Patent Publication Number: US-7908449-B2

Title: Data replication in a storage system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of application Ser. No. 11/907,748, filed Oct. 17, 2007 now U.S. Pat. No. 7,689,792; which is a continuation of application Ser. No. 10/879,471, filed Jun. 30, 2004, now U.S. Pat. No. 7,302,535, which claims priority from Japanese application JP 2004-115693 filed on Apr. 9, 2004, the content of which is hereby incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a storage system and a data replication method, and more particularly to a storage system comprising plural storage control units to which plural disk devices are connected, and a data replication method for creation of replication. 
     Recently, demands for the reduction of time necessary for processing to create replication (hereinafter referred to as the backup) of data stored in a storage device of a storage system possessed by corporations in another storage device are increasing. Such demands are backed by the reduction of time allocated to the backup processing because an amount of information possessed by the corporations is increasing and the time for backup is increasing while business hours of corporations are extended. 
     The prior art enabling to backup data stored in a storage device without stopping the daily jobs of the corporations includes, for example, snapshot technologies proposed as described in JP-A-7-210439 and JP-A-2001-318833. The snapshot is a function to copy a storage area possessed by the storage device at a specified time to a storage device without through a computer connected to the storage device. Using such a snapshot function, a user can use the original storage area for jobs and the data stored in the copied storage area for backup. 
     As a technology for improving the scalability of the storage device connected to a network, a storage system of a cluster configuration is considered. The storage system of the cluster configuration is a system having a conventional storage system such as a disk array device as one cluster and one storage system configured of plural clusters. 
     Conventionally, no technology of performing snapshot in the storage system of a cluster configuration is known. Where the storage system of the cluster configuration and the snapshot technology of the above-described prior art are simply combined, copy of a storage area is performed only in one cluster. 
     As described above, where the storage system of the cluster configuration and the snapshot technology of the above-described prior art are combined, a storage area cannot be copied between different clusters, so that a storage area which can be used as a copy destination of a storage area and a storage area which cannot be used are formed within the storage system of one cluster configuration, causing a problem that the scalability of the storage system of the cluster configuration originally intended is impaired. 
     In the storage system of the cluster configuration, when it is made possible to copy a logical volume (hereinafter referred to as the volume) over clusters, namely when a copy source volume and a copy destination volume are on different clusters, a device configuration in that a cluster (hereinafter referred to as the original cluster) having a volume of the copy source cannot refer to a common memory within a cluster (hereinafter referred to as the copy cluster) having a volume of the copy destination or a device configuration in that reference can be made but access performance between the clusters is low has a problem that an efficiency of preparing a copy volume between different clusters is degraded. Therefore, the system configured as described above has a limited use that a volume of the copy destination is selected within the same cluster as the copy source. Thus, there is also a problem that the device configuration is different from the prior art and the ease-of-use by a user is changed. 
     SUMMARY OF THE INVENTION 
     The present invention has been made under the circumstances described above to remedy the problems of the above-described prior arts and provides a storage system of a cluster configuration having plural storage control units to which plural disk devices are connected, which can generate a copy of the storage area without conscious of different storage control units not only when replication is created in a volume within the disk devices connected to the same storage control unit but also when replication is created in a volume within the disk devices connected to different storage control units, and a data replication method. 
     According to the present invention, the above-described advantage is achieved by a storage system comprising a plurality of storage control units to which plural disk devices are connected and a data replication method thereof, wherein each of the plural storage control units includes a replication creation unit which creates replication of data of a volume in the disk devices and pair information which is information about a volume of a replication source and a volume of a replication destination; and wherein, when the replication creation unit in one of the plural storage control units is to create replication in a volume within the disk devices connected to another storage control unit, all data in the volume of the replication source is copied to the volume of the replication destination, and when the storage control unit having the volume of the replication source receives a data update request to the volume of the replication source, the received data is stored in a cache memory of the storage control unit, the data is stored in a cache memory of the storage control unit of the replication destination on an extension of processing of the update request, and the update of data is reflected to the replication destination to prepare a replication volume. 
     According to the present invention, a storage system of a cluster configuration having plural storage control units to which disk devices are connected can create a copy of a storage area without degrading the performance by minimizing accesses to control information among the control units when replication is created in a volume within the disk devices connected to different storage control units. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of the computer system including the storage system according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram showing a configuration of the computer system including the storage system according to a second embodiment of the present invention; 
         FIG. 3  is a block diagram showing a configuration of the computer system including the storage system according to a third embodiment of the present invention; 
         FIG. 4  is a diagram showing the configuration within a memory; 
         FIG. 5  is a block diagram showing the configuration of a user input/output apparatus; 
         FIGS. 6A and 6B  are diagrams showing example configurations of a volume pair information table; 
         FIG. 7  is a diagram showing an example configuration of a volume information table; 
         FIGS. 8A and 8B  are diagrams illustrating examples of a differential bitmap; 
         FIGS. 9A and 9B  are diagrams showing an example arrangement of differential bitmaps of the first to third embodiments of the present invention; 
         FIG. 10  is a flowchart illustrating a processing operation to prepare replication by the storage systems according to the first to third embodiments of the present invention; 
         FIG. 11  is a flowchart illustrating a processing operation to prepare replication in the same storage control unit by the processing in step  5030  shown in  FIG. 10 ; 
         FIG. 12  is a flowchart illustrating a processing operation to prepare replication between different storage control units by the processing in step  5040  shown in  FIG. 10 ; 
         FIG. 13  is a flowchart illustrating an operation when a write request is made during the replication creation processing in the same storage control unit illustrated with reference to  FIG. 11 ; 
         FIG. 14  is a flowchart illustrating an operation when a write request is made during the replication creation processing between the different storage control units illustrated with reference to  FIG. 12 ; 
         FIG. 15  is a diagram showing a data transfer path when a write request is made during the replication creation processing between the different storage control units illustrated with reference to  FIG. 14 ; 
         FIG. 16  is a flowchart illustrating a processing operation for resynchronization of a pair between different storage control units by the storage systems according to the first to third embodiments of the present invention; 
         FIGS. 17A and 17B  are diagrams showing an example arrangement of differential bitmaps according to a fourth embodiment of the present invention; 
         FIG. 18  is a flowchart illustrating an operation of the fourth embodiment when a write request is made during a replication creation processing between different storage control units; 
         FIG. 19  is a flowchart illustrating a high-speed split processing operation in the fourth embodiment of the present invention; 
         FIG. 20  is a flowchart illustrating an operation of performing write processing to an original volume when falling in a high-speed split status in the fourth embodiment of the present invention; 
         FIG. 21  is a flowchart illustrating an operation of write processing of a copy volume when falling in a high-speed split status in the fourth embodiment of the present invention; 
         FIG. 22  is a diagram showing a data transfer path when a write request is made during the replication creation processing between different storage control units in the forth embodiment of the present invention; 
         FIG. 23  is a diagram showing a data transfer path when a write request is made during the replication creation processing between different storage control units in the second embodiment of the present invention shown in  FIG. 2 ; and 
         FIG. 24  is a diagram showing a data transfer path when a write request is made during the replication creation processing between different storage control units in the third embodiment of the present invention shown in  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of a storage system and a data replication method replication method according to the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a hardware configuration of a computer system including a storage system according to a first embodiment of the present invention. In  FIG. 1 , numeral  10  indicates a host, numerals  20 A and  20 B indicate storage control units, numerals  21 A and  21 B indicate CPUs, numerals  22 A and  22 B indicate memories, numerals  23 A and  23 B indicate cache memories, numeral  24  indicates a hub, numerals  25 A and  25 B indicate switches, numeral  31  indicates a storage group, numeral  40  indicates an I/F adaptor, numeral  70  indicates a storage system, and numeral  80  indicates a user input/output apparatus. 
