Patent Publication Number: US-8112600-B2

Title: Creating a snapshot based on a marker transferred from a first storage system to a second storage system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 12/786,464, filed May 25, 2010, now U.S. Pat. No. 7,953,947 which is a continuation of U.S. application Ser. No. 11/527,446, filed Sep. 27, 2006 (now U.S. Pat. No. 7,725,668). This application relates to and claims priority from Japanese Patent Application No. 2006-213896, filed on Aug. 4, 2006. The entirety of the contents and subject matter of all of the above is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to a storage system and a snapshot creation method thereof. 
     Conventionally, a computer system that connects a host computer and a storage system with a network, and stores data processed with the host computer in the storage system by sending and receiving such data via the network has been put into practical application. 
     In this kind of computer system, remote copy technology is known for preventing the loss of data even when a disaster occurs in the storage system of a local site by configuring a volume group of the storage system of the local site and a volume group of the storage system of a remote site in a pair relationship for executing volume-to-volume copy, and copying (replicating) data from the volume group of the local site storage system to the volume group of the remote site storage system. 
     As background art related to remote copy technology, for instance, IBM REDBOOKS—Disaster Recovery with DB2 UDB for zOS; November 2004, and IBM REDBOOKS—The IBM Total Storage DS8000 Series: Copy Services with IBM Eserver zSeries; February 2006, disclose async remote copy technology for creating a backup of the volume group of the local site storage system in the volume group of the remote site storage system by periodically compiling and transferring the change difference of the volume group of the local site storage system asynchronously to the remote site storage system. 
     Nevertheless, with the foregoing async remote copy technology, upon transferring the change difference of the volume group of the local site storage system to the volume group of the remote site storage system, consistency regarding the writing sequence by the host computer in the volume or volume group of the local site storage system is not guaranteed. 
     Thus, with this async remote copy technology, when a disaster or the like occurs in the local site storage system during the transfer of data, such data may be damaged since the consistency of the volume group of the remote site storage system is not guaranteed. 
     Thus, also disclosed is remote copy technology which combines the async remote copy technology and the local copy technology in order to guarantee the existence of a volume with consistency regarding the writing sequence from the host computer in the remote site storage system at an arbitrary point in time. Specifically, with this remote copy technology, async remote copy is performed from a first volume group of the local site storage system to a second volume group of the remote site storage system, and, after transferring all change differences, local copy is performed from the second volume group to a fourth volume group of the remote site storage system. Like this, by alternately performing async remote copy and local copy, existence of a volume group having consistency will be guaranteed in the fourth volume group when transferring the change difference of the first volume group of the local site storage system to the second volume group of the remote site storage system. 
     Further, Using Asynchronous Replication for Business Continuity Between Two or More Sites; December 2004 discloses remote copy technology capable of configuring a volume group having consistency regarding the writing sequence from the host computer to the local site storage system even when a disaster or the like occurs in the local site storage system and subsequent data transfer cannot be performed by primarily storing difference data in a difference buffer in the remote site storage system. 
     Nevertheless, with the foregoing remote copy technology that combines async remote copy technology and local copy technology, when the second volume group and the fourth volume group are used for testing or the like in the remote site storage system during the system operation of the remote copy technology, it is necessary to discontinue the system operation in the remote copy technology until such testing is complete. Further, when data is written in the second volume group and the fourth volume group while such testing is being conducted, there is a possibility that such data cannot be backed up properly. 
     Thus, it is conceivable to newly prepare a third volume group in the remote site storage system and create a snapshot of the second volume group as the third volume group. Nevertheless, when creating a snapshot of the second volume group as the third volume group, there may be cases where the second volume group is copying the change difference, and, therefore, consistency of the writing sequence from the host computer in the local site storage system will not be guaranteed. 
     Here, in order to create a snapshot where the consistency is guaranteed, it is necessary to discontinue the system operation in the foregoing remote copy technology, issue a command to the local site for guaranteeing the consistency, create a snapshot of the second volume, and thereafter make the local site issue a command for resuming the system operation in the remote copy technology. Thus, complex operations such as the discontinuance and resumption of the system operation in the remote copy technology are required by the local site storage system, and will affect the system operation. 
     Moreover, even with the remote copy technology described in Non-Patent Document 3, when creating a snapshot of the second volume group in the remote site storage system during the system operation in the remote copy technology, there may be cases where the consistency is not guaranteed since it may be applying data of the difference buffer. 
     SUMMARY 
     The present invention was devised in view of the foregoing points. Thus, an object of the present invention is to propose a computer system and a snapshot creation method thereof capable of a simple and fast system operation. 
     In order to overcome the foregoing problems, the present invention provides a computer system including a first storage system having a first volume for storing data sent from a host system, and a second storage system having a second volume for storing the data sent from the first storage system. The first storage system comprises a data transfer unit for transferring the data stored in the first volume to the second volume of the second storage system. The second storage system comprises a snapshot creation unit for creating a snapshot of the second volume in a third volume based on a snapshot creation command. When the snapshot creation unit receives the snapshot creation command while transferring the data from the first volume to the second volume with the data transfer unit, the snapshot creation unit delays the creation of the snapshot of the second volume in the third volume until the transfer of the data from the first volume to the second volume is complete. 
     Accordingly, even when testing or the like is conducted during the operation of the remote copy system, it is possible to effectively prevent complex operations such as the discontinuance or resumption of the system operation from occurring, and create a snapshot having consistency without having to discontinue the system operation. 
     The present invention further provides a snapshot creation method of a computer system including a first storage system having a first volume for storing data sent from a host system, and a second storage system having a second volume for storing the data sent from the first storage system. The snapshot creation method comprises a first step of transferring the data stored in the first volume to the second volume of the second storage system, and a second step of creating a snapshot of the second volume in a third volume based on a snapshot creation command. At the second step, when the snapshot creation command is received while transferring the data from the first volume to the second volume at the first step, creation of the snapshot of the second volume in the third volume is delayed until the transfer of the data from the first volume to the second volume is complete. 
     Accordingly, even when testing or the like is conducted during the operation of the remote copy system, it is possible to effectively prevent complex operations such as the discontinuance or resumption of the system operation from occurring, and create a snapshot having consistency without having to discontinue the system operation. 
     According to the present invention, when the snapshot creation command is received while transferring the data from the first volume to the second volume, creation of the snapshot of the second volume in the third volume is delayed until the transfer of the data from the first volume to the second volume is complete. Therefore, even when testing or the like is conducted during the operation of the remote copy system, it is possible to effectively prevent complex operations such as the discontinuance or resumption of the system operation from occurring, and create a snapshot having consistency without having to discontinue the system operation. Consequently, it is possible to realize a computer system and a snapshot creation method thereof capable of a simple and fast system operation. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the schematic configuration of a computer system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing the schematic configuration inside the computer system; 
         FIG. 3  is a conceptual diagram explaining an update bitmap and a difference bitmap; 
         FIG. 4  is a conceptual diagram explaining pair information; 
         FIG. 5  is a conceptual diagram explaining a side file; 
         FIG. 6  is a conceptual diagram explaining local copy control data; 
         FIG. 7  is a conceptual diagram explaining the operation of a remote copy system; 
         FIG. 8  is a conceptual diagram explaining the operation of a remote copy system; 
         FIG. 9  is a conceptual diagram explaining the update status of the first to fourth volume groups; 
         FIG. 10  is a flowchart showing an I/O access processing routine; 
         FIG. 11  is a flowchart showing an I/O access processing routine; 
         FIG. 12  is a flowchart showing an I/O access processing routine; 
         FIG. 13  is a flowchart showing an I/O access processing routine; 
         FIG. 14  is a flowchart showing a copy management processing routine; 
         FIG. 15  is a flowchart showing a copy management processing routine; 
         FIG. 16  is a flowchart showing a remote copy processing routine; 
         FIG. 17  is a flowchart showing a local copy processing routine; 
         FIG. 18  is a flowchart showing a snapshot processing routine; 
         FIG. 19  is a flowchart showing a background copy processing routine; 
         FIG. 20  is a flowchart showing an extended snapshot processing routine; 
         FIG. 21  is a block diagram showing the schematic configuration of a computer system according to a second embodiment of the present invention; 
         FIG. 22  is a block diagram showing the schematic configuration inside a computer system; 
         FIG. 23  is a conceptual diagram explaining a difference buffer; 
         FIG. 24  is a conceptual diagram explaining the update status of the first to third volume groups; 
         FIG. 25  is a flowchart showing an I/O access processing routine; 
         FIG. 26  is a flowchart showing an I/O access processing routine; 
         FIG. 27  is a flowchart showing an I/O access processing routine; 
         FIG. 28  is a flowchart showing a copy management processing routine; 
         FIG. 29  is a flowchart showing a copy management processing routine; 
         FIG. 30  is a flowchart showing a remote copy processing routine; 
         FIG. 31  is a flowchart showing a local copy processing routine; 
         FIG. 32  is a conceptual diagram explaining a marker; 
         FIG. 33  is a flowchart showing a marker processing routine; 
         FIG. 34  is a flowchart showing a copy management processing routine; and 
         FIG. 35  is a flowchart showing an extended snapshot processing routine. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is now explained in detail with reference to the attached drawings. 
