Patent Publication Number: US-7908446-B2

Title: Copy system and method using differential bitmap

Description:
CROSS REFERENCES 
     This application relates to and claims priority from Japanese Patent Application No. 2007-69235, filed on Mar. 16, 2007, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a copy system and a copy method, and, for example, can be suitably applied to a storage system. 
     From the past, remote copy of copying data written in a logical volume of a storage apparatus set as a primary side (this is hereinafter referred to as the “primary storage apparatus”) to a logical volume of a storage apparatus set as a secondary side has been widely employed. 
     In remote copy, after setting a copy pair with a logical volume of the copy source in the primary storage apparatus as a primary volume, and a logical volume of the copy destination in the secondary storage apparatus as a secondary volume, all areas of the primary volume are copied to the secondary volume (this is hereinafter referred to as the “initial copy”). Subsequently, when new data is written in the primary volume, such data is transferred from the primary storage apparatus to the secondary storage apparatus synchronously or asynchronously with the writing of data so as to write this data in the same location as the primary volume in the secondary volume. According to this kind of remote copy, it is possible to mirror and retain data, and prevent the loss of data caused by disasters or the like. 
     Incidentally, Japanese Patent Laid-Open Publication No. 2005-141381 (Patent Document 1) and Japanese Patent Laid-Open Publication No. 2001-337939 (Patent Document 2) describe a remote copy method in a case when a secondary storage apparatus succeeds the operations of a primary storage apparatus for maintenance or other reasons, including the steps of managing the differential between the primary volume and the secondary volume with a differential bitmap, and, when resuming the operations of the primary storage apparatus, using this differential bitmap to copy only the differential from the secondary volume to the primary volume. 
     Further, Japanese Patent Laid-Open Publication No. 2006-268740 (Patent Document 3) describes a remote copy method, including the steps of creating a backup of a primary volume, copying such backup to a secondary volume, managing the update of the primary volume after the creation of a backup with a differential bitmap, copying the backup data to the secondary volume, and using a differential bitmap to copy only the differential from the primary volume to the secondary volume. 
     SUMMARY 
     Meanwhile, the number of volumes and the volume capacity in storage apparatuses in recent years are ever-increasing. Consequently, the time required to perform the initial copy is increasing, and much time is required for the initial copy to be performed prior to the remote copy. 
     Thus, by applying the technology of copying only the differential (differential copy) disclosed in foregoing Patent Document 1 to Patent Document 3 in performing the initial copy, the time required for the initial copy can be shortened. 
     Nevertheless, in foregoing Patent Document 1 to Patent Document 3, the differential data between the primary volume and the secondary volume is merely sent unilaterally from the copy source to the copy destination, and it is not possible to confirm whether the differential data is accurately reflected in the primary volume of the copy destination (Patent Document 1 and Patent Document 2) or the secondary volume of the copy destination (Patent Document 3). 
     Therefore, even if the technology of differential copy disclosed in Patent Document 1 to Patent Document 3 is applied to the initial copy as is, there is a problem in that the initial copy cannot be performed with high reliability. 
     The present invention was made in view of the foregoing points. Thus, an object of the present invention is to propose a copy system and a copy method capable of performing initial copy in a short amount of time and with high reliability. 
     In order to achieve the foregoing object, the present invention provides a copy system for copying data written from a host into a first volume on a primary side to a second volume on a secondary side synchronously or asynchronously with the writing of data. This copy system comprises a first control unit provided to the primary side for using a first bitmap to manage the area in the first volume to which data was written from the host, creating a second bitmap reflecting the contents of the first bitmap during the initial copy that matches the contents of the first and second volumes to be performed prior to the copy, sending the second bitmap to the secondary side, and copying to the second volume only the valid data containing data written by the host in the first volume based on the second bitmap, and a second control unit provided to the secondary side for creating a third bitmap to which the second bitmap sent from the primary side is merged, and managing the differential between the first and second volumes during the initial copy based on the third bitmap. 
     Thereby, with this copy system, it is possible to determine the status of valid data that has not been copied to the second volume based on a third bitmap. 
     The present invention further provides a copy method for copying data written from a host into a first volume on a primary side to a second volume on a secondary side synchronously or asynchronously with the writing of the data. This copy method comprises a first step on the primary side of using a first bitmap to manage the area in the first volume to which data was written from the host, a second step on the primary side of creating a second bitmap reflecting the contents of the first bitmap during the initial copy that matches the contents of the first and second volumes to be performed prior to the copy, and sending the second bitmap to the secondary side on the primary side, and on the secondary side of creating a third bitmap to which the second bitmap sent from the primary side is merged, and a third step on the primary-side of copying to the second volume only the valid data containing data written by the host in the first volume based on the second bitmap, and on the secondary side of managing the differential between the first and second volumes during the initial copy based on the third bitmap. 
