Patent Publication Number: US-7725651-B2

Title: Storage system and data management method

Description:
CROSS REFERENCES 
     This application relates to and claims priority from Japanese Patent Application No. 2006-324147, filed on Nov. 30, 2006, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present invention generally relates to a storage system and a data management method, and, for instance, can be suitably applied to a storage system that performs a full backup or a backup using a snapshot. 
     This storage system manages data using a logical volume (hereinafter referred to as a “logical volume”) formed in a storage extent provided by a hard disk drive. 
     As a method of managing data, there is a method of periodically executing a full backup of a volume in which a user reads and writes data in a cycle of, for instance, every day or every week. As another method of managing data, there is a method of using a snapshot function loaded in a storage apparatus and periodically creating a snapshot formed from a data image of a volume at a certain point in time in a cycle of, for instance, every 12 hours or every day. 
     The backup data obtained by performing a full backup and the differential data obtained by using a snapshot are obtained through different methods. As a result of separately using the backup data and the differential data obtained through different methods, the volume or data stored in such volume can be restored. 
     Japanese Patent Laid-Open Publication No. H7-84728 discloses a data management method using technology of performing a full backup and technology of performing backup using differential data. This document proposes a data management method of deciding in a storage system of whether to perform a full backup by comparing a predetermined threshold value and the amount of differential, or perform backup using differential data. 
     SUMMARY 
     Nevertheless, in order to back up data as described above, it is necessary to separately prepare software for performing full backup and software for performing backup using differential data. Thus, for instance, during initialization or when changing the configuration of the storage system, it is necessary to configure an environment based on different types of software, respectively. 
     Further, since the two types of software are independent and separate software, it is not possible to combine the backup data obtained through a full backup and the differential data obtained through a snapshot so as to restore data. 
     Moreover, since the operation of this kind of storage system will become complicated, it is likely that a smooth operation will not be possible, and there is a problem in that the burden on the system administrator will become significant. 
     Thus, an object of the present invention is to propose a storage system and a data management method enabling the easy operation of the storage system and capable of alleviating the burden on the system administrator by managing the full backup and snapshot in the same storage extent. 
     In order to achieve the foregoing object, the present invention provides a storage system which reads data from a host system in a primary volume as a virtual volume mapped with a logical volume formed in one or more physical storage extents, and multiplexes the data in the primary volume and a secondary volume as a virtual volume pair-configured with the primary volume. This storage system comprises a creation unit for associating a part or the whole of a storage extent of the secondary volume and creating a pool volume to be supplied with a dynamic memory storage extent from the logical volume, a first storage unit for storing differential data corresponding to the primary volume as backup data in the pool volume in order to store, based on a write command of data from the host system in an arbitrary address in the primary volume, the data in the primary volume and update the primary volume, and a second storage unit for storing data that is not updated in the primary volume based on a snapshot command from the host system for acquiring a snapshot on a primary volume at a prescribed timing as differential data or a differential data group in the pool volume. 
     Thereby, it is possible to store the full backup and snapshot in a pool volume, which is the same memory storage extent, and manage these in the storage system. 
     The present invention also provides a data management method of a storage system which reads data from a host system in a primary volume as a virtual volume mapped with a logical volume formed in one or more physical storage extents, and multiplexes the data in the primary volume and a secondary volume as a virtual volume pair-configured with the primary volume. This data management method comprises a creation step for associating a part or the whole of a storage extent of the secondary volume and creating a pool volume to be supplied with a dynamic memory storage extent from the logical volume, a first storage step for storing differential data corresponding to the primary volume as backup data in the pool volume in order to store, based on a write command of data from the host system in an arbitrary address in the primary volume, the data in the primary volume and update the primary volume, and a second storage step for storing data that is not updated in the primary volume based on a snapshot command from the host system for acquiring a snapshot on a primary volume at a prescribed timing as differential data or a differential data group in the pool volume. 
     Thereby, it is possible to store the full backup and snapshot in a pool volume, which is the same memory storage extent, and manage these in the storage system. 
     Accordingly to the present invention, since the backup data and differential data can be stored and managed in the same storage extent as data having consistency in a storage system, backup can be performed at an arbitrary timing. 
     Further, since the easy operation of the storage system is realized, burden on the system administrator can be alleviated. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing an overall configuration of a storage system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing a channel adapter according to an embodiment of the present invention; 
         FIG. 3  is a conceptual diagram showing a logical configuration of a storage system according to an embodiment of the present invention; 
         FIG. 4  is a block diagram showing a disk adapter according to an embodiment of the present invention; 
         FIG. 5  is a conceptual diagram showing a differential bitmap and a mirroring differential bitmap according to an embodiment of the present invention; 
         FIG. 6  is a conceptual diagram showing a pool area management table according to an embodiment of the present invention; 
         FIG. 7  is a conceptual diagram showing a generation management table according to an embodiment of the present invention; 
         FIG. 8  is a conceptual diagram showing backup data in a pool volume according to an embodiment of the present invention; 
         FIG. 9  is a conceptual diagram showing differential data in a pool volume according to an embodiment of the present invention; 
         FIG. 10  is an explanation diagram showing simulated differential data in a pool volume according to an embodiment of the present invention; 
         FIG. 11  is a diagram explaining the operation of a storage system according to an embodiment of the present invention; 
         FIG. 12  is a diagram explaining the operation of a storage system according to an embodiment of the present invention; 
         FIG. 13  is a diagram explaining the operation of an I/O loop back processing unit according to an embodiment of the present invention; 
         FIG. 14  is a flowchart for distinguishing an I/O from a host system and a simulated I/O according to an embodiment of the present invention; 
         FIG. 15  is a flowchart for generating a simulated I/O according to an embodiment of the present invention; 
         FIG. 16  is an explanatory diagram showing backup processing of a storage system according to an embodiment of the present invention; 
         FIG. 17A to 17D  are conceptual diagrams showing various tables explaining backup processing of a storage system according to an embodiment of the present invention; 
         FIG. 18  is an explanatory diagram showing restore processing of a storage system according to an embodiment of the present invention; 
         FIG. 19  is a conceptual diagram showing various tables explaining restore processing of a storage system according to an embodiment of the present invention; 
         FIG. 20  is a conceptual diagram in a pool volume showing restore processing of a storage system according to an embodiment of the present invention; 
         FIG. 21  is a block diagram showing an overall configuration of a storage system according to another embodiment of the present invention; 
         FIG. 22  is an explanatory diagram showing backup processing of a storage system according to another embodiment of the present invention; 
         FIG. 23  is a block diagram showing an overall configuration of a storage system according to yet another embodiment of the present invention; 
         FIG. 24  is an explanatory diagram showing an overall configuration of a storage system according to still another embodiment of the present invention; 
         FIG. 25  is an explanatory diagram showing an overall configuration of a storage system according to still another embodiment of the present invention; and 
         FIG. 26A to 26C  are conceptual diagrams showing various tables explaining backup processing of a storage system according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is now explained in detail with reference to the attached drawings. 