     (1) First Embodiment 
       FIG. 1  shows a schematic configuration of a computer system  100  according to a first embodiment of the present invention. The computer system  100  is configured by connecting a host computer  101 , a first storage system  102 A, and a second storage system  102 B via a network  103 . The computer system  100  is configured by further connecting a management terminal  104  for referring to the internal status to the first storage system  102 A and the second storage system  102 B via the network  103 , or directly. 
     The first storage system  102 A is set on a physical storage extent provided by one or more hard drives  211 A,  221 B described later, and has one or more first volume groups  111 A for storing data. Further, the second storage system  102 B has a second volume group  111 B, a third volume group  111 C and a fourth volume group  111 D configured the same as the first volume group  111 A. 
     The first volume group  111 A and the second volume group  111 B form a pair relationship  121  for executing a volume-to-volume copy of a remote copy. The computer system  100  executes a remote copy  122  between the first volume group  111 A and the second volume group  111 B. Further, the second volume group  111 B and the fourth volume group  111 D form a pair relationship  123  for executing a volume-to-volume copy of a local copy, and the computer system  100  executes a local copy  124  between the second volume group  111 B and the fourth volume group  111 D. Like this, the computer system  100  configures a remote copy system with two pair relationships. 
     The third volume group  111 C is a volume group to become a target of a snapshot  125  of the second volume group  111 B. The third volume group  111 C, which is a volume group after the snapshot  125  has been executed, is access from the host computer  101  or the like for testing and other applications. 
     The first storage system  102 A has two bitmaps corresponding to the first volume group  111 A; namely, an update bitmap  112 A and a difference bitmap  112 B. The update bitmap  112 A represents the portions that are updated by data being written in the first volume group  111 A. The difference bitmap  112 B represents difference data to be transferred to the second volume group  111 B of the second storage system  102 B. Further, the second storage system  102 B has an update bitmap  112 C corresponding to the second volume group  111 B, and an update bitmap  112 D corresponding to the third volume group  111 C. 
     Generally, the first storage system  102 A is physically installed at the local site, and the second storage system  102 B is physically installed at the remote site away from the local site to avoid being influenced by a disaster or the like. 
       FIG. 2  shows a schematic configuration inside the computer system  100 . The first storage system  102 A is configured from a storage control unit  201 A, and a storage unit  202 A connected to the storage control unit  201 A. The storage unit  202 A is configured from a plurality of hard drives  211 A. The storage control unit  201 A is configured from an I/O unit  221 A connected to the network  103 , a management I/O unit  222 A connected to the management terminal  104  and the network  103 , a maintenance terminal  223 A to be used by a user or the like for giving commands to the storage control unit  201 A, a memory  224 A storing various programs and data, and a processor  225 A for primarily operating the various programs stored in the memory  224 A. Incidentally, the foregoing hard drive  211 A is not limited to a magnetic storage medium such as a hard disk, and may also be a semiconductor memory or the like. Further, the storage control unit can also be configured with the memory  224 A as the storage medium. In such a case, the storage unit  202 A can configured the storage system without being connected. Moreover, another storage system can be connected via a fibre channel or the like as a substitute for the storage unit  202 A so as to operate as a storage unit. 
     The second storage system  102 B is configured the same as the first storage system  102 A excluding the point that the various programs and information stored in the memory  224 B are different, and the respective constituent elements of the second storage system  102 B that correspond to the constituent elements of the first storage system  102 A are given a subscript of “B” in substitute for the subscript of “A” next to the same reference numerals. Incidentally, with the computer system  100 , from the perspective that the second storage system  102 B is a backup storage system, the hard drive  211 A of the first storage system  102 A can be configured from an expensive hard disk drive such as a SCSI (Small Computer System Interface) disk, and the hard drive  211 B of the second storage system  102 B can be configured from an inexpensive hard disk drive such as a SATA (Serial AT Attachment) disk or an optical disk. 
     The memory  224 A of the first storage system  102 A stores a copy management program  231 A for controlling the execution of the remote copy in coordination with a copy management program  231 B described later, a remote copy program  232  for controlling the intermittent transfer of difference data from the first volume group  111 A of the first storage system  102 A to the second volume group  111 B of the second storage system  102 B, an I/O (Input/Output) control program  233 A for controlling the data access (reading and writing) from and to the first volume group  111 A, a marker control program  234  for controlling the writing of a marker described later, a plurality of update bitmaps  112 A, a plurality of difference bitmaps  112 B, pair information  235 A, and a side file  236  to be used for retaining write-target data. 
     Further, the memory  224 B of the second storage system  102 B stores a copy management program  231 B for controlling the execution of the remote copy and local copy in coordination with the copy management program  231 A, a local copy program  237 , an I/O control program  233 B for controlling the data access from and to the second to fourth volume groups  111 B to  112 D, a snapshot extension program  238 , a snapshot program  239 , a local copy control data  240 , a plurality of update bitmaps  112 C,  112 D, and pair information  235 B. 
       FIG. 3  to  FIG. 6  show the data structure of data retained inside the first and second storage systems  102 A,  102 B. 
       FIG. 3  is a schematic explanation of the update bitmaps  112 A,  112 C,  112 D and the difference bitmap  112 B. The update bitmaps  112 A,  112 C,  112 D and the difference bitmap  112 B exist in correspondence to the first to third volume groups  111 A to  111 C, and are an aggregate of bits  301  corresponding one-on-one with a storage extent  302  of a fixed size with respect to all storage extents of the first to third volume groups  111 A to  111 C. The respective bits  301  possess information regarding whether or not the data stored in the storage extent  302  corresponding to the first to third volume groups  111 A to  112 C has been updated from a certain period in time. The update bitmaps  112 A,  111 C,  111 D are used for recording the storage extent  303  to which data has been written in the corresponding first to third volume groups  111 A to  112 C within a specified time, or for determining the necessity of performing data copy to the storage extent  302 . The difference bitmap  112 B is used for referring to data to be copied upon performing the remote copy from the storage extent  303  of the first volume group  111 A. Details will be explained in the operation of the programs described later. 
       FIG. 4  shows a table structure of the pair information  235 A. The pair information  235 A has the items of a pair identifier  311 A, a source volume name  312 A as a volume of the copy source, a target volume name  313 A as a volume of the copy destination, and a pair relationship  314 A representing the pair relationship of the remote copy, and is configured as a list. Incidentally, the pair information  235 B is configured the same as the pair information  235 A other than the point that the pair relationship of the remote copy, local copy, and snapshot regarding the pair relationship  314 B is different. 
       FIG. 5  shows a table structure of the side file  236 . The side file  236  has the items of a volume name  321 , a storage extent address  322 , and a stored data  323 , and is configured from a list. The side file  236  is primarily used for temporarily storing data for the purpose of storing data before it is written in the target storage extent. For instance, when transferring data to the second storage system  102 B while referring to the difference bitmap  112 B and in parallel writing data from the host computer  101  to the second volume  111 B, since the storage extent corresponding to the difference bitmap  112 B may be changed, the side file  236  can be used to temporarily store data that is currently written in such storage extent, and used as a reference upon transferring data if needed. Further, in addition to being stored in the memory  224 A, the side file  236  may also exist in the storage extent of the storage unit  202 A. Details will be explained in the operation of the programs described later. 
       FIG. 6  shows a table structure of the local copy control data  240 . The local copy control data  240  has the items of a volume name  331 , a local copy control flag  332 , and an execution queue flag  333 , and is configured from a list. The local copy control flag  332  is used for determining whether it is possible to create a snapshot in the relevant volume. Here, if the local copy control flag  332  has been turned on (ON), this means that a local copy can be executed since the remote copy is complete. Meanwhile, if the local copy control flag  332  has been turned off (OFF), this means that a local copy cannot be executed since the remote copy is in execution. The execution queue flag  333  is used for preventing a local copy end report from being sent to the first storage system  102 A since the snapshot is in execution. Further, in the foregoing case, if the execution queue flag  333  has been turned on (ON), this means that a remote copy cannot be executed (a local copy end report cannot be sent) since the snapshot is in execution. Meanwhile, if the execution queue flag  333  has been turned off (OFF), this means that a remote copy can be executed (a local copy end report can be sent) since the snapshot is complete. 
       FIG. 7  and  FIG. 8  show the outline of the operation of the remote copy system. The first storage system  102 A, as shown in  FIG. 7 , executes the remote copy  122  for compiling the write-target data which has been written in the first volume group  111 A within a specified time, and transferring such compiled data from the first volume group  111 A to the second volume group  111 B. Subsequently, the second storage system  102 B, as shown in  FIG. 8 , executes the local copy  124  for transferring the compiled data from the second volume group  111 B to the fourth volume group  111 D. With this computer system  100 , it is possible to configure a remote copy system capable of guaranteeing the existence of a volume having data consistency in the second storage system by alternately executing the foregoing remote copy  122  and local copy  124  in cycles. Data consistency will be described in detail later. 
       FIG. 9  shows the update status of the first to fourth volume groups  111 A to  111 D in a time series. Here, reference number  341 A represents the update status of the first volume group  111 A in a time series, reference number  341 B represents the update status of the second volume group  111 B in a time series, reference number  341 C represents the update status of the third volume group  111 C in a time series, and reference number  341 D represents the update status of the fourth volume group  111 D in a time series. 