     The computer system including the storage system according to the first embodiment of the present invention is configured with the host  10  connected to the storage system  70  via the I/F adaptor  40 . The storage system  70  is configured of the plural storage control units  20 A,  20 B, the I/F adaptor  40  connected to the individual storage control units  20 A,  20 B, the user input/output apparatus  80  connected to the individual storage control units  20 A,  20 B via a management network, and the storage group  31  connected to the I/F adaptor  40 . The I/F adaptor  40  is a channel connection part independent of the storage control units  20 A,  20 B. The shown storage system is connected to the storage system  70  and the host  10  or the storage group  31  via a different board not shown. The storage group  31  is a group of storage devices having a plurality of storage devices such as magnetic disk devices. 
     The storage control units  20 A,  20 B each have the CPUs  21 A,  21 B, the memories  22 A,  22 B and the cache memories  23 A,  23 B for temporarily storing I/O data from the host  10 . The CPUs  21 A,  21 B, the memories  22 A,  22 B and the cache memories  23 A,  23 B are multiplexed and mutually connected by the switches  25 . The storage control units  20 A,  20 B each are configured to allow the CPU in them to access the memory and the cache memory therein. 
     According to the first embodiment of the present invention configured as described above, the I/F adapter  40  having received an I/O request from the host  10  sends the pertinent request to the storage control units  20 A,  20 B. The CPUs  21 A,  21 B in the storage control units  20 A,  20 B obtain and analyze the command, and if the request is read, judge whether the cache memories  23 A,  23 B have the object data therein; if the data is available, the CPUs  21 A,  21 B send the data to the host  10  via the hub  24  within the I/F adapter  40  but, if the cache memory  23  does not have the data, secure a region in the cache memories  23 A,  23 B, read out data from the storage group  31 , execute staging in the region secured in the cache memories  23 A,  23 B and send the data to the host  10 . 
       FIG. 2  is a block diagram showing a hardware configuration of a computer system including a storage system according to a second embodiment of the present invention, and numerals in  FIG. 2  are same as those in  FIG. 1 . 
     The computer system including the storage system according to the second embodiment of the present invention shown in  FIG. 2  is different from the first embodiment of the present invention shown in  FIG. 1  on the point that the switch  25 A in the storage control unit  20 A and the switch  25 B in the storage control unit  20 B are mutually connected, and the storage control units  20 A and  20 B are mutually connected. Thus, the storage control units  20 A,  20 B are mutually connected via the switches  25 A,  25 B in the second embodiment, so that the individual CPUs in the storage control units  20 A,  20 B can access the memory in the other storage control unit. 
     For example, the CPU  21 A of the storage control unit  20 A can access the memory  21 B in the storage control unit  20 B via the switch  25 . A connection line  64  between the switch  25 A of the storage control unit  20 A and the switch  25 B of the storage control unit  20 B may be a bus or a network. But, when the storage control units mutually access the memory and cache memory in the other storage control unit in the second embodiment shown in  FIG. 2 , the connection line  64  (a bus or a network configured of hardware) has a slow access speed in performance as compared with the case of accessing the memory and cache memory in the same storage control unit. 
       FIG. 3  is a block diagram showing a structure of the storage system according to a third embodiment of the present invention. In  FIG. 3 , numeral  25  indicates a device interface, numeral  32  indicates a storage group, numeral  50  indicates a processor, numerals  60  to  63  indicate networks, numeral  510  indicates a configuration information table, and other numerals are same as those in  FIG. 1 . 
     The computer system including the storage system according to the third embodiment of the present invention shown in  FIG. 3  is configured with a first storage system  70 A connected to the host  10  via the network  60  and the first storage system  70 A and a second storage system  70 B connected via the network  61 . A user input/output apparatus  80  can be connected to each part in the first storage system  70 A via the management networks  62 ,  63 . The first and second storage systems  70 A,  70 B are configured in the same way, but only the first storage system  70 A is shown its inside structure in  FIG. 3 , and the inside structure of the storage system  70 B is omitted. Here, it is shown that the second storage system  70 B is connected, but the present invention may have the structure without connecting the second storage system  70 B. 
     The first and second storage systems  70 A,  70 B are basically storage systems having the same functional structure as those described with reference to  FIG. 1  and  FIG. 2 . The first storage system  70 A is provided with plural I/F adaptors  40  which are channel connection portions independent of the storage control units  20 A,  20 B and treat protocols in conformity with LAN (Local Area Network), public line, dedicated line and ESCON (Enterprise Systems Connection); the plural I/F adaptors  40  and the plural storage control units  20 A,  20 B are connected via the network  63 . According to the third embodiment, the processor  50  having the configuration information table  510  is connected to the network  63 , and the storage group  31  is connected to the storage control unit  20 A via the device interface  25 A. 