     (1) First Embodiment 
     (1-1) Configuration of Storage System in First Embodiment 
     (1-1-1) Physical Configuration of Storage System 
       FIG. 1  shows an overall storage system  1  according to the present embodiment. The storage system  1  is configured by a host system  2  being connected to a storage apparatus  4  via a network  3 . 
     The host system  2  is a computer device comprising information processing resources such as a CPU (Central Processing Unit) and a memory, and, for instance, is configured from a personal computer, a workstation, a mainframe or the like. Further, the host system  2  comprises an information input device (not shown) such as a keyboard, a switch, a pointing device, a microphone or the like, and an information output device (not shown) such as a monitor display, a speaker, or the like. 
     The network  3 , for instance, is configured from a SAN (Storage Area Network), LAN (Local Area Network), internet, public line, dedicated line or the like. Communication between the host system  2  and the storage apparatus  4  via the network  3  is conducted, for example, according to a fibre channel protocol when the network  3  is a SAN, and according to a TCP/IP (Transmission Control Protocol/Internet Protocol) protocol when the network  3  is a LAN. 
     The storage apparatus  4  comprises a disk drive unit configured from a plurality of hard disk drives  50 , and a controller  6  for managing the plurality of hard disk drives  50  according to a RAID (Redundant Array of Independent/Inexpensive Disks) system. 
     The hard disk drives  50 , for instance, are configured from expensive disk drives such as SCSI (Small Computer System Interface) disks, or inexpensive disk drives such as SATA (Serial AT Attachment) disks or optical disk drives. 
     The controller  6  comprises a plurality of channel adapters  7 , a connection  8 , a shared memory  9 , a cache memory  10 , a plurality of disk adapters  11 , and a service processor  12 . 
     The respective channel adapters  7 , as shown in  FIG. 2 , are configured as a microcomputer system comprising a microprocessor  70 , a memory  71 , a communication interface and the like, and include ports  72 ,  73  for connecting to the network  3 . The respective channel adapters  7  interpret various commands sent from the host system  2  and execute necessary processing. The ports  72 ,  73  of each channel adapter  7  are allocated with a network address (for instance, an IP address or WWN) for identifying the respective channel adapters  7 , and the channel adapters  7  are thereby able to independently function as a NAS (Network Attached Storage). The memory  71  of each channel adapter  7  includes a write command processing unit  74  for processing write commands from the host system  2 , and an I/O loop back processing unit  75  for returning a simulated I/O generated from the disk adapter  11  back to the disk adapter  11 . In addition, the respective channel adapters  7  are connected to the connection via an internal bus adapter (not shown). 
     In addition to the foregoing channel adapters  7 , the connection  8  is also connected to the shared memory  9 , the cache memory  10 , and the disk adapters  11 . The transfer of data and commands among the channel adapters  7 , the shared memory  9 , the cache memory  10 , and the disk adapters  11  is conducted via the connection  8 . The connection  8  is configured from a switch such as an ultra-fast crossbar switch or a bus for performing data transfer with high-speed switching. 
     The shared memory  9  is a storage memory to be shared by the channel adapters  7  and the disk adapters  11 . The shared memory  9  is primarily used for storing system configuration information and various control programs read from the system volume when the power of the storage apparatus  4  is turned on, and commands sent from the host system  2 . 
     The cache memory  10  is also a storage memory to be shared by the channel adapters  7  and the disk adapters  11 . The cache memory  10  is primarily used for temporarily storing user data to be input and output to and from the storage apparatus  4 . 
     The respective disk adapters  11 , as shown in  FIG. 3 , are configured as a microcomputer system comprising a microprocessor  110 , a memory  111  and the like, and functions as an interface for controlling the protocol during communication with the disk drive unit  5 . The disk adapters  11  are connected to the corresponding disk drive unit  5  via, for example, a fibre channel cable, and sends and receives data to and from the disk drive unit  5  according to the fibre channel protocol. 
     The service processor  12  is a computer device to be operated for performing maintenance to or managing the storage apparatus  4 , and, for instance, is configured from a laptop personal computer or the like. The service processor  12  is connected to the host system  2  via the network  3 , and is capable of receiving data or commands from the host system  2 . The service processor  12  is able to monitor the occurrence of failures in the storage apparatus  4  and display such failures on a display screen (not shown). 