     The first storage system  102 A, at timing T 1 , switches the contents of the update bitmap  112 A and the difference bitmap  112 B corresponding to the first volume group  111 A. The update status of the first volume group  111 A at this point in time in the first storage system  102 A is referred to as an update status A. 
     Subsequently, the remote copy program  232  of the first storage system  102 A executes the remote copy  122 A for referring to the contents of the difference bitmap  112 B and transferring difference data to the second storage system  102 B in order to make the second volume group  111 B become an update status A. During a difference application period D 1 , which is the application period of applying difference data to the second volume group  111 B of the second storage system  102 B, since data is written in the second volume group  111 B independent from the writing sequence from the host computer  101 , the data consistency regarding the writing sequence of the second volume group  111 B is not guaranteed. 
     Eventually, the second volume group  111 B becomes an update status A when the remote copy  122 A is complete and the difference application period D 1 , which is the application period of difference data for making the update status become an update status A, is complete (timing T 2 ). Thereby, the data consistency of the first volume group  111 A and the second volume group  111 B will be guaranteed. 
     Subsequently, the local copy program  221  of the second storage system  102 B executes the local copy  124 A of transferring difference data from the second volume group  111 B to the fourth volume group  111 D in order to make the update status of the fourth volume group  111 D become an update status A. During a difference application period D 2 , which is the application period of applying difference data to the fourth volume group  111 D, the data consistency regarding the write-target data which has been written in the fourth volume group  111 D may not be guaranteed. 
     Eventually, the fourth volume group  111 D becomes an update status A when the local copy  124 A is complete and the difference application period D 2 , which is the application period of difference data, is complete (timing T 3 ). Thereby, the data consistency of the second volume group  111 B and the fourth volume group  111 D will be guaranteed. 
     As described above, with the computer system  100 , data consistency of the fourth volume group  111 D is guaranteed when the second volume group  111 B is in the difference application period D 1 , and data consistency of the second volume group  111 B is guaranteed when the fourth volume  112 D is in the difference application period D 2 . Thereby, it is possible to constantly guarantee the existence of a volume having data consistency in the second storage system  102 B during the operation of the remote copy system. 
     Further, with the computer system  100 , a local copy end report is sent to the first storage system  102 A when the local copy  124 A is complete ( 342 ), and the remote copy program  232  of the first storage system  102 A will thereby complete a single loop. Thereafter, the first storage system  102 A, at timing T 4 , once again switches the contents of the update bitmap  112 A and the difference bitmap  112 B corresponding to the first volume group  111 A. Then, after making the update status of the first volume group  111 A of the first storage system at such point in time become an update status B, the first storage system  102 A and the second storage system  102 B, as with the case described above, intermittently executes the remote copy  122 B and the local copy  124 B in order to make the update status of the second volume group  111 B and the fourth volume group  111 D become an update status B. 
     Here, when a snapshot creation command is issued from the second volume group  111 B to the third volume group  111 C at timing T 5  (difference application period D 1  of the second volume group  111 B) during the operation of such remote copy system, the second storage system  102 B delays the snapshot execution until the difference application period D 1  of the second volume group  111 B is complete (delay period D 3 ). Then, the second storage system  102 B executes a snapshot when the difference application period D 1  of the second volume group  111 B is complete ( 343 ). Here, the second storage system  102 B instantaneously ends the execution of the snapshot itself, and executes a background copy of copying all data of the second volume group  111 B to the third volume group  111 C within a time limit ( 344 ). 
     Further, when data is written in the second volume group  111 B during the background copy, the second storage system  102 B executes the I/O control program  216 B so as to copy data of the relevant storage extent in the second volume group  111 B to the corresponding storage extent of the third volume group  111 C, and thereafter executes the writing of data in such storage extent of the second volume group  111 B. Operation of the I/O control program  216 B will be explained later with reference to a flowchart. 
     An example of I/O access processing (at the local site) in the first storage system  102 A is now explained.  FIG. 10  is a flowchart showing a specific processing routine of the processor  225 A regarding the I/O access processing in the first storage system  102 A. 
     When the processor  225 A receives an I/O access (write request or read request) from the host computer  101 , it checks whether the type of such I/O access is a write request according to a first I/O access processing routine RT 1  shown in  FIG. 10  by executing the I/O control program  233 A, which is a program for executing the I/O access (S 1 ). 
     When the type of I/O access is not a write request (S 1 : NO), the processor  225 A determines that the type of I/O access is a read request, and reads read-target data from the storage extent of the volume corresponding to the read request (S 2 ), and thereafter ends the first I/O access processing routine RT 1  shown in  FIG. 10  (S 8 ). 
     Contrarily, when the type of I/O access is a write request (S 1 : YES), the processor  225 A checks whether the volume corresponding to the write request is a source volume of the remote copy by referring to the pair information  235 A (S 3 ). 
     When the volume corresponding to the write request is not a source volume of the remote copy (S 3 : NO), the processor  225 A writes the write-target data in a storage extent of a volume corresponding to the write request (S 7 ), and thereafter ends the first I/O access processing routine RT 1  shown in  FIG. 10  (S 8 ). 
     Contrarily, when the volume corresponding to the write request is not a source volume of the remote copy (S 3 : YES), the processor  225 A changes the bit  301  of the update bitmap  112 A corresponding to the storage extent of a volume corresponding to the write request to “ON” (copy required (with update)) ( FIG. 3 ) (S 4 ). 
     Subsequently, the processor  225 A checks whether the bit  301  of the update bitmap  112 A corresponding to the storage extent of a volume corresponding to the write request is “ON”, and whether old data stored in the storage extent of a volume corresponding to the write request is not stored in the side file  236  (S 5 ). 
     In this case, the processor  225 A can also determine the existence of storage in the side file  236  and refer to data by recording whether old data has been stored in the side file  236  in the update bitmap  112 A or the difference bitmap  1128 , and recording the referrals (represented by addresses and the like) to the respective elements of the corresponding side file  236 . 
     When the bit  301  of the difference bitmap  1128  corresponding to the storage extent of a volume corresponding to the write request is “OFF” (copy not required (with no update)) ( FIG. 3 ), or the old data that has been stored in the storage extent of a volume corresponding to the write request is stored in the side file  236  (S 5 : NO), the processor  225 A writes the write-target data in the storage extent of a volume corresponding to the write request (S 7 ), and thereafter ends the first I/O access processing routine RT 1  shown in  FIG. 10  (S 8 ). 
     Contrarily, when the bit  301  of the difference bitmap  112 B corresponding to the storage extent of a volume corresponding to the write request is “ON” (copy required (with update)) ( FIG. 3 ), and the old data that has been stored in the storage extent of a volume corresponding to the write request is not stored in the side file  236  (S 5 : YES), the processor  225 A reads the old data stored in the storage extent of a volume corresponding to the write request and stores this in the side file  236  (S 6 ). 
     In the foregoing case, the processor  225 A specifically stores in the side file  236 , together with the read old data, the volume name  321  and the storage extent address  322  representing the position of the storage extent of a volume corresponding to the write request. 
     Eventually, the processor  225 A writes the write-target data in the storage extent of a volume corresponding to the write request (S 7 ), and thereafter ends the first I/O access processing routine RT 1  shown in  FIG. 10  (S 8 ). 
     Incidentally, the processor  225 A is able to read the read-target data from the storage extent of a volume corresponding to the read request according to a method of making the host computer  101  or a program refer to the data disposed in a cache of the memory  224 A, or a method of reading data from a corresponding hard drive of the storage unit  202 A and making the host computer  101  or various program refer to such data. 
     Furthermore, the processor  225 A realizes the writing of write-target data by writing the write-target data in a cache of the memory  224 A, notifying the end of writing to the host computer  101  and various programs, and writing the write-target data asynchronously in a corresponding hard drive of the storage unit  202 A. Further, the processor  225 A can also realize the writing of write-target data by synchronously writing the write-target data in a corresponding hard drive of the storage unit  202 A. 
     An example of I/O access processing (at the remote site) in the second storage system  102 B is now explained.  FIG. 11  to  FIG. 13  are flowcharts showing a specific processing routine of the processor  225 B regarding the I/O access processing in the second storage system  1028 . 
     When the processor  225 B receives an I/O access from the host computer  101 , or receives an I/O access request as a result of the remote copy program  232  of the first storage system  102 A being executed, it executes the I/O control program  223 B, which is a program for executing the I/O access, for checking whether the volume corresponding to the I/O access is a source volume of a local copy by referring to the pair information  235 B according to a second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 11 ). 
     When the volume corresponding to the I/O access is not a source volume of a local copy (S 11 : NO), the processor  225 B checks whether the volume corresponding to the I/O access is a snapshot target volume by referring to the pair information  235 B (S 12 ). 
     When the volume corresponding to the I/O access is not a snapshot target volume (S 12 : NO), the processor  225 B executes the reading and writing of data from and in the storage extent of a volume corresponding to the I/O access (S 13 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the volume corresponding to the I/O access is a source volume of a local copy (S 11 : YES), the processor  225 B checks whether the type of I/O access is a write request (S 21 ) ( FIG. 12 ). 