     According to the third embodiment of the present invention configured as described above, the I/F adaptor  40  receives an I/O request from the host  10 , analyzes the command to perform protocol conversion, judges LU (Logical Unit), in which data demanded by the command is stored, whether it is managed under control of either of the storage control units  20 A and  20 B or by the storage system  70 B, and sends the I/O request to the judged location. It is judged which device manages the LU storing the above-described request data with reference to the configuration information table  510  stored in the memory within the processor  50  connected via the network  63 . 
     The user input/output apparatus  80  recognizes each part within the first storage system  70 A via the network  62  but can be configured to directly connect through a dedicated line. 
     The storage control unit  20 A has a CPU  21 A, a memory  22 A, a cache memory  23 A for temporarily storing I/O data from the host  10 , the hub  24  for connection to the network  63  and a device interface  25  for controlling sending/receiving of data to/from the storage group  31 , which are mutually connected via an internal bus. The storage control unit  20 B also has the same configuration. 
     The hardware configuration of the computer system provided with the storage system according to the first to third embodiments of the present invention were briefly described above. Individual components common to the individual embodiments will be described below. 
       FIG. 4  is a diagram showing an internal structure of the memory  22 A (the memory  22 B also has the same structure, and these memories are simply referred to as the memory  22  below). In  FIG. 4 ,  200  is a RAID (Redundant Array of Inexpensive Disks) control program,  201  is a replication creation program,  210  is a management agent,  220  is a volume pair information table,  230  is a volume information table, and  240  is a differential bitmap. 
     As shown in  FIG. 4 , the memory  22  stores various programs to be executed by the CPU  21 . Specifically, they are the RAID control program  200  for controlling the operation of the storage systems  70 ,  70 A,  70 B and the management agent  210  for managing the storage system configuration. The memory  22  also stores various kinds of management information. Specifically, they are the volume pair information table  220  for recording information about a data copy source and copy destination, the volume information table  230 , the differential bitmap  240  and a configuration information table (not shown) that the storage system  70 B provides the storage system  70 A with its own LU as the LU of the storage system  70 A. 
     The RAID control program  200  has a functional portion (not shown) for issuing a command to the storage group  31 , and the RAID control program  200  has therein as a sub-program the replication creation program  201  for creating replication of data within the storage system  70 . To execute the replication of data, there are variations of synchronization (the completion is reported to a higher device upon the completion of copy) and asynchronization (the completion is reported to the higher device without the completion of copy), but they are not particularly distinguished in the embodiments of the present invention. The management agent  210  is a program for setting information about storage device (hereinafter referred to as storage device information) upon receiving input from the user input/output apparatus  80  and outputting storage device information to the user input/output apparatus  80 . 
       FIG. 5  is a block diagram showing a structure of the user input/output apparatus  80 , which will be described below. The user input/output apparatus  80  includes a CPU  81 , a main storage  82 , an input unit (keyboard, etc.)  83 , an output unit (display device, etc.)  84 , a management I/F  85  for connection with an outside device and a storage unit  86 , and they are mutually connected via an internal bus as shown in  FIG. 5 . 
     The host  10  is, for example, a personal computer, a workstation, a general-purpose computer or the like and provided with HBA (Host Bus Adaptor) as an FC interface for connection to the outside. The HBA is provided with a WWN. 
       FIG. 6A  and  FIG. 6B  are diagrams showing an example configuration of the volume pair information table  220 . The volume pair information table  220  is information for managing a pair of volumes (hereinafter referred to as the pair) for holding the copied data within the storage system  70  (the same is also applied to  70 A and  70 B;  70  is used to indicate the storage system unless otherwise specified) and includes the fields for a pair number  221 , original volume information  222 , copy volume information  224 , and a pair status  226 . And, the volume pair information table  220  includes a volume pair information table  220 A in the same storage control unit which is an information table at the time of creation of replication within the same control storage control unit as shown in  FIG. 6A  and a volume pair information table  220 B in a different storage control unit which is an information table at the time of creation of replication within a different storage control unit as shown in  FIG. 6B . 
     In the tables  220 A,  220 B, the pair number  221  indicates an identifier arbitrarily allocated to the pair of original and copy volumes. The original volume information  222  indicates volume numbers allocated to the original volume among the pairs to which the identifier is given in the table  220 A and a storage control unit number  227  and a volume number  223  which are allocated to the original volume among the pairs to which the identifier is given in the table  220 B. The copy volume information  224  indicates the volume numbers allocated to the copy volume among the pairs to which the identifier is given in the table  220 A and indicates a storage control unit number  228  and a volume number  225  which are allocated to the copy volume among the pairs to which the identifier is given in the table  220 B. The pair status  226  indicates the present status of the pair. For example, such a status includes a status that data stored in the individual volumes of the pair are synchronized and the contents of the stored data match (hereinafter referred to as a Pair status), a status that data are not synchronized among the pairs (hereinafter referred to as a Split status), and the like. 
     The storage system  70  can change, for example, a pair in the Pair status into the Split status in a prescribed time. At this time, data possessed by the pair at the prescribed time is stored in the copy volume (this processing is called “an acquisition of snapshot”). Then, the host  10  reads out data from the copy volume and writes in another storage device, e.g., a tape device, so that data stored in the pair at the time when the snapshot is acquired can be backed up. After the acquisition of the snapshot, the copy volume itself may be stored as backup of the data. 
     Information of the pair having a copy source and a copy destination in the storage control unit  20 A is stored in the memory  22 A within the storage control unit  20 A, and information of the pair having a copy source and a copy destination in the storage control unit  20 B is stored in the memory  22 B within the storage control unit  20 B. Information of the pair between the storage control units having different copy source and copy destination in the storage control units  20 A and  20 B is stored in the memory  22 A of the storage control units  20 A and  20 B and volume pair information table  220 B in  20 B. 
     For example, when the volume pair information table  220 A in the same storage control unit is in a storage control unit No.  1 , it is seen that pair number  0  is a pair of volume numbers  100  and  10  in the storage control unit No.  1 . It is also seen that pair No.  1  is a pair of volume No.  110  of the storage control unit No.  1  and volume No.  120  of the storage control unit No.  2 . 
       FIG. 7  is a diagram showing an example structure of the volume information table  230 . This volume information table  230  is registered with information for managing the volume under control by the storage control unit  20 A and is stored in the memory within the storage control unit  20 A and includes the fields for a volume number  231 , original/copy  232  indicating the original and copy of a volume, a pair volume number  236  and a volume status  235  indicating whether the volume is being used or not. 
