251 , which is a copy of the differential bitmap table  151 , and a withdrawal destination address management table  252 , which is a copy of the withdrawal destination address management table  152 , respectively, in the reserved storage region. The generational management information duplicating section  260  records the storage positions of the respective tables  251 ,  252 , in the generational management information storage destination address information  261 . 
     The generational management information storage destination address information  261  is formed by associating, for example, the name of the element constituting the generational management information (the name of the partial information), information for identifying the destination storage device (for example, the device number, or the like), a header address, and a data size.  FIG. 6  shows a case where the respective tables  151 ,  152  stored in a distributed fashion in the disk array device  100  are stored together in the cache memory of the disk array device  200 . It is also possible to adopt a composition wherein the tables  251 ,  252  in the generational management information  253  are stored in a distributed fashion in the disk array device  200 . Furthermore, the storage destination device in the generational management information  253  is not limited to being a cache memory, and may also be one or a plurality of disks. 
     The outline of processing in the storage system is now described with reference to  FIG. 7  and  FIG. 8 .  FIG. 7  is a flowchart showing an overview of back-up processing for backing up data to a secondary site, including the generational management carried out in the primary site. 
     Firstly, the disk array device  100  transfer the storage contents of the main volume  110 , to the disk array device  200  (S 11 ). Thereupon, the disk array device  100  transfers the storage contents of the main pool  120  to the disk array device  200  (S 12 ). 
     Upon receiving the data of the main volume  110 , the disk array device  200  of the secondary site stores this data in a prescribed position of the secondary volume  210 , and thereby creates a copy of the main volume  110  (S 21 ). Moreover, upon receiving the data of the main pool  120 , the disk array device  200  of the secondary site stores this data in a prescribed position of the secondary pool  220 , and thereby creates a copy of the main pool  120  (S 22 ). 
     The disk array device  100  in the primary site acquires the respective data sizes of the differential bitmap table  151  and the withdrawal destination address management table  152  constituting the generational management information  153 , and it reports these respective data sizes to the disk array device  200  (S 13 ). 
     The disk array device  200  of the secondary site reserves the required storage region, on the basis of the data size of the differential bitmap table  151  and the data size of the withdrawal destination address management table  152 . The disk array device  200  records the storage destination addresses of the differential bitmap table  251  and the withdrawal destination address management table  252 , in the generational management information storage destination address information  261  (S 23 ). 
     The disk array device  100  in the primary site transmits the data of the differential bitmap table  151  and the withdrawal destination address management table  152  constituting the generational management information  153 , respectively, to the disk array device  200  (S 14 ). 
     The disk array device  200  in the secondary site creates respective copies of the differential bitmap table  151  and the withdrawal destination address management table  152 , on the basis of the data thus received, and it stores these copies at prescribed positions in the generational management information storing section  250  (S 24 ). 
     The disk array device  100  of the primary site monitors data update requests from the host  30  (S 15 ). If there has been an update request (S 15 : YES), then the disk array device  100  manages the differential generated by this update (S 16 ). If, for example, the amount of differential data reaches a prescribed volume, or if a prescribed transfer timing has been reached, (S 17 : YES), then the disk array device  100  repeats the processing in S 11 -S 14 . 
     Here, the differential management carried out in S 16  will be described in simple terms. When the host  30  seeks to update data in the main volume  110 , the disk array device  100  refers to the differential bitmap table  151  which manages the most recent snapshot of the main volume  110 . If the update flag for the data block to be updated has already been set to on (set to “1”), then the original data stored in that data block has already been withdrawn to the main pool  120 . Consequently, in this case, the disk array device  100  writes over the new data, without withdrawing the data in the main volume  110 . 
     If the update flag for the data block to be updated is set to off (if it is set to “0”), then it is necessary for the data stored in that data block to be withdrawn before overwriting the data. Therefore, the disk array device  100  reads out the data stored in the data block to be copied, from the main volume  110 , and copied the data to the main pool  120 . Once copying has been completed, the disk array device  100  records the withdrawal destination address of the data, in the withdrawal destination address management table  152 . Furthermore, the disk array device  100  sets the update flag for that block in the differential bitmap table  151 , to on. 
       FIG. 8  is a flowchart showing an overview of generation restore processing. Firstly, the disk array device  200  in the secondary site monitors whether or not there has been an instruction to restore a data generation, from the host  30  (S 31 ). 
     If an instruction has been received from the host  30  indicating that a virtual volume for the prescribed generation is to be created (S 31 : YES), then the disk array device  200  refers to the generational management information storage destination address information  261 , and acquires the respective storage addresses of the differential bitmap table  251  and the withdrawal destination address management table  252  (S 32 ). The disk array device  200  acquires the generational management information  253  (the differential bitmap table  251  and the withdrawal destination address management table  252 ), from the generational management information storing section  250  (S 33 ). 
     On the basis of the generational management information  253 , the disk array device  200  creates a virtual volume  240  of the generation designated by the host  30 , from the storage contents of the secondary volume  210  and the secondary pool  220  (S 34 ). Here, the virtual volume  240  is, for example, a read out conversion table for restoring the storage contents of the designated generation, in a virtual fashion. It is recorded in the virtual volume  240  which of the secondary volume  210  and the secondary pool  220  the target data is stored in. If the target data is stored in the secondary volume  210 , then this data is read out from the secondary volume  210 . If the target data is stored in the secondary pool  220 , then this data is read out from the secondary pool  220 . 