     When the type of I/O access is not a write request (S 21 : NO), the processor  225 B determines that the type of I/O access is a read request, reads the read-target data from the storage extent of a volume corresponding to the read request (S 22 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the type of I/O access is a write request (S 21 : YES), the processor  225 B checks whether the volume corresponding to the write request is a snapshot source volume by referring to the pair information  235 B (S 23 ). 
     When the volume corresponding to the write request is not a snapshot source volume (S 23 : NO), the processor  225 B changes the bit  301  of the update bitmap  112 C corresponding to the storage extent of a volume corresponding to the write request to “ON” (S 24 ), writes the write-target data in the storage extent of a volume corresponding to the write request (S 25 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the volume corresponding to the write request is a snapshot source volume (S 23 : YES), the processor  225 B checks whether the bit  301  of the update bitmap  112 D corresponding to the storage extent of such snapshot target volume is “ON” (S 26 ). 
     When the bit  301  of the update bitmap  112 D corresponding to the storage extent of such snapshot target volume is not “ON” (S 26 : NO), the processor  225 B changes the bit  301  of the update bitmap  112 C corresponding to the storage extent of a volume corresponding to the write request to “ON” (S 24 ), writes the write-target data in the storage extent of a volume corresponding to the write request (S 25 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the bit  301  of the update bitmap  112 D corresponding to the storage extent of such snapshot target volume is “ON” (S 26 : YES), the processor  225 B copies the data currently stored in the storage extent of a volume corresponding to the write request in the storage extent of the snapshot target volume (S 27 ). 
     Subsequently, the processor  225 B changes the bit  301  of the update bitmap  112 D corresponding to the storage extent of a snapshot target volume to “OFF” (S 28 ). Eventually, the processor  225 B changes the bit  301  of the update bitmap  112 C corresponding to the storage extent of a volume corresponding to the write request to “ON” (S 24 ), writes the write-target data in the storage extent of a volume corresponding to the write request (S 25 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the volume corresponding to the I/O access is a snapshot target volume (S 13 : YES), the processor  225 B checks whether the bit  301  of the update bitmap  112 D of the storage extent of a volume (snapshot target volume) corresponding to the I/O access is “ON” (S 31 ) ( FIG. 13 ). 
     When the bit  301  of the update bitmap  112 D of the storage extent of a volume corresponding to the I/O access is not “ON” (S 26 : NO), the processor  225 B executes the reading and writing of data from and in the storage extent of a volume corresponding to the I/O access (S 32 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Contrarily, when the bit  301  of the update bitmap  112 D of the storage extent of a volume corresponding to the I/O access is “ON” (S 26 : NO), the processor  225 B copies the data currently stored in the storage extent of snapshot source volume corresponding to the storage extent of a volume corresponding to the write request to the storage extent of a volume corresponding to the I/O access (S 33 ). 
     Subsequently, the processor  225 B changes the bit  301  of the update bitmap  112 D of the storage extent of a volume corresponding to the I/O access to “OFF” (S 34 ). 
     Eventually, the processor  225 B executes the reading and writing of data from and in the storage extent of a volume corresponding to the I/O access (S 32 ), and thereafter ends the second I/O access processing routine RT 2  shown in  FIG. 11  to  FIG. 13  (S 14 ). 
     Incidentally, the processor  225 A realizes the reading and writing of data from and in the storage extent of a volume to read and write data corresponding to the I/O access with the same method as the I/O control program  233 A. 
     An example of copy management processing (at the local site) in the first storage system  102 A is now explained.  FIG. 14  is a flowchart showing a specific processing routine of the processor  225 A regarding the copy management processing in the first storage system  102 A. 
     The processor  225 A executes the remote copy  122  according to a first copy management processing routine RT 3  shown in  FIG. 14  by executing in a prescribed timing the copy management program  231 A, which is a program for executing the copy management (S 41 ). Subsequently, the processor  225 A waits in a standby mode for the remote copy  122  to be completed (S 42 ). When the remote copy  122  is eventually complete (S 42 : YES), the processor  225 A sends a local copy start command to the second storage system  102 B (S 43 ). 
     Subsequently, the processor  225 A waits in a standby mode to receive a local copy end report from the second storage system  1028  (S 44 ). When the processor  225 A eventually receives the local copy end report from the second storage system  102 B (S 44 : YES), it thereafter once again executes the remote copy  122  (S 41 ), returns to a standby mode and waits for the remote copy  122  to be completed (S 42 ), and thereafter repeats the same processing steps (S 41  to S 44 ). 
     An example of copy management processing (at the remote site) in the second storage system  102 B is now explained.  FIG. 15  is a flowchart showing a specific processing routine of the processor  225 B regarding the copy management processing in the second storage system  102 B. 
     The processor  225 B initially waits in a standby mode to receive a local copy start command from the first storage system  102 A according to a second copy management processing routine RT 4  shown in  FIG. 15  by executing the copy management program  231 B, which is a program for executing the copy management (S 51 ). When the processor  225 B eventually receives the local copy start command from the first storage system  102 A (S 51 : YES), it changes the local copy control flag  332  of a volume corresponding to the local copy start command in the local copy control data  240  to “ON” (S 52 ). 
     Subsequently, the processor  225 B executes the local copy  124  of a volume corresponding to the local copy start command (S 53 ). Next, the processor  225 B waits in a standby mode for the designated time standby processing, which is processing for standing by for a predetermined number of hours, to be completed (S 54 ). Eventually, when the designated time standby processing is complete (S 54 : YES), the processor  225 B changes the local copy control flag  332  of a volume corresponding to the local copy start command in the local copy control data  240  to “OFF” (S 55 ). 
     Subsequently, the processor  225 B waits in a standby mode for the local copy  124  to be completed (S 56 ). Eventually, when the local copy  124  is complete (S 56 : YES), the processor  225 B changes the execution queue flag  333  of a volume corresponding to the local copy start command in the local copy control data  240  to “OFF” (S 57 ). 
     Subsequently, the processor  225 B sends a local copy end report to the first storage system  102 A (S 58 ), thereafter returns to a standby mode and waits to receive a local copy start command from the first storage system  102 A (S 51 ), and then repeats the same processing steps (S 51  to S 58 ). 
     Incidentally, the processor  225 B executes designated time standby processing for controlling the switching interval of the update bitmap  112 A and the difference bitmap  112 B in the operation of the remote copy system. Here, the longer the switching interval of the update bitmap  112 A and the difference bitmap  112 B, the amount of write-target data to be overwritten in the update bitmap  111 A will increase. Thus, it is possible to reduce the overall data traffic. Meanwhile, the shorter the switching interval between the update bitmap  112 A and the difference bitmap  112 B, the possibility of recovering data near the failure point will increase. This interval is individually and specifically decided based on a tradeoff in consideration of the demand. 
     An example of remote copy processing in the first storage system  102 A is now explained.  FIG. 16  is a flowchart showing a specific processing routine of the processor  225 A regarding the remote copy processing in the first storage system  102 A. 
     The processor  225 A switches the contents of the update bitmap  112 A and the difference bitmap  112 B according to a remote copy processing routine RT 5  shown in  FIG. 16  by executing in a prescribed timing the remote copy program  232 , which is a program for executing the remote copy  122  with the copy management program  231 A (S 61 ). 
     Subsequently, the processor  225 A makes the first bit  301  of the difference bitmap  112 B become an examination target (S 62 ). Next, the processor  225 A checks whether the bit  301  of the difference bitmap  112 B to be examined is “ON” (S 63 ). When the bit  301  of the difference bitmap  112 B to be examined is not “ON” (S 63 : NO), the processor  225 A proceeds to step S 68 . Contrarily, when the bit  301  of the difference bitmap  1128  to be examined is “ON” (S 63 : YES), the processor  225 A checks whether data of the storage extent corresponding to the bit  301  of the difference bitmap to be examined is stored in the side file  236  (S 64 ). 
     When data of the storage extent corresponding to the bit  301  of the difference bitmap to be examined is not stored in the side file  236  (S 64 : NO), the processor  225 A sends data of the storage extent corresponding to the bit  301  of the difference bitmap  112 B to be examined to the second storage system  102 B (S 65 ). Contrarily, when data of the storage extent corresponding to the bit  301  of the difference bitmap to be examined is stored in the side file  236  (S 64 : YES), the processor  225 A sends data stored in the side file  236  corresponding to the bit  301  of the difference bitmap  112 B to be examined to the second storage system  1028  (S 66 ). 
     The processor  225 A eventually changes the bit  301  of the difference bitmap  112 B to be examined to “OFF” (S 67 ). Subsequently, the processor  225 A checks whether all bits up to the last bit  301  of the difference bitmap  112 B have been made to be an examination target (S 68 ). 
     When all bits up to the last bit  301  of the difference bitmap  1128  have not been made to be an examination target (S 68 : NO), the processor  225 A makes the subsequent bit  301  of the difference bitmap  112 B an examination target (S 69 ), thereafter once again checks whether the bit  301  of the difference bitmap  1128  to be examined is “ON” (S 63 ), and then repeats the same processing steps (S 63  to S 68 ). Contrarily, when all bits up to the last bit  301  of the difference bitmap  112 B have been made to be an examination target (S 68 : YES), the processor  225 A thereafter ends the remote copy processing routine RT 5  shown in  FIG. 16  (S 70 ). 