     (1-1-2) Logical Configuration of Storage System 
     The logical configuration of the foregoing storage system  1  is now explained.  FIG. 4  is a conceptual diagram showing the logical relationship of the host system  2  and the plurality of hard disk drives  50  in the storage system  1 . 
     In the storage system  1 , one or more logical volumes LDEV are defined in a storage extent provided by the plurality of hard disk drives  50 . 
     Each logical volume LDEV is allocated with a unique identifier (LUN: Logical Unit Number). In the case of this embodiment, the input and output of data are performed by combining this identifier and a number (LBA: Logical Block Address) unique to such block allocated to the respective blocks as an address, and designating such address. 
     Further, a pool volume PL to be dynamically supplied from a storage extent provided by the plurality of hard disk drives  50  is defined according to the data volume of the differential data P 1  to Pn. 
     Moreover, with the storage system  1 , a virtual volume mapped with a logical volume LDEV to be accessed by the host system  2  is also defined. As the virtual volume LU, there is a virtual volume LU mapped with a logical volume LDEV as a real volume, and a virtual volume LU mapped with a logical volume LDEV and a pool volume PL. 
     As the virtual volume LU, there is a primary volume PVOL and a secondary volume SVOL. The secondary volume SVOL is a virtual volume LU to be used for the backup of the primary volume PVOL. Meanwhile, even when a failure occurs in the primary volume PVOL, the secondary volume SVOL can be used to promptly recover the primary volume PVOL. 
     In this embodiment, the pool volume PL is a dynamic logical volume to be associated with the secondary volume SVOL. Therefore, the pool volume PL is used as a memory storage extent for backing up the primary volume PVOL. Specifically, the pool volume PL stores differential data P 1  to Pn of a bitmap corresponding to the data acquired at a prescribed timing in the primary volume PVOL. 
     With the storage system  1  configured as described above, when the host system  2  accesses the virtual volume LU, data from the host system  2  is read from and written into the logical volume LDEV associated with the virtual volume LU in block units of a prescribed size. 
     (1-2) Backup Function and Snapshot Function in Present Embodiment 
     The backup function and snapshot function loaded in the storage apparatus  4  of the storage system  1  are now explained. 
     The storage system  1  is characterized in that it is able to write data from the host system or simulated data (this is hereinafter referred to as “simulated data”) generated from the disk adapter  11  in an arbitrary address of the primary volume PVOL, and back up the data written beforehand in such address as backup data PD and store it in the pool volume PL as the memory storage extent of the primary volume PVOL. Further, the storage system  1  is also characterized in that the backup data PD is differential data P 1  to Pn, simulated differential data PS 1  to PSn as a differential data group, or a mirror volume PM. 
     The backup function according to the present embodiment is the function where the storage system  1 , upon receiving an I/O from the host system  2 , flushes out the data pre-stored in an arbitrary address of the primary volume PVOL and stores such data in the pool volume PL. Therefore, data to be backed up and stored in the pool volume PL will be data (old data) that is one older than the latest data. Latest data from the host system  2  is stored in an unused area formed in an arbitrary address of the primary volume PVOL. 
     Incidentally, I/O from the host system refers to the read/write request from the host system  2  or the data to be input or output pursuant to such read/write request. The backup processing according to the present embodiment will be described with reference to the write request from the host system  2  and the data to be output pursuant to such write request. Further, the restore processing according to the present embodiment will be described with reference to the read request from the host system  2  and the data to be input pursuant to such read request. 
     In addition, the snapshot function according to the present embodiment is the function where the storage system  1 , upon receiving a snapshot command from the host system  2 , creates a replication by copying the data of the storage extent of the primary volume PVOL at such time to the pool volume PL as the memory storage extent of the secondary volume SVOL. With the storage system  1 , it is possible to intermittently acquire the replication of data by periodically executing the snapshot function. 
     (1-2-1) Processing Contents of Disk Adapter 
     As a means for realizing the foregoing characteristics, as shown in  FIG. 3 , the memory  111  of the disk adapters  11  in the storage apparatus  4  is provided with a data processing unit  112  for processing an I/O from the host system  2  or a simulated I/O generated by the disk adapters  11 . 
     Incidentally, a simulated I/O is a simulation of the read/write request from the host system  2  and the data to be input and output pursuant to such read/write request, and refers to the simulated read/write request generated from the disk adapters  11  and the simulated data to be input and output pursuant to such simulated read/write request. The backup processing in this embodiment will be described with reference to the simulated write request generated from the disk adapters  11  and the simulated data to be output pursuant to such simulated write request. Further, the restore processing in this embodiment will be described with reference to the simulated read request generated from the disk adapters  11  and the simulated data to be input pursuant to such simulated read request. 
     The data processing unit  112  stores a differential bitmap  113 , a mirroring differential bitmap  114 , a pool area management table  115 , a generation management table  116 , a backup program  117 , and a simulated I/O generation program  118 . 
     The differential bitmap  113 , as shown in  FIG. 5 , is a table for managing the differential data P 1  to Pn in which “0” and “1” corresponding to the respective data stored in the primary volume PVOL are disposed therein. 
     For example, when certain data is backed up from the primary volume PVOL to the pool volume PL as the memory storage extent of the secondary volume SVOL, and, after such storage thereof, “1” is displayed for indicating that the differential data P 1  to Pn have been updated. Updating from “0” to “1”, as shown in  FIG. 5 , is conducted in order from the upper left of the differential bitmap  113 . 