     The volume number  231  is an identifier which is allocated to the volume. The example shown in  FIG. 7  has three pairs set up for volume number  0  of the own storage control unit. The example shown in  FIG. 7  shows that a first pair indicates that the copy volume which is a pair volume is volume No.  20  of the storage control unit No.  1 , and a second pair indicates that the copy volume is volume No.  158  within the same storage control unit (indicated by “-”). A third pair indicates that the copy volume is volume No.  426  of the storage control unit No.  1 . Besides, volume No.  1  of the own storage control unit is being used as the copy volume of the pair, indicating that the original volume is volume No.  3783  of the storage control unit No.  3 . 
     A storage control unit number  233  and a volume number  234  in the pair volume information  236  are pair volume information when they are paired. In a case of the pair in the same storage control unit, only the copy volume number is registered in the volume number  234 . In a case of a pair between different storage control units, the storage control unit number of the copy volume is registered in the storage control unit number  233  and the volume number is registered in the volume number  234 . The volume status  235  is information indicating whether the volume is in use or available. 
       FIG. 8A  and  FIG. 8B  are diagrams illustrating examples of the differential bitmap  240 . The differential bitmap has 1 bit corresponded to data having a predetermined size and a value determined as “1” if even 1 bit in the data having a predetermined size of one of the pair is updated, and indicates for each predetermined data size whether the copy between the pairs has completed. 
     Specifically, the differential bitmap has data having a prescribed data size corresponded to a bit, its value “0” indicates a portion where the copy has completed, and the value “1” indicates a portion where the copy has not completed. For example, when data of 64 KB is corresponded to one bit and even 1 B is updated in the data of 64 KB, the bit is determined to be “0” so that the content is reflected to the copy destination. According to the first to third embodiments of the present invention, only one differential bitmap P 1  is advantageously provided as shown in  FIG. 8A , and in a fourth embodiment of the present invention to be described later, two bitmaps P 1  and P 2  having the same size are provided for one pair as shown in  FIG. 8B . 
       FIG. 9A  and  FIG. 9B  are diagrams showing an example arrangement of the differential bitmaps according to the first to third embodiments of the present invention. 
     When replication is to be created in the same storage control unit as shown in  FIG. 9A , its pair is created in, for example, the storage group  31  connected to the storage control unit  20 A, and the differential bitmap  240  is created as P 1  in the memory  22 A of the storage control unit  20 A. When replication is created in different storage control units as shown in  FIG. 9B , its pair is created in, for example, the storage group  31  to be connected to the storage control unit  20 A and the storage group  31  to be connected to the storage control unit  20 B, the differential bitmap  240  is created as P 1  in the memory  22 A of the storage control unit  20 A and as S 1  in the memory  22 B of the storage control unit  20 B. And, the differential bitmaps P 1  and S 1  are controlled so to match mutually. 
     Then, in the storage systems according to the first to third embodiments of the present invention, an operation of creating a copy volume of a volume, which is in the storage control unit  20 A, in the storage control unit  20 A or  20 B will be described. 
       FIG. 10  is a flowchart illustrating a processing operation to create replication by the storage systems according to the first to third embodiments of the present invention, and this processing operation will be described. The copy volume is created by the replication creation program  201 . The replication creation program  201  checks whether the original and copy volumes of a replication pair are within the same storage control unit or between the different storage control units and performs processing of the pair in the same storage control unit and the pair between the different storage control units. 
     (1) First, one volume is selected as a copy volume from available volumes, wherein the selected volume (the copy volume) is used as a copy destination. And, information about original volume and copy volume configuring the pair is registered in the volume pair information table  220  with the original and copy volumes determined as a pair. In this case, the registration is made in the table  220 A or the table  220 B depending on whether the pair is in the same storage control unit or not (step  5010 ). 
     (2) It is judged whether the pair is in the same storage control unit  20  (step  5020 ), and if the pair is within the storage control unit  20 , the replication creation processing in the same storage control unit is executed (step  5030 ), and if the pair is in the different storage control units  20 , the replication creation processing is executed between the different storage control units (step  5040 ). 
       FIG. 11  is a flowchart illustrating a processing operation for creation of replication in the same storage control unit by the processing in the above-described step  5030 , and this processing operation will be described below. 
     (1) Initial copy processing for whole copy of the content of the original volume into the copy volume is started. And, a differential bitmap P 1  is created as shown in  FIG. 9A , and all differential bits of P 1  of the differential bitmap  240  are set to “1” because this is an initial copy (step  6010 ). 
     (2) It is detected whether the value of an initial differential bit on the differential bitmap is “1” (step  6020 ). If “1” is detected, it is judged whether data at a portion corresponding to the pertinent bit is in the cache memory, namely whether it is a cache hit or not (step  6030 ). 
     (3) If the data is not in the cache memory when judged in the step  6030 , a region is secured in the cache memory (step  6035 ), and corresponding data is read out from the original volume and stored in the region secured in the cache memory. This step is called staging (step  6040 ). 
     (4) When it is found by the judgment made in the step  6030  that the data is in the cache memory or when the staging is executed by the processing in the step  6040 , a copy of the pertinent data is created for data of the copy volume within the cache memory. In this copying operation, redundant information for judging whether data is correct or not is also created newly for the copy volume and attached to the data (step  6050 ). 
     (5) After the copy for data of the copy volume is created in the cache memory, a corresponding differential bit of the differential bitmap is set to “0” (step  6060 ). 
     (6) If the value of the differential bit is not “1” in the step  6020 , namely if the value of the differential bit is “0”, or when it is judged whether there is a next differential bit after the processing in the step  6060  and there is a next differential bit, the processing returns to the step  6020  to repeat the same processing, and when the next differential bit disappears, the processing here is terminated (step  6070 ). 
     (7) Meanwhile, the data for the copy volume copied on the cache memory by the processing in the step  6050  is stored in the copy volume in asynchronization with the above-described processing operation (step  6080 ). 
     In the above-described processing operation, when data is to be read from the original volume into the cache memory, redundant information for the copy volume may be created and stored directly as data for the copy volume in the cache memory. 
       FIG. 12  is a flowchart illustrating a processing operation of the replication creation between the different storage control units by the processing in the above-described step  5040 , and this processing operation will be described below. 