     An example of local copy processing in the second storage system  102 B is now explained.  FIG. 17  is a flowchart showing a specific processing routine of the processor  225 B concerning the local copy processing in the second storage system  102 B. 
     When the processor  225 B receives a local copy start command from the first storage system  102 A, it makes the first bit  301  of the update bitmap  112 C an examination target according to a local copy processing routine RT 6  shown in  FIG. 17  by executing the local copy program  237 , which is a program for executing the local copy  124  with the copy management program  231 B (S 71 ). Subsequently, the processor  225 B checks whether the bit  301  of the update bitmap  112 C to be examined is “ON” (S 72 ). 
     When the bit  301  of the update bitmap  112 C to be examined is not “ON” (S 72 : NO), the processor  225 B proceeds to step S 75 . Contrarily, when the bit  301  of the update bitmap  112 C to be examined is “ON” (S 72 : YES), the processor  225 B stores data of the storage extent corresponding to the bit  301  of the update bitmap  112 C to be examined in the storage extent of the target volume of a local copy (S 73 ). 
     The processor  225 B eventually changes the bit  301  of the update bitmap  112 C to be examined to “OFF” (S 74 ). Subsequently, the processor  225 B checks whether all bits up to the final bit  301  of the update bitmap  112 C have been made to be an examination target (S 75 ). 
     When all bits up to the final bit  301  of the update bitmap  112 C have not been made to be an examination target (S 75 : NO), the processor  225 B makes the subsequent bit  301  of the update bitmap  112 C an examination target (S 76 ), thereafter once again checks whether the bit  301  of the update bitmap  112 C to be examined is “ON” (S 72 ), and then repeats the same processing steps (S 72  to S 75 ). 
     Contrarily, when all bits up to the final bit  301  of the update bitmap  112 C have been made to be an examination target (S 75 : YES), the processor  225 B thereafter ends the local copy processing routine RT 6  shown in  FIG. 17  (S 77 ). 
     An example of snapshot processing in the second storage system  1028  is now explained.  FIG. 18  is a flowchart showing a specific processing routine of the processor  225 B regarding the snapshot processing in the second storage system  102 B. 
     When the processor  225 B receives the input of a snapshot source volume and a snapshot target volume based on a command of a user or a program from the host computer  101  or management terminal  104 , or other maintenance terminals  223 A,  223 B or programs, it creates an update bitmap  112 D where all bits  301  corresponding to the snapshot target volume are all “ON” (changes the update bitmap  112 D to a status where all bits  301  corresponding to the snapshot target volume are “ON”) according to a snapshot processing routine RT 7  shown in  FIG. 18  by executing the snapshot program  239 , which is a program for executing the snapshot  125  (S 81 ). 
     Subsequently, the processor  225 B creates a pair relationship by adding information representing the snapshot source volume name and the snapshot target volume name, and information representing the snapshot pair relationship to the list of the pair information  235 B (S 82 ). Next, the processor  225 B executes background copy processing (S 83 ), and thereafter ends the snapshot processing routine RT 7  shown in  FIG. 18  (S 84 ). 
     An example of background copy processing in the second storage system  102 B is now explained.  FIG. 19  is a flowchart showing a specific processing routine of the processor  225 B concerning the background copy processing in the second storage system  1028 . 
     When a pair relationship is created, the processor  225 B makes the first bit  301  of the update bitmap  112 D an examination according to a background copy processing routine RT 8  shown in  FIG. 19  (S 91 ). Subsequently, the processor  225 B checks whether the bit  301  of the update bitmap  112 D to be examined is “ON” (S 92 ). When the bit  301  of the update bitmap  112 D to be examined is not “ON” (S 92 : NO), the processor  225 B proceeds to step S 95 . Contrarily, when the bit  301  of the update bitmap  112 D to be examined is “ON” (S 92 : YES), the processor  225 B stores data of the storage extent corresponding to the bit  301  of the update bitmap  112 D to be examined in the storage extent of the snapshot target volume (S 93 ). 
     The processor  225 B eventually changes the bit  301  of the update bitmap  112 D to be examined to “OFF” (S 94 ). Subsequently, the processor  225 B checks whether all bits up to the final bit  301  of the update bitmap  112 D have been made to be an examination target (S 95 ). 
     When all bits up to the final bit  301  of the update bitmap  112 D have not been made to be an examination target (S 95 : NO), the processor  225 B makes the subsequent bit  301  of the update bitmap  112 D an examination target (S 96 ), thereafter once again checks whether the bit  301  of the update bitmap  112 D to be examined is “ON” (S 92 ), and then repeats the same processing steps (S 92  to S 95 ). 
     Contrarily, when all bits up to the final bit  301  of the update bitmap  112 D have been made to be an examination target (S 95 : YES), the processor  225 B thereafter ends the background copy processing routine RT 8  shown in  FIG. 19  (S 97 ). Incidentally, the processor  225 B is able to end the background copy processing within a time limit since the update bitmap  112 D will not be turned “OFF” by various control programs or the like, and there is a limited in the size of the update bitmap  112 D. 
     Further, there may be cases where the processor  225 B does not execute the background copy processing since the snapshot target volume can be I/O accessed from the host computer  101  or the like. Further, when the background copy processing is not to be executed or during the execution of the background copy processing, the processor  225 B may be subject to a slight deterioration in performance since an overhead will occur in copying data from the snapshot source volume in the read processing to be performed to the snapshot target volume. 
     An example of extended snapshot processing in the second storage system  102 B is now explained.  FIG. 20  is a flowchart showing a specific processing routine of the processor  225 B regarding the extended snapshot processing in the second storage system  102 B. 
     When the processor  225 B receives a snapshot creation command based on a command of a user or a program from the host computer  101  or management terminal  104 , or other maintenance terminals  223 A,  223 B or programs, it executes the snapshot extension program  238 , which is a program for executing the snapshot  125  based on the snapshot creation command, for checking whether the source volume corresponding to the snapshot creation command is a volume of the second volume group  111 B by referring to the pair information  235 B according to an extended snapshot processing routine RT 9  shown in  FIG. 20  (S 101 ). 
     When the source volume corresponding to the snapshot creation command is not a volume of the second volume group  111 B (S 101 : NO), the processor  225 B thereafter ends the extended snapshot processing routine RT 9  shown in  FIG. 20  (S 106 ). Contrarily, when the source volume corresponding to the snapshot creation command is a volume of the second volume group  111 B (S 101 : YES), the processor  225 B checks whether the local copy control flag  332  of a source volume corresponding to the snapshot creation command in the local copy control data  240  is “ON” (S 102 ). 
     When the local copy control flag  332  of a source volume corresponding to the snapshot creation command in the local copy control data  240  is not “ON” (S 102 : NO), the processor  225 B waits in a standby mode for the local copy control flag  332  of the source volume to be turned “ON”. Contrarily, when the local copy control flag  332  of a source volume corresponding to the snapshot creation command in the local copy control data  240  is “ON” (S 102 : YES), the processor  225 B changes the execution queue flag  333  of a source volume corresponding to the snapshot creation command in the local copy control data  240  to “ON” (S 103 ). 
     Subsequently, the processor  225 B executes the snapshot  125  from a source volume corresponding to the snapshot creation command to the corresponding target volume (S 104 ). Next, the processor  225 B changes the execution queue flag  333  of a source volume corresponding to the snapshot creation command in the local copy control data  240  to “OFF” (S 105 ). Eventually, the processor  225 B thereafter ends the extended snapshot processing routine RT 9  shown in  FIG. 20  (S 106 ). 
     Like this, with the computer system  100  in the first embodiment, as a result of utilizing the fact that the data consistency in the second volume group  111 B is maintained other than during the async remote copy  122  from the first volume group  111 A to the second volume group  111 B, creation of the snapshot is delayed until the transfer of the remote copy  122  is complete when a snapshot creation command from the second volume group  111 B to the third volume group  111 C is issued during such transfer, and a snapshot is created instantaneously when a snapshot creation command is issued when there is no transfer of the remote copy  122 . 
     Accordingly, even when testing or the like is conducted during the operation of the remote copy system, it is possible to effectively prevent complex operations such as the discontinuance or resumption of the system operation from occurring, and create a snapshot having consistency without having to discontinue the system operation. 
     (2) Second Embodiment 
       FIG. 21  shows a schematic configuration of the computer system  100  according to a second embodiment of the present invention. The computer system  100  is configured the same as the computer system  100  in the first embodiment other than that a captured difference buffer  131 A and a transferred difference buffer  131 B are provided to the first storage system  102 A in substitute for the update bitmap  112 A and the difference bitmap  112 B, and a received difference buffer  131 C and an applied difference buffer  131 D are provided to the second storage system in substitute for the update bitmap  112 C and the fourth volume group  111 D, and the captured difference buffer  131 A, the transferred difference buffer  131 B, the received difference buffer  131 C and the applied difference buffer  131 D are used to operate the remote copy system. 
     In the first embodiment, for instance, a volume having data consistency is guaranteed by managing two volume groups in the second storage system  102 B. In the second embodiment, a volume having data consistency is guaranteed by providing the received difference buffer  131 C and the applied difference buffer  131 D for accumulating write-target data in the second storage system  102 B. 