     When creating a virtual volume  240  for the designated generation, the disk array device  200  causes all or a portion of that virtual volume  240  to be transmitted to the disk array device  100  in the primary site (S 35 ). 
     The disk array device  100  in the primary site then stores the data received from the disk array device  200  of the secondary site, in the main volume  110  (S 41 ). 
     By adopting the composition described above, the present embodiment has the following beneficial effects. In the present embodiment, a composition was adopted wherein the generational management information in the copy source is also backed up to the copy destination. Therefore, it is possible to manage the back-up data for a plurality of generations, in the back-up destination (copy destination) also. By this means, protection against failure can be increased. 
     In the present embodiment, a composition is adopted wherein management of a plurality of data generations is carried out on the basis of differential snapshots, in the disk array device  100  in the copy source, and all data, including the generational management information  153  for these respective generations, is backed up to the disk array device  200  in the copy source. Therefore, it is possible to manage a plurality of data generations by means of a small volume data capacity, in comparison with cases where the whole volume is copied for each generation. 
     2. Second Embodiment 
     A second embodiment is now described on the basis of  FIG. 9 . The present embodiment is equivalent to a modification example of the first embodiment. The characteristic feature of the present embodiment lies in the fact that only the back-up data for a previously designated generation is transferred to the secondary site and held in same. 
       FIG. 9  is a flowchart showing an overview of back-up processing. Firstly, the disk array device  100  of the primary site acquires the generation to be backed up (S 51 ). The generation for which data is to be backed up can be instructed to the disk array device  100  by the host  30 . 
     The disk array device  100  transmits the respective storage contents of the main volume  110  and the main pool  120 , to the disk array device  200  in the secondary site (S 52 , S 53 ). Thereupon, the disk array device  100  respective acquires the data sizes of the differential bitmap table  151  and the withdrawal destination address management table  152  contained in the generational management information  153  relating to the designated generation. The disk array device  100  respectively reports the data sizes of the tables  151 ,  152 , to the disk array device  200  (S 54 ). The disk array device  100  transmits the contents of the differential bitmap table  151  and the withdrawal destination address management table  152  relating to the designated generation, to the disk array device  200  (S 55 ). 
     The disk array device  100  monitors update requests from the host  30  (S 56 ), and if an update request has been issued, then the differential generated in the main volume  110  is managed (S 57 ). When a prescribed transfer timing has arrived (S 58 : YES), the disk array device  100  repeats the processing in steps S 52 -S 55 . 
     In this way, in the present embodiment, it is possible to cause the generational management information  153 , or the like, relating to the generation designated by the host  30 , to the disk array device  200  of the secondary site, and hence ease of use is improved. A composition is adopted wherein, when the storage contents of the main pool  120  are transferred to the disk array device  200 , only the portion relating to the storage contents of a generation designated by the user is transferred. 
     3. Third Embodiment 
     A third embodiment is now described on the basis of  FIG. 10  and  FIG. 11 . The present embodiment is equivalent to a modification example of the first embodiment. The characteristic feature of the present embodiment lies in the fact that copies of a main volume, a main pool, and respective generational management information, held in the copy destination are transferred to the copy source, on the basis of an instruction from the user. 
       FIG. 10  is an approximate block diagram showing the general composition of a storage system. The disk array device  200  in the secondary site comprises a restoring section  270 . In  FIG. 10 , in order to facilitate the description, the virtual volume creating section  230  is omitted from the drawing. 
     As shown in  FIG. 11  as well, the restoring section  270  respectively transfers the storage contents of the secondary volume  210 , the storage contents of the secondary pool  220 , and the generational management information  253 , to the disk array device  100  of the primary site, thereby respectively restoring the main volume  110 , the main pool  120  and the generational management information  153 . 
       FIG. 11  is a flowchart showing an overview of restore processing. When the disk array device  200  of the secondary site detects a restore instruction from the host  30  (S 61 : YES), it transfers the storage contents of the secondary volume  210  to the disk array device  100  (S 62 ). The disk array device  100  of the primary site stores the data received from the disk array device  200 , in the main volume  110 , and thereby restores the main volume  110  (S 71 ). 
     Thereupon, the disk array device  200  of the secondary site transfers the storage contents of the secondary pool  220  to the disk array device  100  (S 63 ). The disk array device  100  of the primary site stores the data received from the disk array device  200 , in the main pool  120 , and thereby restores the main pool  120  (S 72 ). 
     The disk array device  200  refers to the generational management information storage address information  261 , and acquires the storage address of the generational management information  253  (S 64 ). The disk array device  200  respectively acquires the differential bitmap table  251  and the withdrawal destination address management table  252 , from the generational management information storing section  250 , on the basis of the acquired address (S 65 ). The disk array device  200  transmits the contents of the differential bitmap table  251  and the withdrawal destination address management table  252 , respectively, to the disk array device  100  (S 66 ). 
     The disk array device  100  of the primary site stores the data received from the disk array device  200 , in a prescribed position of the generational management information storing section  150 , and thereby restores the generational management information  153  (S 73 ). 
     In this way, in the present embodiment, the copies of the main volume  110  and the main pool  120  and the copy of generational management information  153  held in the copy destination disk array device  200  are respectively returned to the copy source disk array device  100 . 
     The present invention is not limited to the embodiments described above. It is possible for a person skilled in the art to make various additions, modifications, or the like, without departing from the scope of the present invention.