     Meanwhile, when certain data has not been backed up from the primary volume PVOL to the pool volume PL, “0” is displayed for indicating that the differential data P 1  to Pn have not yet been updated. Therefore, when there are numerous indications of “1” displayed on the differential bitmap  113 , this shows that many data of the primary volume PVOL have been updated, and, when there are numerous indications of “0”, this shows that not many data of the primary volume PVOL have been updated. 
     The mirroring differential bitmap  114 , as shown in  FIG. 5 , is a table for managing all data in which “0” and “1” corresponding to the respective data stored in the primary volume PVOL are disposed therein. The display method of “0” and “1” is the same as the method described regarding the foregoing differential bitmap  113 , and the explanation thereof is omitted. 
     The pool area management table  115 , as shown in  FIG. 6 , is a table for managing the backup data PD to be backed up in the pool volume PL. The pool area management table  115  is configured from a “sequence number” field  115 A, an “address” field  115 B, and a “data” field  115 C. 
     The “sequence number” field  115 A stores a sequence number in which data is to be sequentially backed up from the primary volume PVOL to the pool volume PL. This sequence number is used for protecting the writing order of data. Therefore, smaller the sequence number, older the stored data since the writing order is early, and, larger the sequence number, newer the stored data. 
     The “address” field  115 B stores addresses in the primary volume PVOL. 
     The “data” field  115 C stores the backup data PD to be backed up from the primary volume PVOL to the pool volume PL. 
     For example, in the pool area management table  115 , when the “sequence number” field  115 A is “3”, this shows that the backup processing into the pool volume PL has been performed thirdly. Here, the backup data PD is data “CCCCC” of address “0x3214” in the primary volume PVOL. Therefore, the larger the sequence number, more recent the backup processing which was performed. 
     Incidentally, an “update time” field for managing the update time of data can be further provided to the pool area management table  115  for managing the writing order of data together with the sequence number. 
     The generation management table  116 , as shown in  FIG. 7 , is a table for managing the generation of data to be backed up from the primary volume PVOL to the pool volume PL based on a snapshot command. 
     Here, the generation of data indicates the data in the primary volume PVOL based on a snapshot command of a certain number. For instance, upon receiving a command of a first snapshot from the host system  2 , this shows that data in the primary volume PVOL at the time of the first command is data of the first generation. Similarly, upon receiving a command of a second . . . Nth snapshot from the host system  2 , this shows that data in the primary volume PVOL at the time corresponding to such command is data of the second generation . . . Nth generation. 
     Further, the generation management table  116  is configured from a “generation number” field  116 A and a “sequence number” field  116 B. 
     The “generation number” field  116 A stores the generation number of data to be backed up from the primary volume PVOL to the pool volume PL based on a snapshot command. 
     The “sequence number”  116 B stores the sequence number for sequentially backing up data from the primary volume PVOL to the pool volume PL. 
     (1-2-2) Configuration of Pool Volume 
     The pool volume PL in this embodiment plays the role as a memory storage extent of the secondary volume SVOL. Therefore, the pool volume PL stores the backup data PD, which is the backup data of the primary volume PVOL. As shown in  FIG. 8 , as the backup data PD, there are differential data P 1  to Pn, simulated differential data PS 1  to PSn, and a mirror volume PM. 
     The differential data P 1  to Pn are backup data PD which is data stored in an arbitrary address of the primary volume PVOL at an arbitrary time to be stored in the pool volume PL as the differential data P 1  to Pn. 
     Specific differential data P 1  is shown in  FIG. 9 . As the differential data to be stored in the pool area, there is an address P 1 A in the primary volume PVOL and a differential data P 1 D of the primary volume PVOL. 
     The simulated differential data PS 1  to PSn are backup data PD in which the unupdated portion is stored in the pool volume PL as the simulated differential data PS 1  to PSn when such unupdated portion of data remains even after data to be disposed in the differential bitmap  113  or the mirroring differential bitmap  114  corresponding to the primary volume PVOL is updated. 
       FIG. 10  shows a conceptual diagram explaining the simulated differential data PS 1  to PSn. 
     When the microprocessor  110  of the disk adapter  11  searches and detects an unupdated portion of data in the differential bitmap  113  or the mirroring differential bitmap  114  corresponding to the primary volume PVOL, it sends a simulated I/O to the address of the unupdated portion. This uses the backup function described above. Normally, the I/O from the host system  2  is sent to an arbitrary address of the primary volume so as to migrate the data stored beforehand in the primary volume PVOL to the pool volume PL. Nevertheless, when there are numerous unupdated portions of data as a result of sending a simulated I/O generated from the disk adapters  11  in the storage apparatus  4  to an arbitrary address of the primary volume PVOL, it is possible to reduce the number of times data is sent from the host system  2 . This is because unupdated data in the primary volume PVOL is stored as the simulated differential data PS 1  to PSn in the pool volume PL, and latest data is stored in the area which was the unupdated portion. Therefore, this means that the pool volume PL stores data that is one older than the latest data as simulated data. 
     Here, as shown in  FIG. 10 , the simulated I/O generated from the disk adapter  11  is configured from an arbitrary address PAn in the primary volume PVOL and an appropriate data FDn. 
     The simulated data FDn could be any type of data since it merely has to play the role of flushing out the data stored beforehand in an arbitrary address of the primary volume PVOL. 
     As described above, the microprocessor  10  of the disk adapter  11 , as shown in  FIG. 10 , stores the address in the primary volume PVOL and the simulated differential data as the backup data of the unupdated portion in the secondary volume. 
     (1-2-3) Operational Outline of Storage System 
     The operational outline of the storage system  1  according to the present embodiment is now explained. 
     Foremost, a case is explained where the backup function and the snapshot function in this embodiment are invalid, and an I/O is sent from the host system  2 . 