     (1) Initial copy processing for the whole copy of the content of the original volume into the copy volume is started. To create replication between different storage control units, the differential bitmap is created as P 1 , S 1  in the memories of the storage control units  22 A,  22 B as shown in  FIG. 9B . And, all bits of P 1 ,  51  of the differential bitmap  240  are set to “1” because this is an initial copy (step  8010 ). 
     (2) It is detected whether the value of the initial differential bit on the differential bitmap on the side of the original volume is “1” or not (step  8020 ). If the value “1” is detected, it is judged whether data at a portion corresponding to its bit is in the cache memory or not, namely whether it is a cache hit or not (step  8030 ). 
     (3) If data is not in the cache memory when judged in the step  8030 , a region is secured in the cache memory (step  8035 ), corresponding data is read out of the original volume and read into the region secured in the cache memory. This procedure is called staging (step  8040 ). 
     (4) If data is in the cache memory, namely if it is a cache hit, when judged in the step  8030 , or after the processing in the step  8040 , the storage control unit  20 A having the original volume issues a write request to the storage control unit  20 B which creates a copy volume (step  8050 ). The storage control unit  20 B on the copy side having received the write request secures a cache memory for storage of write data (step  8060 ) and reports the storage control unit  20 A on the original side that the cache memory has been secured (step  8070 ). 
     (6) Upon receiving the report about the ensurance of the cache memory in the step  8070 , the storage control unit  20 A on the original side transfers the write data to the storage control unit  20 B on the copy side (step  8080 ) and receives a transfer completion report from the storage control unit  20 B on the copy side (step  8090 ). 
     (7) The storage control unit  20 A on the original side sets the value of a corresponding bit of P 1  of the differential bitmap  240  of the own unit to “0” (step  8100 ). 
     (8) If the value of the differential bit is not “1” in the step  8020 , namely if the value of the differential bit is “0”, or it is judged whether there is a next differential bit (step  8110 ) after the processing in the step  8100  and, if there is a next differential bit, the process returns to the step  8020  to repeat the processing. When the next differential bit disappears, the processing here is terminated. 
     (9) In asynchronization with the above-described processing operation, the storage control unit  20 B on the copy side stores data for the copy volume stored in the cache memory into the copy volume (step  8120 ). 
     When the initial copy processing is completed by performing the above-described flows of  FIG. 11  and  FIG. 12 , the differential bitmap has all differential bits with a value “0”. 
     During the processing of the initial copy according to the flows of  FIG. 11  and  FIG. 12  described above, there is a possibility that a normal read/write request arrives. Then, a processing upon the reception of a write request during the processing of the initial copy will be described. 
       FIG. 13  is a flowchart illustrating an operation in case of a write request during the processing for creation of replication in the same storage control unit described with reference to  FIG. 11 , and this operation will be described below. 
     (1) When the storage control unit  20 A receives a write request from the host  10  (step  9010 ), the storage control unit  20 A sets a differential bit at a portion corresponding to write the object data of P 1  of the differential bitmap  240  to “1” (step  9020 ). 
     (2) The storage control unit  20 A secures a cache memory region for write data storage (step  9030 ), receives the write data transferred from the host  10  and stores it in the cache memory (step  9040 ). 
     (3) The storage control unit  20 A returns a write completion report to the host  10  (step  9050 ), stores write data in the cache memory into the original volume in asynchronization with the write request from the host  10  (step  9060 ). 
     (4) Meanwhile, the processing for reflecting the content of the write data into the copy volume sequentially refers to the differential bitmaps, detects a differential bit “1” (step  9070 ) and, if “1” is not detected, namely if all differential bits of the differential bitmap are “0”, the processing is terminated without doing anything. 
     (5) If the differential bit “1” is detected in the step  9070 , the same processing as in the steps  6030  to  6060  and  6080  described with reference to the flow shown in  FIG. 11  is executed, data is asynchronously written in the copy volume (step  9090 ), it is judged whether there is a next differential bit, namely next data (step  9110 ), and if there is, the processing returns to the step  9070  and the processing is repeated, and if there is not, the processing here is terminated. 
     The storage systems according to the first to third embodiments of the present invention employ a method not using the differential bitmap for a pair extending over the storage control units in response to a read/write request during the initial copy. And, at the time of the initial copy for creation of replication between the different storage control units, “1” is registered for all differential bits of P 1  of the differential bitmap  240 , and the same processing as that described above is executed. If a write request occurs during this processing, the storage control unit  20 A stores data in the cache memory of the own device and transfers data to the storage control unit  20 B as a continuation of the same write request. 
       FIG. 14  is a flowchart illustrating an operation in case of a write request during the processing for creation of replication between the different storage control units described with reference to  FIG. 12 , and this operation will be described below. 
     (1) Upon receiving a write request from the host  10  (step I 2010 ), the storage control unit  20 A secures a memory region in the cache memory for storage of write date (step  12020 ) and receives the write data from the host  10  to store in the region secured in the cache memory (step  12030 ). 
     (2) It is detected whether the differential bit corresponding to write data of the differential bitmap is “0” (copied) or not (step  12035 ). If the differential bit is “0”, the storage control unit  20 A issues a write request to the storage control unit  20 B having a copy volume in the same way as the processing in the steps  8050  to  8090  described with reference to  FIG. 12  and transfers the data (steps  12040  to  12080 ). 
     (3) If the differential bit is not “0” when detected in the step  12035 , namely if it is “1”, corresponding data is data not having completed the initial copy, so that the write data is stored in the cache memory of the storage control unit  20 A and copies to the storage control unit  20 B on the copy side at the time of the initial copy processing. Then, the storage control unit  20 A reports the completion to the host  10  and terminates the processing (step  12090 ). 
     (4) The data stored in the cache memory of the storage control unit  20 A on the original side is asynchronously written (destaged) to the original volume by the same processing as in the step  6080  (step  12100 ), and the data stored in the cache memory of the storage control unit  20 B on the copy side is asynchronously written (destaged) to the copy volume by the same processing as in the step  8120  (step  12110 ). 
       FIG. 15  is a diagram showing a data transfer path in case of a write request during the processing for creation of replication between the different storage control units described with reference to  FIG. 14 .  FIG. 15  also shows an arrangement of the differential bitmap. 
     It is apparent from the description with reference to  FIG. 14  and also from the data transfer path shown in  FIG. 15 , if there is a write request during the processing for creation of replication between the different storage control units, the reflection of update to the copy volume can be executed as a continuation of the same I/O processing when a write request is received. The write request for update to the copy volume in this case is sent from the CPU  21 A to the CPU  21 B through the same route as the data transmission line. 