     The operation in the second embodiment is now schematically explained. The first storage system  102 A simultaneously stores the write-target data from the host computer  101  or the like in the first volume group  111 A and in the captured difference buffer  131 A ( 141 ). Subsequently, the first storage system  102 A switches the captured difference buffer  131 A and the transferred difference buffer  131 B ( 142 ). Here, after the foregoing switch, the first storage system  102 A once again simultaneously stores the write-target data from the host computer  101  or the like in the first volume group  111 A and in the captured difference buffer  131 A ( 141 ). Subsequently, the first storage system  102 A transfers the contents of the transferred difference buffer  131 B to the received difference buffer  131 C of the second storage system  102 B (remote copy  143 ). Next, the second storage system  102 B switches the received difference buffer  141 C and the applied difference buffer  141 D ( 144 ). Here, after the foregoing switch, the first storage system  102 A once again transfers the contents of the transferred difference buffer  131 B to the received difference buffer  131 C of the second storage system  102 B (remote copy  143 ). Subsequently, the second storage system  102 B applies the contents of the applied difference buffer  141 D to the second volume group  1412 B (local copy  145 ). 
     With this computer system  100 , by periodically executing step S 112 , it is possible to configure a remote copy system capable of constantly configuring a volume having data consistency in the second storage system  102 B. The computer system  100  thereafter creates a snapshot  125  where the data consistency of the second volume group  111 B is maintained in the third volume group  111 C during the operation of the foregoing remote copy system. Data consistency will be described in detail later. 
       FIG. 22  shows a schematic configuration inside the computer system  100  in the second embodiment. This computer system  100  is configured the same as the computer system  100  in the first embodiment other than that a captured difference buffer  131 A and a transferred difference buffer  131  are provided in substitute for the update bitmap  112 A, the difference bitmap  112 B and the side file  236 B in the memory  224 A of the storage control unit  201 A in the first storage system  102 A, a received difference buffer  131 C and an applied difference buffer  131 D are provided in substitute for the update bitmap  112 C in the memory  224 B of the storage control unit  201 B in the second storage system  102 B, and the processing contents of the copy management programs  231 A,  231 B, the remote copy program  232 , the I/O control programs  233 A,  233 B, and the local copy program  237  are different from the programs of the first embodiment. These programs will be explained in detail later with reference to flowcharts. 
       FIG. 23  shows a table structure of the captured difference buffer  131 A. The captured difference buffer  131 A has the items of a volume name  351 A, a target address  352 A showing the corresponding storage extent of the volume, and a write-target data  353 A, and is configured from a list. 
     With this computer system  100 , as another loading method of the captured difference buffer  131 A, there is a method of retaining a pointer, in substitute for the write-target data, in the cache of the memory  224 A. Here, when the I/O control program  233 A is to retain the write-target data in the cache, the used memory volume can be reduced by retaining the write-target data in the cache until the transfer of the remote copy or the I/O access to the storage extent is complete, and retaining a pointer to data on the cache in the captured difference buffer  131 A so as to avoid the retention of redundant data. 
     Incidentally, the table structure of the transferred difference buffer  131 B, the received difference buffer  131 C and the applied difference buffer  131 D is the same as the table structure of the captured difference buffer  131 A. 
       FIG. 24  shows the update status of the first to third volume groups  111 A to  111 C and the captured difference buffer  131 A, the transferred difference buffer  131 B, the received difference buffer  131 C and the applied difference buffer  131 D in a time series. Here, reference number  361 A represents the update status of the first volume group  111 A in a time series, reference number  361 B represents the update status of the second volume group  111 B in a time series, reference number  361 C represents the update status of the third volume group  111 C in a time series, reference number  362 A represents the difference data status of the captured difference buffer  131 A in a time series, reference number  362 B represents the difference data status of the transferred difference buffer  131 B in a time series, reference numeral  362 C represents the difference data status of the received difference buffer  131 C in a time series, and reference number  362 D represents the difference data status of the applied difference buffer  131 D in a time series. 
     With the first storage system  102 A, if the update status of the first volume group  111 A is an update status A at timing T 1  of switching the captured difference buffer  131 A and the transferred difference buffer  131 B, contents of the transferred difference buffer  131 B after switching the captured difference buffer  131 A and the transferred difference buffer  131 B will be an aggregate of write-target data for making the second volume group  111 B become an update status A ( 363 ). 
     After switching the captured difference buffer  131 A and the transferred difference buffer  131 B, the first storage system  102 A ongoingly stores the write-target data from the host computer  101  in the captured difference buffer  131 A ( 364 ). Here, when data is written in the same storage extent upon storing the write-target data in the captured difference buffer  131 A, the first storage system  102 A is able to reduce the data traffic during the execution of remote copy by deleting the data stored in such storage extent. Further, the first storage system  102 A may also manage the referral to from the target address showing the storage extent to the elements in the captured difference buffer  131 A with a bitmap. Incidentally, elements in the difference buffer mean the status of data being stored in a prescribed storage extent of the difference buffer. 
     Subsequently, the first storage system  102 A transfers the write-target data of the transferred difference buffer  131 B to the received difference buffer  131 C of the second storage system  102 B (remote copy  143 ). In the second storage system  102 B, after the execution of the remote copy  143 , contents of the received difference buffer  131 C become an aggregate of write-target data for making the second volume group  111 B to become update status A ( 365 ). 
     Subsequently, the first storage system  102 A sends a difference buffer switch command to the second storage system  102 B ( 366 ). Thereby, the second storage system  102 B switches the received difference buffer  131 C and the applied difference buffer  131 D. 
     Subsequently, the second storage system  102 B sends a difference buffer switch end report to the first storage system  102 A ( 367 ). Thereby, the first storage system  102 A will be able to once again switch the captured difference buffer  131 A and the transferred difference buffer  131 B ( 142 ). At timing T 2 , the captured difference buffer  131 A and the transferred difference buffer  131 B are switched again, and the same processing steps are thereafter repeated. 
     Meanwhile, in the second storage system  102 B, contents of the applied difference buffer  131 D are applied to the second volume group  111 B (local copy  115 A). Thereupon, since the second storage system  102 B applies the write-target data of the applied difference buffer  131 D to the second volume group  111 B independent from the writing sequence from the host computer  101 , data consistency regarding the writing sequence of the second volume group  111 B may not be guaranteed (difference application period D 11 ). When the application of write-target data from the applied difference buffer  131 D is eventually complete, the update status of the second volume group  111 B will become an update status A ( 368 ). Thereby, the data consistency of the first volume group  111 A and the second volume group  111 B will be guaranteed. 
     As described above, with the computer system  100 , since the second storage system  102 B has the received difference buffer  131 C and the applied difference buffer  131 D for accumulating write-target data, it is retaining the necessary write-target data for guaranteeing the data consistency of the second volume group  111 B. Thereby, it is possible to configure a remote copy system capable of constantly configuring a volume having data consistency. 
     Here, when a snapshot creation command is issued from the second volume group  111 B to the third volume group  111 C at timing T 13  (difference application period D 11  of the second volume group  111 B) during the operation of such remote copy system, the second storage system  102 B delays the snapshot execution until the difference application period D 11  of the second volume group  111 B is complete (delay period D 12 ). Then, the second storage system  102 B executes a snapshot when the difference application period D 11  of the second volume group  111 B is complete ( 369 ). Here, the second storage system  102 B instantaneously ends the execution of the snapshot itself, and executes a background copy of copying all data of the second volume group  111 B to the third volume group  111 C within a time limit ( 370 ). 
     Further, when data is written in the second volume group  111 B during the background copy, the second storage system  102 B applies the write-target data to the second volume group  111 B even during the execution of the background copy processing by executing the I/O control program  216 B as in the first embodiment described above (S 115 B). 
     An example of I/O access processing (at the local site) of the first storage system  102 A in the second embodiment is now explained.  FIG. 25  is a flowchart showing a specific processing routine of the processor  225 A regarding the I/O access processing in the first storage system  102 A. 
     When the processor  225 A receives an I/O access from the host computer  101 , it checks whether the type of such I/O access is a write request according to a first I/O access processing routine RT 10  shown in  FIG. 25  by executing the I/O control program  233 A, which is a program for executing the I/O access (S 111 ). 
     When the type of I/O access is not a write request (S 111 : NO), the processor  225 A determines that the type of I/O access is a read request, and reads read-target data from the storage extent of the volume corresponding to the read request (S 112 ), and thereafter ends the first I/O access processing routine RT 10  shown in  FIG. 25  (S 116 ). 
     Contrarily, when the type of I/O access is a write request (S 111 : YES), the processor  225 A checks whether the volume corresponding to the write request is a source volume of the remote copy by referring to the pair information  235 A (S 113 ). 
     When the volume corresponding to the write request is not a source volume of the remote copy (S 113 : NO), the processor  225 A proceeds to step S 115 . Contrarily, when the volume corresponding to the write request is a source volume of the remote copy (S 113 : YES), the processor  225 A stores the write-target data corresponding to the write request in the captured difference buffer  131 A (S 114 ). 
     Eventually, the processor  225 A writes the write-target data in the storage extent of a volume corresponding to the write request (S 115 ), and thereafter ends the first I/O access processing routine RT 10  shown in  FIG. 25  (S 116 ). 