     As shown in  FIG. 11 , when an I/O sent from the host system enters one port  72  of the channel adapter  7  of the storage apparatus  4  ( 1 ), the microprocessor  70  of the channel adapter  7  receives data ( 2 ). The microprocessor  70  of the channel adapter  7 , through the write command processing unit  74  ( 3 ), sends data to the disk adapter  11  ( 4 ). 
     The microprocessor  110  of the disk adapter  11  that received the data refers to the backup program  117  in the data processing unit  112  and confirms whether the backup function and snapshot function are valid or invalid ( 5 ). When the microprocessor  110  of the disk adapter  11  confirms that the backup function and snapshot function are invalid, it writes the received data in an arbitrary address of the primary volume PVOL ( 6 ). 
     A case is now explained where the backup function and snapshot function in this embodiment are valid, and a simulated I/O is to be generated from the disk adapter  11 . 
     As shown in  FIG. 12 , when the microprocessor  110  of the disk adapter  11  refers to the differential bitmap  113  or the mirroring differential bitmap  114  corresponding to the primary volume PVOL and recognizes the unupdated portion of data, it boots the simulated I/O generation program and generates a simulated I/O ( 1 ). 
     The microprocessor  110  of the disk adapter  11  sends the generated simulated I/O to the channel adapter  7  ( 2 ). 
     The microprocessor  70  of the channel adapter  7  that received the simulated I/O executes processing in the I/O loop back processing unit  75  for making a round of the simulated I/O in the channel adapter  7  ( 3 ). 
     Specifically, as shown in  FIG. 13 , the I/O loop back processing unit  75  constructs a round of processing by assuming two channel adapters  7 A,  7 B and migrating the simulated I/O in such two channel adapters  7 A,  7 B. 
     The operation for constructing a round of processing is as follows. When the microprocessor  70 A of the channel adapter  7 A receives a simulated I/O from the disk adapter  11 , it sends the simulated I/O to another channel adapter  7 B via the port  72 A. When the microprocessor  70 B of the other channel adapter  7 B receives the simulated I/O via the port  72 B of the other channel adapter  7 B, it sends the simulated I/O as is to the disk adapter  11  ( 4 ). 
     Like this, the microprocessor  70  of the channel adapter  7  makes a round of the simulated I/O with the I/O loop back processing unit  75  of the channel adapter. 
     When the microprocessor  110  of the disk adapter  11  receives the simulated I/O, it determines whether to use the differential bitmap  113  or the mirroring differential bitmap  114 , and selects the bitmap to be used ( 5 ). The microprocessor  110  of the disk adapter  11  thereafter refers to the backup program  117 , and confirms that the backup function and snapshot function are valid ( 6 ). 
     When there is unupdated data in the differential bitmap  113  or the mirroring differential bitmap  114 , the microprocessor  110  of the disk adapter  11  sends the simulated I/O to an arbitrary address of the primary volume PVOL, and reads and acquires data from the arbitrary address of the primary volume PVOL ( 7 ). 
     The microprocessor  110  of the disk adapter  11  stores such data as backup data PD in the pool volume PL ( 8 ). Here, the microprocessor  110  of the disk adapter  11  treats the unupdated portion in the differential bitmap  113  or the mirroring differential bitmap  114  as having been updated. 
     (1-2-4) Processing Contents of Microprocessor 
     In the operational outline of this storage system  1 , the difference in the processing when the microprocessor  110  of the disk adapter  11  receives an I/O from the host system  2  or a simulated I/O generated from the disk adapter  11  is now explained. This processing is executed by the microprocessor  110  of the disk adapter  11  based on the backup program  117 . Incidentally, although this processing explains a case of the microprocessor  110  using the mirroring differential bitmap  114 , the same processing may be performed in the case of using the differential bitmap  113 . 
     Foremost, as shown in  FIG. 14 , the processing is started by the microprocessor  110  of the disk adapter  11  receiving from the channel adapter  7  an I/O from the host system  2  or a simulated I/O generated from the disk adapter  11  (SP 0 ). 
     Subsequently, the microprocessor  110  of the disk adapter  11  determines whether the mirroring differential bitmap  114  is unupdated (SP 1 ). 
     When it is determined that the mirroring differential bitmap  114  is unupdated (SP 1 : YES), the microprocessor  110  of the disk adapter  11  treats the mirroring differential bitmap  114  as having been updated (SP 2 ). The microprocessor  110  of the disk adapter  11  thereafter reads and acquires data from the primary volume PVOL (SP 3 ), and writes the read data in a mirror volume as backup data PD (SP 4 ). 
     Meanwhile, when the microprocessor  110  of the disk adapter  11  determines at step SP 1  that the mirroring differential bitmap  114  has been updated (SP 1 : NO), it performs the subsequent processing at step SP 5 . 
     Subsequently, the microprocessor  110  of the disk adapter  11  determines whether an I/O has been sent from the host system (SP 5 ), and, when it determines that it is an I/O from the host system (SP 5 : YES), the microprocessor  110  of the disk adapter  11  writes the latest data from the host system in the primary volume (SP 6 ), and ends this processing (SP 7 ). 
     Meanwhile, when the microprocessor  110  of the disk adapter  11  that it is not an I/O from the host system but rather a simulated I/O (SP 5 : NO), it ends this processing directly (SP 7 ). 
     (1-2-5) Simulated I/O Generation Processing 
     Processing for generating a simulated I/O from the disk adapter  11  is now explained. This processing is executed by the microprocessor  110  of the disk adapter  11  based on the simulated I/O generation program  118 . 