     When a pair is set up between the different storage control units and a synchronized state of the contents of the original and copy volumes in a pair status becomes a split status after the termination of the initial copy, if there is a write request from the host  10 , the storage control unit  20 A updates the differential bit of the differential bitmap P 1 , and the storage control unit  20 B updates the differential bit of the differential bitmap S 1 . The embodiments of the present invention have one each of the original and a copy of the differential bitmap, and when the pair is in the split status, the replication processing can be realized by updating the differential bitmap in each storage control unit. If there is a write request to the pair in the same storage control unit, the update position may be stored in the differential bitmap P 1 . 
       FIG. 16  is a flowchart illustrating a processing operation of resynchronization of the pair between the different storage control units of the storage system according to the first to third embodiments of the present invention, and this processing operation will be described below. The resynchronization processing (Resync) is a processing for synchronizing the copy volume with the content of the original volume, namely a processing for copying the content of the original volume at that time to the copy volume. 
     (1) Upon receiving a Resync request from the host  10  (step  20010 ), the storage control unit  20 A issues a read request of the differential bitmap to the storage control unit  20 B (step  20020 ). 
     (2) The storage control unit  20 A secures a cache memory for a differential bitmap to be received (step  20030 ), and the storage control unit  20 B transfers the differential bitmap S 1  to the storage control unit  20 A (step  20040 ). 
     (3) The storage control unit  20 A merges two differential bitmaps P 1 , S 1  to create a new differential bitmap on the side of the storage control unit  20 A. Specifically, the storage control unit  20 A creates a new bitmap on P 1  with a bit having “1” set up for either of the differential bits at the same position of the differential bitmaps P 1  and  51  determined as “1” and one having “0” set up for both differential bits determined as “0” (step  20050 ). 
     (4) After the new bitmap has been created, the storage control unit  20 A refers to the differential bitmap P 1  from its beginning and, if the bit has “1” set up, performs a copy processing, and if the bit has “0” set up, does not perform the copy processing and refers to a next bit to perform the copy processing (step  20060 ). 
     The copy processing is executed by the same processing as the initial copy described above. The bitmap merged by the above-described processing in the step  20050  may be disposed other than the storage control unit  20 A. And, the merge processing may be executed by any unit other than the storage control unit  20 A. 
     According to the above-described Resync method, the Resync processing can be achieved without increasing the differential bitmap volume. According to this method, the management of the link between the storage control units  20 A and  20 B can also be simplified. 
     The configurations and operations of the storage systems according to the first to third embodiments of the present invention were described above. Then, the storage system according to a fourth embodiment of the present invention will be described below. In the fourth embodiment of the present invention, the structure in terms of hardware may be the same as in the first to third embodiments of the present invention described above except that two differential bitmaps each are disposed in both of the storage control unit  20 A having the original volume and the storage control unit  20 B having the copy volume. 
       FIG. 17A  and  FIG. 17B  are diagrams showing example arrangements of the differential bitmap according to the fourth embodiment of the present invention. 
     When replication is to be created in the same storage control unit, the pair of the original and a copy is created in, for example, a storage group  31  to be connected to the storage control unit  20 A as shown in  FIG. 17A , and a differential bitmap  240  is previously created in two as P 1 , P 2  in the memory  22 A of the storage control unit  20 A. Where the replication is created in different storage control units, the pair of the original and a copy is created in, for example, the storage group  31  connected to the storage control unit  20 A and the storage group  31  connected to the storage control unit  20 B as shown in  FIG. 17B , and the differential bitmap  240  is created in two as P 1 , P 2  in the memory  22 A of the storage control unit  20 A and as S 1 , S 2  in the memory  22 B of the storage control unit  20 B. And, the differential bitmaps P 1 , S 1  are created at the time of the initial copy, and the differential bitmaps P 2 , S 2  are used in the processing after the pair is split. 
     According to the fourth embodiment of the present invention, the initial copy may be executed by the same processing as that according to the flowcharts described with reference to  FIG. 11  and  FIG. 12 , and the processing in response to a write request during the copy processing may be executed for the pair in the same storage control unit in the same way as the processing according to the flowchart described with reference to  FIG. 13 . 
       FIG. 18  is a flowchart illustrating an operation according to the fourth embodiment of the present invention when there is a write request during the processing for creation of the replication between different storage control units, and this operation will be described below. 
     (1) Upon receiving a write request from the host  10  (step  22010 ), the storage control unit  20 A secures a memory region in the cache memory for storage of write data (step  22020 ), receives the write data being transferred from the host  10  and stores in the memory region secured in the cache memory (step  22030 ). 
     (2) A differential bit corresponding to the write data of the differential bitmap P 1  is set to “1”. When the differential bit has already a value “1”, the initial copy to the storage area corresponding to the differential bit has not completed, so that the write data is stored in the cache memory within the storage control unit  20 A, and the write data is copied to the copy side at the time of the initial copy processing. When the bit is “0”, the differential bit P 1  is repeatedly searched to detect the bit “1” and copied to the copy side when the copy processing is executed (step  22040 ). 
     (3) Then, the storage control unit  20 A reports the completion to the host  10 , and the processing here is terminated (step  22050 ). 
     (4) The data stored in each cache memory is written (destaged) in the original volume in asynchronization with the above-described processing (step  22060 ) and written (destaged) in the copy volume (step  22070 ). 
     The split is generally executed when the initial copy is terminated and the contents of the original volume and the copy volume are synchronized. On the other hand, there is a technology called “high-speed Split” in that if a Split request is received during the initial copy, the split completion is immediately reported to the host  10 , and the remaining copy is performed in the background. 
       FIG. 19  is a flowchart illustrating an operation of high-speed split processing according to the fourth embodiment of the present invention, and this operation will be described below. 
     (1) The host  10  issues “high-speed split” (step  14010 ), the storage control unit  20 A receives it, and the storage control unit  20 A switches the differential bitmap  240 , which stores a write request location from the host  10 , from P 1  to P 2 . In other words, the storage control unit  20 A has 1 bit of information indicating which of the bitmaps P 1 , P 2  is used and switches to use alternately the bitmaps P 1 , P 2  (step  14020 ). 