     Incidentally, the processor  225 A is able to read the read-target data from the storage extent of a volume corresponding to the read request according to a method of making the host computer  101  or a program refer to the data disposed in a cache of the memory  224 A, or a method of reading data from a corresponding hard drive of the storage unit  202 A and making the host computer  101  or various program refer to such data. 
     Incidentally, when the processor  225 A is to store the write-target data in the captured difference buffer  131 A, if the write-target data has already been stored in the same storage extent of the difference capture buffer  131 A, it is able to reduce the data volume of the captured difference buffer  131 A by overwriting such elements with the write-target data, and reduce the data traffic to the second storage system  1028 . Furthermore, the processor  225 A realizes the reading and writing of data with the same method as the first embodiment. 
     An example of I/O access processing (at the remote site) of the second storage system  102 B in the second embodiment is now explained.  FIG. 26  and  FIG. 27  are flowcharts showing a specific processing routine of the processor  225 B regarding the I/O access processing in the second storage system  1028 . 
     When the processor  225 B receives an I/O access from the host computer  101 , or receives an I/O access request as a result of the remote copy program  232  of the first storage system  102 A being executed, it executes the I/O control program  2238 , which is a program for executing the I/O access, for checking whether the volume corresponding to the I/O access is a target volume of a remote copy by referring to the pair information  235 B according to a second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27  (S 121 ). 
     When the volume corresponding to the I/O access is not a target volume of a remote copy (S 121 : NO), the processor  225 B checks whether the volume corresponding to the I/O access is a snapshot target volume by referring to the pair information  235 B (S 122 ). 
     When the volume corresponding to the I/O access is not a snapshot target volume (S 122 : NO), the processor  225 B executes the reading and writing of data from and in the storage extent of a volume corresponding to the I/O access (S 123 ), and thereafter ends the second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27 . 
     Contrarily, when the volume corresponding to the I/O access is a target volume of a remote copy (S 121 : YES), the processor  225 B checks whether the type of I/O access is a write request (S 131 ) ( FIG. 27 ). 
     When the type of I/O access is not a write request (S 131 : NO), the processor  225 B determines that the type of I/O access is a read request, reads the read-target data from the storage extent of a volume corresponding to the read request (S 132 ), and thereafter ends the second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27  (S 124 ). 
     Contrarily, when the type of I/O access is a write request (S 131 : YES), the processor  225 B checks whether the volume corresponding to the write request is a snapshot source volume by referring to the pair information  235 B (S 133 ). 
     When the volume corresponding to the write request is not a snapshot source volume (S 133 : NO), the processor  225 B writes the write-target data in the storage extent of a volume corresponding to the write request (S 134 ), and thereafter ends the second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27  (S 124 ). 
     Contrarily, when the volume corresponding to the write request is a snapshot source volume (S 133 : YES), the processor  225 B checks whether the storage extent of such snapshot target volume is “ON” (S 135 ). 
     When such snapshot target volume is not “ON” (S 135 : NO), the processor  225 B writes the write-target data in the storage extent of a volume corresponding to the write request (S 134 ), and thereafter ends the second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27  (S 124 ). 
     Contrarily, when such snapshot target volume is “ON” (S 135 : YES), the processor  225 B copies the data currently stored in the storage extent of a volume corresponding to the write request in the storage extent of the snapshot target volume (S 136 ). 
     Subsequently, the processor  225 B changes the storage extent of a snapshot target volume to “OFF” (S 137 ). 
     Eventually, the processor  225 B writes the write-target data in the storage extent of a volume corresponding to the write request (S 134 ), and thereafter ends the second I/O access processing routine RT 11  shown in  FIG. 26  and  FIG. 27  (S 124 ). Incidentally, the processor  225 B realizes the reading and writing of data with the same method as the first embodiment. 
     An example of copy management processing (at the local site) of the first storage system  102 A in the second embodiment is now explained.  FIG. 28  is a flowchart showing a specific processing routine of the processor  225 A regarding the copy management processing in the first storage system  102 A. 
     The processor  225 A executes the remote copy  143  according to a first copy management processing routine RT 12  shown in  FIG. 28  by executing in a prescribed timing the copy management program  231 A, which is a program for executing the copy management (S 141 ). Subsequently, the processor  225 A waits in a standby mode for the remote copy  143  to be completed (S 142 ). When the remote copy  143  is eventually complete (S 142 : YES), the processor  225 A sends a difference buffer switch start command to the second storage system  102 B (S 143 ). 
     Subsequently, the processor  225 A waits in a standby mode to receive a difference buffer switch end report from the second storage system  1028  (S 144 ). When the processor  225 A eventually receives the difference buffer switch end report from the second storage system  102 B (S 144 : YES), it thereafter once again executes the remote copy  143  (S 141 ), returns to a standby mode and waits for the remote copy  143  to be completed (S 142 ), and thereafter repeats the same processing steps (S 141  to S 144 ). 
     An example of copy management processing (at the remote site) of the second storage system  102 B in the second embodiment is now explained.  FIG. 29  is a flowchart showing a specific processing routine of the processor  225 B regarding the copy management processing in the second storage system  102 B. 
     The processor  225 B initially waits in a standby mode to receive a difference buffer switch start command from the first storage system  102 A according to a second copy management processing routine RT 13  shown in  FIG. 29  by executing the copy management program  231 B, which is a program for executing the copy management (S 151 ). When the processor  225 B eventually receives the difference buffer switch start command from the first storage system  102 A (S 151 : YES), it switches the contents of the received difference buffer  131 C and the applied difference buffer  131 D (S 152 ). 
     Subsequently, the processor  225 B sends a difference buffer switch end report to the first storage system  102 A (S 153 ). Next, the processor  225 B changes the local copy control flag  332  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  to “OFF” (S 154 ). Then, the processor  225 B executes the local copy  145  of a volume corresponding to the difference buffer switch start command (S 155 ). 
     Subsequently, the processor  225 B waits in a standby mode for the local copy  145  to be completed (S 156 ). Eventually, when the local copy  145  is complete (S 156 : YES), the processor  225 B changes the local copy control flag  332  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  to “ON” (S 157 ). 
     Subsequently, the processor  225 B checks whether the execution queue flag  333  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  is “OFF” (S 158 ). 
     When the execution queue flag  333  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  is not “OFF” (S 158 : NO), the processor  225 B waits in a standby mode for the copy control flag  332  of the source volume to be turned “OFF”. Contrarily, when the execution queue flag  333  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  is “OFF” or is turned “OFF” (S 158 : YES), the processor  225 B thereafter once again returns to a standby mode and waits to receive a difference buffer switch start command from the first storage system  102 A (S 151 ), and then repeats the same processing steps (S 151  to S 158 ). 
     Incidentally, the processor  225 B changes the local copy control flag  332  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  to “OFF” at step S 154 , and changes the local copy control flag  332  of a volume corresponding to the difference buffer switch start command in the local copy control data  240  to “ON” at step S 157 . Like this, the second embodiment is opposite to the second copy management processing routine RT 4  shown in  FIG. 15  of the first embodiment, and the reason for this is because the execution of the local copy  145  is the application of contents of the applied difference buffer  131 D to the second volume group  111 B, and, since the second volume group  111 B will be a snapshot source volume group, data consistency during the execution of the local copy  145  is not guaranteed. 
     An example of remote copy processing of the first storage system  102 A in the second embodiment is now explained.  FIG. 30  is a flowchart showing a specific processing routine of the processor  225 A regarding the remote copy processing in the first storage system  102 A. 
     The processor  225 A switches the contents of the captured difference buffer  131 A and the transferred difference buffer  131 B according to a remote copy processing routine RT 14  shown in  FIG. 30  by executing in a prescribed timing the remote copy program  232 , which is a program for executing the remote copy  143  with the copy management program  231 A (S 161 ). 
     Subsequently, the processor  225 A checks whether there are elements of the transferred difference buffer  131 B (S 162 ). When there are no elements of the transferred difference buffer  131 B (S 162 : NO), the processor  225 A thereafter ends the remote copy processing routine RT 14  shown in  FIG. 30  (S 168 ). Contrarily, when there are elements of the transferred difference buffer  131 B (S 162 : YES), the processor  225 A makes the first element of the transferred difference buffer  131 B an examination target (S 163 ). 
     Subsequently, the processor  225 A sends data of elements of the transferred difference buffer  131 B to be examined to the second storage system  102 B (S 164 ). Next, the processor  225 A deletes data of elements of the transferred difference buffer  131 B to be examined (S 165 ). 
     Subsequently, the processor  225 A checks whether all elements up to the last element of the transferred difference buffer  131 B have been made to be an examination target (S 166 ). When all elements up to the last element of the transferred difference buffer  131 B have not been made to be an examination target (S 166 : NO), the processor  225 A makes the subsequent element of the transferred difference buffer  131 B an examination target (S 167 ), thereafter once again sends the data of the element of the transferred difference buffer  131 B to be examined to the second storage system  102 B (S 164 ), and then repeats the same processing steps (S 164  to S 166 ). 
     Contrarily, when all elements up to the last element of the transferred difference buffer  131 B have been made to be an examination target (S 166 : YES), the processor  225 A thereafter ends the remote copy processing routine RT 14  shown in  FIG. 30  (S 168 ). 