     Foremost, as shown in  FIG. 15 , the microprocessor  110  of the disk adapter  11  starts this processing upon using the differential bitmap  113  or the mirroring differential bitmap  114  corresponding to the primary volume in the operational outline of the storage system  1  described above (SP 10 ). 
     The microprocessor of the disk adapter  11  selects one address at a time from the top address in the differential bitmap  113  or the mirroring differential bitmap  114  (SP 11 ). 
     Subsequently, the microprocessor  110  of the disk adapter  11  determines whether the selected address in the differential bitmap  113  or the mirroring differential bitmap  114  is data of an unupdated portion (SP 12 ). In other words, the microprocessor  110  of the disk adapter  11  determines whether the selected address in the differential bitmap  113  or the mirroring differential bitmap  114  is “0” or “1”. 
     When the microprocessor  110  of the disk adapter  11  determines that the selected address in the differential bitmap  113  or the mirroring differential bitmap  114  is data of an unupdated portion (SP 12 : YES), is sends a simulated I/O to such selected address (SP 13 ). This is in order to flush out data of such unupdated portion from the primary volume and create an unused area. 
     Meanwhile, at step SP 12 , when the microprocessor  110  of the disk adapter  11  determines that the selected address in the differential bitmap  113  or the mirroring differential bitmap  114  is not data of an unupdated portion (SP 12 : NO), it proceeds to the processing at step SP 14 . 
     The microprocessor  110  of the disk adapter  11  determines whether addresses up to the last address in the differential bitmap  113  or the mirroring differential bitmap  114  have been selected (SP 14 ). 
     When the microprocessor  110  of the disk adapter  11  determines that addresses up to the last address in the differential bitmap  113  or the mirroring differential bitmap  114  have been selected (SP 14 : NO), it returns to step SP 11  and selects the subsequent address. 
     Meanwhile, when the microprocessor  110  of the disk adapter  11  determines that addresses up to the last address in the differential bitmap  113  or the mirroring differential bitmap  114  have not been selected (SP 14 : YES), it sends a simulated I/O to data of all target unupdated portions, and, since data of the unupdated portion in the primary volume has been read, it ends this processing (SP 15 ). 
     In particular, when the microprocessor  110  of the disk adapter  11  selects and uses the mirroring differential bitmap  114 , it sends a simulated I/O to an address at the upper left of the bitmap, and treats data of all unupdated portions in the mirroring differential bitmap  114  has having been updated. 
     (1-2-6) Backup Processing 
     Sequential backup processing in the storage system  1  is now explained regarding how the data stored in the primary volume PVOL is backed up in the pool volume PL using the various tables and operational outline described above. 
       FIG. 16  shows a time series of the I/O from the host system  2 , snapshot command from the host system  2 , and simulated I/O generated from the disk adapter  11  in the vertical axis t, and the conceptual diagram of the primary volume PVOL and the pool volume PL corresponding to such time series. 
     Foremost, when an I/O from the host system  2  has been sent and a snapshot command has not yet been issued (t 1  to t 4 ), after the performance of the operation order ( 1  to  6 ) of the storage system  1  explained with reference to  FIG. 11 , data from the host system  2  is written in the primary volume PVOL. 
     When the host system  2  issues a first generation snapshot command at an arbitrary timing (t 5 ), the microprocessor  110  of the disk adapter  11  initializes the pool volume PL. To initialize the pool volume PL means to initialize the pool area management table  115  and the generation management table  116  as shown in  FIG. 17A . Backup processing of data in the primary volume PVOL is then started. 
     Subsequently, when the host system  2  sends an I/O to an arbitrary address of the primary volume PVOL (t 6  to t 8 ), it uses the backup function and snapshot function of the storage system  1  to read the data stored beforehand in the primary volume PVOL, and store such data as differential data P 1  to Pn in the pool volume PL. As shown in  FIG. 17B , the pool area management table  115  stores, for instance, differential data P 1  acquired at time t 6 . 
     Thereafter, when the host system  2  issues a second generation snapshot command (t 9 ), the disk adapter  11  sends a simulated I/O for backing up all data stored in the primary volume PVOL (t 10  to t 13 ). Since data of the primary volume PVOL is stored beforehand in the pool volume PL as differential data P 6  to P 8  at time t 6  to time t 8 , here, data of the unupdated portion among the data of the primary volume PVOL is stored as simulated differential data PS 1  to PSn. When generating a simulated I/O, the backup efficiency will improve in comparison to sending an I/O from the host system  2  since backup processing can be performed in just the storage apparatus  4 . 
     As shown in  FIG. 17C , simulated differential data acquired at time t 10  to time t 13  is stored as differential data groups P 10  to P 13  in the pool area management table  115 . Since the first backup processing is not yet complete at time t 10  to time t 13 , data during this period is the first generation backup data PD. In  FIG. 16 , the first generation backup data PD has a generation of “1” and a sequence number of “7”, but the subsequent generation number is entered in the “generation number” field  116 A of the generation management table  116 . Thus, a generation number of “2” and a sequence number of “7” as the simulated differential data of the end of the first generation are stored in the generation management table  116 . 
     When the first backup is complete, the disk adapter  11  is able to acquire the first generation backup data. As shown in  FIG. 17C , the area of the first generation backup data will be where the sequence number is 1 to 7. 
     Like this, the storage of the differential data P 1  to P 3  and the differential data P 4  to P 7  as simulated differential data in a pool volume results in the backup of all data of the first generation primary volume PVOL. 
     Subsequently, at time t 13  to time t 17 , data stored in the primary volume PVOL is stored as differential data P 13  to P 17  in the pool volume PL pursuant to the I/O sent from the host system  2 . 