     (2) The storage control unit  20 A judges whether the pair is a pair between different storage control units (step  14030 ), and if the pair is a pair between the different storage control units, transfers the differential bitmap P 2  to the storage control units  20 B (step I 4040 ), and the storage control unit  20 B stores the received differential bitmap P 2  in the differential bitmap S 2  (step  14050 ). 
     (3) After the processing in the step  14050 , or if the pair was judged not to be a pair between different storage control units in the step I  4030 , the storage control unit  20 A changes the pair status in the volume pair information table. Thus, the original volume and the copy volume can accept a read/write request. The storage control unit  20  performs the processing of copying dirty data to the copy volume in the background according to the bitmap P 2  (step  14060 ). 
     (4) For the processing to reflect to the copy volume, the differential bits of the differential bitmap are sequentially referred in order to judge whether bit “1” is detected (step I 4070 ), and if “1” is detected for a differential bit of the differential bitmap, it is judged whether it is a pair in the same storage control unit (step  14080 ). 
     (5) If it is judged as the pair in the same storage control unit in the step I 4080 , the same processing as in the steps  6030  to  6060  and  6080  described with reference to the flow shown in  FIG. 11  is performed (step  14090 ). 
     (6) If it is judged as the pair between the different storage control units in the step  14080 , the same processing as in the steps  8030  to  8100  (a difference in  8100  is P 2 ) and step  8120  described with reference to the flow shown in  FIG. 12  is performed (step  14100 ). 
     (7) After reporting the completion in the step  8090  during the processing in the step  14100 , the corresponding bit of the differential bitmap S 2  is also set to “0” (step  14110 ). 
     (8) After completing the processing described above, it is judged whether a next differential bit of the differential bitmap is searched or not (step  14120 ), and if the next differential bit is to be searched, the procedure returns to the processing in the step  14070  and the same processing is repeated. Otherwise, the processing here is terminated. 
     In the above-described processing operation, the processing in the step  14070  and afterward is a processing for performing the remaining copy in the background (the copy processing having been performed before falling in the split status in order to have the same content between the original and copy volumes). 
     In the above-described processing, the differential bitmaps P 2  and S 2  have their contents always matched. 
       FIG. 20  is a flowchart illustrating an operation to perform a write processing to the original volume when falling in a high-speed split status according to the fourth embodiment of the present invention, and this operation will be described below. 
     (1) The storage control unit  20 A receives from the host  10  a write request to the original volume (step  15010 ) and sets a differential bit at a portion corresponding to the write object data of the differential bitmap P 1  to “1” (step  15020 ). 
     (2) Then, the storage control unit  20 A observes a differential bit of a portion corresponding to the write object data of the differential bitmap P 2  (namely, a portion corresponding to old data to be updated by write data) to check whether its value is “1” (step  15030 ). If the value “1” is detected, it indicates that data corresponding to the differential bit has not been copied to the copy volume, so that the storage control unit  20 A judges whether the old data to be written is present in the cache memory (step  15040 ). 
     (3) If the old data is not in the cache memory when judged in the step  15040 , the storage control unit  20 A secures a storage area in the cache memory (step  15050 ), reads the old data to be written from the original volume and performs staging (step  15060 ). 
     (4) If the old data is in the cache memory when judged in the step  15040  (a cache hit), or after the staging of the old data to the cache memory by the processing in the step  15060 , the storage control unit  20 A issues a write request of old data subject to writing to the storage control unit  20 B having the copy volume (step  15070 ). 
     (5) The storage control unit  20 B receives the write request from the storage control unit  20 A and secures a cache memory area (step  15080 ), and the storage control unit  20 A receives a cache memory area assurance completion report from the storage control unit  20 B (step  15090 ). 
     (6) The storage control unit  20 A transfers the old data to be written to the storage control unit  20 B (step  15100 ). Meanwhile, the storage control unit  20 B stores the transferred data into the cache memory and also sets a differential bit corresponding to the old data to-be-written of the differential bitmap S 2  to “0” (step  15110 ). 
     (7) The storage control unit  20 A receives a write completion report of the old data to-be-written from the storage control unit  20 B (step  15120 ) and sets a differential bit corresponding to the old data to-be-written of the differential bitmap P 2  to “0” (step  15130 ). 
     (8) After the processing in the step  15130 , or if the differential bit “1” is not detected but “0” is detected in the step  15030 , the storage control unit  20 A receives write data being transferred from the host  10  (step  15140 ), returns the write completion report to the host  10  and terminates the processing here (step  15150 ). 
     (9) The storage control unit  20 B stores the data from the storage control unit  20 A into the cache memory by the processing in the step  15100  and stores the data into the copy volume in asynchronization with the copy processing from the storage control unit  20 A (step  15160 ). 
     (10) The storage control unit  20 A receives the data from the host  10  by the processing in the step  15140  and stores the data into the original volume in asynchronization with the write request from the host (step  15170 ). 
       FIG. 21  is a flowchart illustrating an operation to perform write processing to the copy volume when falling in the high-speed split status according to the fourth embodiment of the present invention, and this operation will be described below. 
     (1) The storage control unit  20 B receives a write request to the copy volume from the host  10  (step  16010 ) and sets the differential bit at a portion corresponding to the data to-be-written of the differential bitmap S 1  to “1” (step  16020 ). 
     (2) The storage control unit  20 B detects whether a differential bit at a portion (namely, a portion corresponding to the old data to be updated by write data) corresponding to the data to-be-written of the differential bitmap S 2  is “1” (step  16030 ), and if it can not be detected that the differential bit is “1”, secures a cache memory area (step  16035 ) and proceeds to step  16160  to be described later. 
     (3) If the differential bit has “1” set up when detected in the step  16030 , it indicates that data corresponding to that bit has not been copied from the original volume to the copy volume, so that the storage control unit  20 B judges whether the data to-be-written does not match the whole range of data corresponding to the differential bit (step  16040 ). 
     (4) If the data to-be-written matches the whole range of data corresponding to the differential bit when judged in the step  16040 , the copying from the original volume is meaningless because all data are rewritten, so that the copying may be omitted. The storage control unit  20 B secures a cache memory area (step  16080 ) and receives data from the host (step  16190 ). 
     (5) Then, the storage control unit  20 B sets a differential bit corresponding to the old data to-be-written of the differential bitmap P 2  to “0” (step  16200 ), sets the differential bit corresponding to old data to-be-written of the differential bitmap S 2  to “0” and proceeds to step  16170  to be described later (step  16210 ). 