     An example of local copy processing of the second storage system  102 B in the second embodiment is now explained.  FIG. 31  is a flowchart showing a specific processing routine of the processor  225 B concerning the local copy processing in the second storage system  102 B. 
     When the processor  225 B receives a difference buffer switch start command from the first storage system  102 A, it checks whether there are elements of the applied difference buffer  131 D according to a local copy processing routine RT 15  shown in  FIG. 31  by executing the local copy program  237 , which is a program for executing the local copy  145  with the copy management program  231 B (S 171 ). When there are no elements of the applied difference buffer  131 D (S 171 : NO), the processor  225 B thereafter ends the local copy processing routine RT 15  shown in  FIG. 31  (S 177 ). Contrarily, when there are elements of the applied difference buffer  131 D (S 171 : YES), the processor  225 B makes the first element of the applied difference buffer  131 D an examination target (S 172 ). 
     Subsequently, the processor  225 B stores data of elements of the applied difference buffer  131 D to be examined in the storage extent of a volume showing such elements (S 173 ). Next, the processor  225 A deletes data of elements of the applied difference buffer  131 D to be examined (S 174 ). 
     Subsequently, the processor  225 B checks whether all elements up to the last element of the applied difference buffer  131 D have been made to be an examination target (S 175 ). When all elements up to the last element of the applied difference buffer  131 D have not been made to be an examination target (S 175 : NO), the processor  225 A makes the subsequent element of the applied difference buffer  131 D an examination target (S 176 ), thereafter once again sends the data of the element of the applied difference buffer  131 D to be examined to the storage system of a volume showing such elements (S 173 ), and then repeats the same processing steps (S 173  to S 175 ). 
     Contrarily, when all elements up to the last element of the applied difference buffer  131 D have been made to be an examination target (S 175 : YES), the processor  225 B thereafter ends the local copy processing routine RT 15  shown in  FIG. 31  (S 177 ). As described above, with the computer system  100  in the second embodiment, since the second storage system  102 B has the received difference buffer  131 C and the applied difference buffer  131 D for accumulating write-target data, it is retaining the necessary write-target data for guaranteeing the data consistency of the second volume group  111 B. Thereby, it is possible to configure a remote copy system capable of constantly configuring a volume having data consistency. 
     (3) Third Embodiment 
     A method of using a marker will be explained in the third embodiment. The computer system in the third embodiment is configured the same as the computer system in the first or second embodiment excluding the point of using a marker. A marker is an identifier representing the status of a volume, and the host computer  101  or the like issues a marker creation command to the first storage system  102 A at a certain point in time. For example, when a condition such as “certain important data has been written” in a prescribed volume group is satisfied, the host computer  101  issues a designated marker creation command. Thereafter, upon creating a snapshot of the second volume group  111 B during the operation of the remote copy system with the second storage system  1028  at the remote site, the computer system  100  creates a snapshot of a volume group satisfying a specified condition via a method using a marker. 
       FIG. 32  shows the structure of a marker in the third embodiment. A marker is used for representing the status of a prescribed volume group. The storage extent  302  corresponding one-on-one with a prescribed volume group  111  is allocated as a marker area. For instance, with the computer system  100 , a method of securing the storage extent  302  in the memory  224 A of the storage control unit  201 A, or a method of securing the storage extent  302  in a part of the volume group  111  can be considered. 
       FIG. 33  is a flowchart showing a specific processing routine of the processor  225 A regarding the marker processing of the first storage system  102 A in the third embodiment. When the processor  225 A receives a marker creation command based on a command of a user or a program from the host computer  101  or management terminal  104 , or other maintenance terminals  223 A,  223 B or programs, it writes a designated marker in the marker area of a volume corresponding to the marker creation command according to a marker processing routine RT 16  shown in  FIG. 33  by executing the marker control program  234 , which is a program for executing the marker creation processing (S 181 ), and thereafter ends the marker processing routine RT 16  shown in  FIG. 33  (S 182 ). 
     An example of copy management processing (at the local site) of the first storage system  102 A in the third embodiment is now explained.  FIG. 34  is a flowchart showing a specific processing routine of the processor  225 A regarding the copy management processing in the first storage system  102 A. 
     The processor  225 A executes the remote copy  122  according to a first copy management processing routine RT 17  shown in  FIG. 34  by executing in a prescribed timing the copy management program  231 A, which is a program for executing the copy management (S 191 ). Subsequently, the processor  225 A waits in a standby mode for the remote copy  122  to be completed (S 192 ). When the remote copy  122  is eventually complete (S 192 : YES), the processor  225 A applies data of a marker area of the source volume of the remote copy to a marker area of the target volume of the remote copy (S 193 ). 
     Here, for instance, when the processor  225 A allocated a marker area to the storage extent of the volume  111 , it applies this to the marker area as with the transfer of the write-target data to the second storage system  1028  in the remote copy. Further, for instance, when the processor  225 A allocated a marker area to the storage extent  303  in the memory  224 A, it applies this to the marker area by transferring the data of the marker area at the time of application to the second storage system  1028  via the network  103 , and writing data transferred to the second storage system  102 B from the first storage system  102 A in the storage extent  302  in the memory  224 B before the execution of the local copy. Further, the processor  225 A applies this to the marker area even during the execution of the remote copy. 
     Subsequently, the processor  225 A sends a local copy start command to the second storage system  1028  (S 194 ). Next, the processor  225 A waits in a standby mode to receive a local copy end report from the second storage system  1028  (S 195 ). When the processor  225 A eventually receives the local copy end report from the second storage system  1028  (S 195 : YES), it thereafter once again executes the remote copy  122  (S 191 ), returns to a standby mode and waits for the remote copy  122  to be completed (S 192 ), and thereafter repeats the same processing steps (S 191  to S 195 ). 
     Incidentally, in this embodiment, although a case was explained where the specific processing routine of the processor  225 A regarding the copy management processing in the first embodiment was explained, the same applies to a case of employing the specific processing routine of the processor  225 A regarding the copy management processing in the second embodiment. 
     An example of extended snapshot processing of the second storage system  102 B in the third embodiment is now explained.  FIG. 35  is a flowchart showing a specific processing routine of the processor  225 B regarding the extended snapshot processing in the second storage system  102 B. 
     When the processor  225 B receives the input of a designated marker based on a command of a user or a program from the host computer  101  or management terminal  104 , or other maintenance terminals  223 A,  223 B or programs, it executes the snapshot extension program  238 , which is a program for executing the snapshot  125  based on the snapshot creation command, for checking whether the source volume corresponding to the designated marker is a volume of the second volume group  111 B by referring to the pair information  235 B according to an extended snapshot processing routine RT 18  shown in  FIG. 35  (S 201 ). 
     When the source volume corresponding to the input of the designated marker is not a volume of the second volume group  111 B (S 201 : NO), the processor  225 B thereafter ends the extended snapshot processing routine RT 18  shown in  FIG. 35  (S 208 ). Contrarily, when the source volume corresponding to the input of the designated marker is a volume of the second volume group  111 B (S 201 : YES), the processor  225 B checks whether the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is “ON” (S 202 ). 
     When the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is not “ON” (S 202 : NO), the processor  225 B waits in a standby mode for the local copy control flag  332  of the source volume to be turned “ON”. Contrarily, when the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is “ON” or is turned “ON” (S 202 : YES), the processor  225 B checks whether the data representing the marker and the designated marker are equivalent in the volume of the second volume group  111 B (S 203 ). 
     When the data representing the marker in the volume of the second volume group  111 B and the designated marker are not equivalent (S 203 : NO), the processor  225 B checks whether the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is “OFF” (S 204 ). When the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is not “OFF” (S 204 : NO), the processor  225 B waits in a standby mode for the local copy control flag  332  of such source volume to be turned “OFF”. Contrarily, when the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is “OFF” or is turned “OFF” (S 204 : YES), the processor  225 B checks whether the local copy control flag  332  of a source volume corresponding to the input of the designated marker in the local copy control data  240  is “ON” (S 202 ), and then repeats the same processing steps (S 202  to S 204 ). 
     Contrarily, when the data representing the marker in the volume of the second volume group  111 B and the designated marker are equivalent (S 203 : YES), the processor  225 B changes the execution queue flag  333  of a source volume corresponding to the input of the designated marker in the local copy control data  240  to “ON” (S 205 ). 
     Subsequently, the processor  225 B executes the snapshot  125  from a source volume corresponding to the input of the designated marker to the corresponding target volume (S 206 ). Next, the processor  225 B changes the execution queue flag  333  of a source volume corresponding to the input of the designated marker in the local copy control data  240  to “OFF” (S 207 ). Eventually, the processor  225 B thereafter ends the extended snapshot processing routine RT 18  shown in  FIG. 35  (S 208 ). 
     Accordingly, with the computer system  100  in the third embodiment, it is possible to create a snapshot when the data representing the marker in the volume of the second volume group  111 B and the designated marker coincide, and the local copy control flag  332  corresponding to the input of the designated marker in the local copy control data  240  is “ON”. 
     The present invention can be broadly applied to a computer system in which the host computer and the storage system are connected via a network, and which stores data processed with the host computer in the storage system by sending and receiving such data via the network.