     When the host system issues a third generation snapshot command (t 18 ), at time t 19  to time t 22 , the disk adapter  11  generates a simulated I/O and stores simulated differential data as the differential data group P 19  to P 22  in the pool volume PL. Here, when an I/O is sent from the host system  2  (t 23 ), the disk adapter  11  stores this as differential data in the pool volume PL. At time t 24  to time t 27 , the disk adapter  11  generates a simulated I/O and stores simulated differential data as the differential data group P 24  to P 27  in the pool volume. 
     When the second backup is completed as described above, the disk adapter  11  is able to acquire the second generation backup data. As shown in  FIG. 17D , the area of the second generation backup data will be where the sequence number is 8 to 27. 
     Subsequently, the third generation backup data can be acquired similarly. 
     In the present embodiment, although only data of the unupdated portions was stored in the pool volume PL as the simulated differential data PS 1  to PSn each time a snapshot command from the host system  2  was given to the primary volume, the primary volume PVOL storing beforehand all data may also be stored as a mirror volume PM in the pool volume PL. 
     Needless to say, operation of the storage system  1  of storing the primary volume PVOL as the mirror volume PM in the pool volume PL may be performed in parallel with the operation of storing the differential data and simulated differential data in the pool volume PL. 
     Even when storing the primary volume PVOL as the mirror volume PM in the pool volume PL, as described above, backup will be completed by generating a simulated I/O and migrating all data. 
     When an I/O is sent from the host system  2  while the backup data PD is being stored in the pool volume PL based on a simulated I/O from the disk adapter  11 , priority may be given to either the I/O from the host system or the simulated I/O upon storing the backup data PD in the pool volume PL. If the backup efficiency is to be improved, priority is given to the simulated I/O for storing the backup data PD. 
     (1-2-7) Restore Processing 
     The processing for restoring backup data PD stored in the pool volume PL based on backup processing into the primary volume PVOL at an arbitrary timing is now explained. Sequential backup processing in the storage system  1  is now explained regarding how the data stored in the primary volume PVOL is backed up in the pool volume PL using the various tables and operational outline described above. Incidentally, differential data P 1  to Pn will be used as the backup data PD in the explanation. 
     As shown in  FIG. 18 , for instance, a case is explained where the area of second generation data is restored from the pool volume PL to the primary volume PVOL. 
     As shown in  FIG. 19 , differential data P 1  to Pn are managed with the pool area management table  115  and the generation management table  116 . Since it is evident from the generation management table  116  that the second generation is immediately after sequence number 7, the area of the second generation data will be from a sequence number of 8 to 21. Further, since the sequence number shows the writing order of data, restoration must be performed from new data to old data. Therefore, as order of performing restoration, differential data P 8  to P 21  should be restored in order from the sequence number of 21 to 8. 
     Upon performing this restoration, as shown in  FIG. 20 , it is also possible to group and store the backup data PD to be stored beforehand in the pool volume PL. 
     For example, a user sets failure groups G 1 , G 2  beforehand in the pool volume PL. Differential data P 1  to Pn and simulated differential data PS 1  to PSn are stored in one failure group G 1 , and a mirror volume PM is stored in the other failure group G 2 . As a result of the foregoing storage, even when the primary volume PVOL, failure group G 1  or failure group G 2  is damaged, damaged data can be recovered by restoring one of the remaining two. Specifically, when a failure occurs in the primary volume PVOL and data in such volume PVOL is damaged, the primary volume PVOL can be recovered by restoring the backup data PD in one failure group among the failure groups G 1 , G 2 . 
     Like this, since it is possible to recover the primary volume PVOL by using the differential data P 1  to Pn and the simulated differential data PS 1  to PSn stored in the pool volume PL, it could be said that the differential data P 1  to Pn and the simulated differential at a PS 1  to PSn are all data at an arbitrary point in time stored beforehand in the primary volume PVOL. 
     (1-3) Effect of First Embodiment 
     According to the present embodiment, it is possible to store and manage the differential data as backup data, the simulated differential data and the mirror volume in a pool volume of the storage system as data with consistency, and backup processing and restore processing can be performed at an arbitrary timing. 
     (2) Second Embodiment 
       FIG. 21  shows an overall storage system  200  according to the present embodiment. 
     The storage system  200  of this embodiment is configured by connecting a tape library device  15  comprising a plurality of tapes  150  to the storage apparatus  4  of the first embodiment as shown in  FIG. 21  which is given the same reference numerals for the portions corresponding to those shown in  FIG. 1 . The plurality of tapes  150  of the tape library device  15  are external recording mediums. 
     Incidentally, the remaining configuration is the same as the configuration of the storage system  1  according to the first embodiment, and the explanation thereof is omitted. Further, the same reference numeral is given to the same components as in the first embodiment. 
     Processing for further backing up the tape library device  15  in order to store the backup data PD stored in the pool volume PL of the first embodiment for a long period of time is now explained. 
     As shown in  FIG. 22 , the disk adapter  11  transfers all backup data stored in the pool volume PL of the first embodiment to the tape library device  15 , and replicates such backup data in the tapes  150 . In addition, the disk adapter  11  also transfers the pool area management table  115  and the generation management table  116  to the tape library device  15 . The pool area management table  115  and the generation management table  116  are managed in the tape library device  15 . 
     Incidentally, processing for storing the backup data PD in the pool volume has been explained in the first embodiment, and the explanation thereof is omitted. 
     When restoring the backup data replicated in the tapes  150  of the tape library device  15 , a logical volume of data of the corresponding generation is created from the pool area management table  115  and the generation management table  116  managed in the tape library device  15 , and the backup data, and such volume is replicated in the primary volume. 