     (6) If the write range does not fully match in the step  16040 , the storage control unit  20 B issues a to-be-written request to the storage control unit  20 A. The issue of the to-be-written request means that the storage control unit  20 B asks the storage control unit  20 A to issue a write request to the storage control unit  20 B (step  16015 ). 
     (7) The storage control unit  20 A judges whether the old data to-be-written hits the cache memory (step  16050 ) and, if it does not hit, secures the cache memory (step  16060 ) and stages the old data (step  16070 ). 
     (8) If the old data to-be-written hits the cache memory of the storage control unit  20 A when judged in the step  16050 , or after the old data is staged by the processing in the step  1670 , the storage control unit  20 A issues an old data write request to the storage control unit  20 B (step  16080 ). 
     (9) The storage control unit  20 B secures a cache memory area (step  16090 ) and, if secured, reports the cache memory assurance completion to the storage control unit  20 A (step  16100 ). 
     (10) The storage control unit  20 A transfers the old data to the storage control unit  20 B (step  16110 ), the storage control unit  20 B sets a corresponding bit of the differential bitmap S 2  to “0” (step  16120 ) and reports the completion to the storage control unit  20 A (step  16130 ). 
     (11) The storage control unit  20 A which has transferred the old data sets the differential bitmap P 2  to “0” (step  16140 ) and reports the completion of the to-be-written request to the storage control unit  20 B (step  16150 ). 
     (12) The storage control unit  20 B receives write data from the host  10  (step  16160 ), sends a completion report to the host  10  and terminates the processing here (step  16170 ). 
     (13) The storage control unit  20 B having received the write data from the host  10  by the processing in the step  16160  stores the data into the copy volume asynchronously (step  16220 ). 
     The processing in response to the write request to the copy volume was described above. In a case of a read request to the copy volume, the storage control unit  20 B receives old data from the storage control unit  20 A and transfers to the host  10  in the same way as above. 
     The processing at a time when the original volume receives a write request to an unreflected region to the copy volume from the host  10  when falling in the above-described high-speed split status is called the “previous copy processing”, and the processing at a time when the copy volume receives from the host  10  a read/write request to an uncopied region from the original volume is called the “prefetch copy processing”. 
     The fourth embodiment of the present invention realizes the prefetch copy processing, as a form of the to-be-written request from the copy volume to the original volume as described with reference to the flow of  FIG. 21 , so that a deadlock involved in the cache memory area assurance within the opposite-side storage control unit between the prefetch copy processing and the previous copy processing can be prevented. In other words, the processing among the high-speed Split, the previous copy and the prefetch copy can be performed by the processing having secured the lock of the data to-be-copied, and it becomes possible to perform exclusion control. 
     After the high-speed Split, the differential bitmap is switched from P 1  to P 2 , the bitmap P 2  is used to continue copying in the background, but the bitmap P 2  is not used for the update of data, and it is made not to turn ON the bit of the bitmap P 2 . And, the opportunity to turn OFF the bit of the differential bitmap P 1  is determined to be after the background copy, previous copy and prefetch copy complete copying the data to-be-copied from the original volume to the copy volume. Thus, a case that the bit of the differential bitmap P 1  is unfavorably turned OFF can be excluded. And, the differential bitmap S 2  can set all bits to “1” (ON) before the split so to exclude a case “the bit of the differential bitmap S 2  is OFF, but the bit of the differential bitmap P 2  is ON”. A region where the differential bitmap P 2  has not been copied to the differential bitmap S 2  is judged to be necessary to perform a prefetch copy with reference to the differential bitmap S 2 , and the “to-be-written request” is performed, but when the differential bitmap P 2  is referred to, there is a case that the copy is actually not necessary. In such a case, the bit of the differential bitmap S 2  is turned OFF, and the request of the copy volume may be performed. 
     Besides, the storage control unit  20 A determines a frequency of sending depending on a response time for the cache memory assurance from the storage control unit  20 B and performs sending. It is because the processing is in series from the storage control unit  20 A to the storage control unit  20 B, so that it is necessary to limit a flow rate of processing. 
     The Resync processing in the fourth embodiment of the present invention can be performed in the same way as those in the first to third embodiments of the present invention. 
       FIG. 22  is a diagram showing a data transfer path in case of a write request during the replication creation processing between different storage control units according to the fourth embodiment of the present invention.  FIG. 22  also shows an arrangement of the differential bitmap. 
     It is seen from the data transfer path shown in  FIG. 22  that, if there is a write request during the replication creation processing between the different storage control units, the reflection of the update to the copy volume when the write request is received can be performed on an extension of the same I/O processing in the same way as in the above-described first to third embodiments of the present invention. The write request for update to the copy volume in this case is sent from CPU  21 A to CPU  21 B through the same route as the data transmission line, and the differential bitmap at the time of merging of the differential bitmap is also sent through the same route as the data transmission line. 
       FIG. 23  is a diagram showing a data transfer path in case of a write request during the replication creation processing between different storage control units in the second embodiment of the present invention shown in  FIG. 2 , and  FIG. 24  is a diagram showing a data transfer path in case of a write request during the replication creation processing between different storage control units in the third embodiment of the present invention shown in  FIG. 3 . 
     The data transfer paths shown in  FIG. 23  and  FIG. 24  are different form that shown in  FIG. 15  because the hardware has a different structure, but the other portions are not different from those in  FIG. 15 , and the write request for update to the copy volume is sent from the CPU  21 A to the CPU  21 B through the same route as the data transmission line. When the above-described fourth embodiment of the present invention is applied to the embodiments of the present invention shown in  FIG. 2  and  FIG. 3 , the write request for update to the copy volume is sent from the CPU  21 A to the CPU  21 B through the same route as the data transmission line in the same way as described with reference to  FIG. 22 . And, the differential bitmap at the time of merging the differential bitmap is also sent through the same route as the data transmission line. 
     According to the first to fourth embodiments of the present invention described above, when replication is to be created in the volume within the disk devices connected to different control units in the storage system having the plural storage control units to which the plural disk devices are connected, a copy of the storage area can be created without degrading the performance by minimizing the access of control information between the control units. And, where a pair is divided, the control units are physically divided, and the I/O of the other control unit does not affect, so that the storage area can be copied without degrading the performance. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.