     According to the storage system of the present embodiment, in addition to the effect of the first embodiment, it is possible to replicate backup data of the pool volume in the external recording medium without having to be aware of the type of backup data. Further, since the various tables for managing the backup data are also replicated in the external recording medium, it is possible to restore the desired data with only the data in the external recording medium. 
     (3) Third Embodiment 
       FIG. 23  shows an overall storage system  300  according to the present embodiment. 
     The storage system  300  according to the present embodiment is configured such that the channel adapter  7  of the storage apparatus  4 ′ is connected to an external storage apparatus  17  via a network  16  as shown in  FIG. 23  which is given the same reference numerals for the portions corresponding to those shown in  FIG. 1 . The external storage apparatus  17  is provided with an external volume  170 . 
     The storage apparatus  4 ′ of this embodiment is equipped with a pool volume PL′. The pool volume PL′ further stores metadata PM 1  to PMn as data related to the differential data P 1  to Pn. The metadata PM 1  to PMn are data concerning the creation date and time, creator, data format and the like of the differential data P 1  to Pn. 
     Incidentally, the remaining configuration is the same as the configuration of the storage system  1  according to the first embodiment, and the explanation thereof is omitted. Further, the same reference numeral is given to the same components as in the first embodiment. 
     Processing for further backing up the external storage apparatus  17  in order to store the differential data P 1  to Pn stored in the pool volume PL′ of this embodiment for a long period of time is now explained. 
     As shown in  FIG. 24 , a part or the whole of the old differential data among the differential data P 1  to Pn stored in the pool volume PL′ is transferred to and stored in the external volume  170  of the external storage apparatus  17 . 
     Incidentally, as the recording medium for storing the differential data, in addition to the external volume  17 , a storage apparatus, tape or WORM area to be newly and separately provided may also be used. 
     According to the storage system of the present embodiment, in addition to the effect of the first embodiment, it is possible to store only the old differential data in the external storage apparatus since metadata relating to the backup data of the pool volume is stored. Further, it is also possible to delete only the old differential data. 
     (4) Fourth Embodiment 
       FIG. 1  shows an overall storage system  400  according to the present embodiment. 
     Incidentally, the storage system  400  according to this embodiment is configured the same as the storage system  1  of the first embodiment, and the explanation thereof is omitted. 
     As shown in  FIG. 25 , the method of storing the backup data in the pool volume PL is as follows. 
     Foremost, the disk adapter  11  creates synthesized data PC 1 , PC 2  by synthesizing a plurality of differential data P 1  to Pn. For example, when referring to the generation management table  116  shown in  FIG. 26A , the second generation differential data P 3 , P 4  have a sequence number of 3 and 4, and the third generation differential data P 5 , P 6  have a sequence number of 5 and 6. When these differential data P 3  to P 6  are synthesized, the synthesized data PC 1  will become differential data P 3  to P 6  having sequence numbers of 3 to 6. The same synthesizing method may be used to synthesize differential data P 1  to Pn of a plurality of generations. 
     Then, subsequently, the disk adapter  11  compresses data among the synthesized data PC 1 , PC 2  . . . and creates compressed data PP 1  . . . . For example, when referring to the pool area management table  115  shown in  FIG. 26B , addresses having a sequence number of 3 and 5 are overlapping. Thus, number 3 as the smallest number among the sequence numbers of the overlapped addresses is preserved and synthesized data corresponding to the other sequence number 5 is deleted, whereby it is possible to create compressed data PP 1  . . . as shown in  FIG. 26C . 
     The disk adapter  11  is able to further encrypt the compressed data created as described above and store it as data PE 1  . . . 
     According to the storage system of the fourth embodiment, in addition to the effect of the first embodiment, when it is necessary to store old backup data for a long period of time, such old backup data can be efficiently stored in the pool volume. Further, since the old backup data is encrypted, it is possible to prevent such data from being falsified or stolen. 
     (5) Other Embodiments 
     In the foregoing embodiments, although the disk adapters  11  and the plurality of hard disk drive  50  of the storage apparatus  4  were used as the creation unit for associating a part or the whole of the storage extent of the secondary volume SVOL and forming a pool volume to be supplied with a dynamic memory storage extent from the logical volume, the disk adapters and the hard disk drives of the externally provided storage apparatus may also be used as the creation unit. 
     Although the disk adapters  11  and the plurality of hard disk drives  50  in the storage apparatus  4  were used as the first storage unit for storing the differential data P 1  to Pn corresponding to the primary volume PVOL in the pool volume PL, the disk adapters and the hard disk drives of the externally provided storage apparatus may also be used as the first storage unit. 
     Although the unupdated data in the primary volume PVOL was made to be the simulated differential data PS 1  to PSn, and the disk adapters  11  and the plurality of hard disk drives  5  in the storage apparatus  4  were used as the second storage unit for storing the simulated differential data PS 1  to PSn in the pool volume, the disk adapters and the hard disk drives of the externally provided storage apparatus may also be used as the second storage unit. 
     Although add data in the primary volume PVOL was made to be the mirror volume PM and the disk adapters  11  and the plurality of hard disk drives  50  in the storage apparatus  4  were used as the third storage unit for storing the pool volume PL, the disk adapters and the hard disk drives of the externally provided storage apparatus may also be used as the third storage unit. 
     Although the management unit for managing the order of writing the differential data P 1  to Pn in the pool volume PL was provided in the memory  111  of the disk adapter  11 , the management unit can also be provided as a separate hardware configuration. 
     The present invention can be broadly applied to a storage system having one or more storage apparatuses, as well as storage systems of other modes.