     Thereby, with this copy method, it is possible to determine the status of valid data that has not been copied to the second volume based on a third bitmap. 
     According to the present invention, it is possible to effectively prevent the copy omission of valid data, and thereby realize a copy system and a copy method capable of performing initial copy in a short amount of time and with high reliability. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the overall configuration of a storage system according to an embodiment of the present invention; 
         FIG. 2  is a conceptual diagram explaining the outline of the remote copy function according to an embodiment of the present invention; 
         FIG. 3  is a conceptual diagram showing an update bitmap management table; 
         FIG. 4A  and  FIG. 4B  are schematic diagrams showing a volume creation screen; 
         FIG. 5  is a block diagram explaining data write reflection processing; 
         FIG. 6  is a flowchart showing the processing contents of data write reflection processing; 
         FIG. 7  is a flowchart showing the processing contents of a configuration controller concerning the first non-reflection condition; 
         FIG. 8  is a flowchart showing the processing contents of a disk controller concerning the second non-reflection condition; 
         FIG. 9  is a chart explaining the initial copy method; 
         FIG. 10  is a chart explaining the initial copy method; 
         FIG. 11  is a flowchart showing the processing contents of first initial copy prestart processing; 
         FIG. 12  is a flowchart showing the processing contents of second initial copy prestart processing; 
         FIG. 13  is a flowchart showing the processing contents of differential bitmap creation processing; 
         FIG. 14  is a flowchart showing the processing contents of first initial copy data guarantee processing; 
         FIG. 15  is a flowchart showing the processing contents of second initial copy data guarantee processing; 
         FIG. 16  is a flowchart showing the processing contents of first erase processing; and 
         FIG. 17  is a flowchart showing the processing contents of second erase processing. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is now explained in detail with reference to the attached drawings. 
     (1) Configuration of Storage System in Present Embodiment 
       FIG. 1  shows the overall storage system  1  according to the present embodiment. The storage system  1  is configured by two storage apparatuses  2 A,  2 B equipped with the remote copy function being connected via a remote copy path  3 , and the hosts  4 A,  4 B being connected to the two storage apparatuses  2 A,  2 B, respectively. 
     Each storage apparatus  2 A,  2 B comprises a plurality of disk devices  10 A,  10 B that provide storage areas for storing data, and control units  11 A,  11 B for controlling the disk devices  10 A,  10 B. 
     Each disk device  10 A,  10 B is configured, for example, from an expensive disk such as a SCSI (Small Computer System Interface) disk, or an inexpensive disk such as a SATA (Serial AT Attachment) disk or an optical disk. The disk devices  10 A,  10 B are operated by the control units  11 A,  11 B according to a RAID (Redundant Array of Inexpensive Disks) system. Incidentally, as the disk devices  10 A,  10 B, for instance, a semiconductor memory such as a flash memory may also be used. 
     One or more logical volumes VOL are established on a physical storage area provided by one or more disk devices  10 A,  10 B, and data is read from and written into the logical volumes VOL in block units of a prescribed size. 
     Each logical volume VOL is allocated a unique volume ID (for instance, a LUN (Logical Unit Number)). In the case of this embodiment, access to data stored in the storage apparatuses  2 A,  2 B is conducted by combining the volume ID and a unique block ID number (for instance, an LBA (Logical Block Address)) to be allocated to each block as the address, and designating this address. 
     The control units  11 A,  11 B comprise channel controllers  12 A,  12 B, remote copy controllers  13 A,  13 B, command controllers  14 A,  14 B, cache memories  15 A,  15 B, shared memories  16 A,  16 B, configuration controllers  17 A,  17 B and disk controllers  18 A,  18 B. 
     The channel controllers  12 A,  12 B interface with the hosts  4 A,  4 B and perform protocol control during communication with the hosts  4 A,  4 B. Based on the protocol control function of the channel controllers  12 A,  12 B, data and commands can be sent and received between the hosts  4 A,  4 B and the storage apparatuses  2 A,  2 B according to an FC protocol, an iSCSI protocol, or a TCP/IP (Transmission Control Protocol/Internet Protocol) protocol. 
     The remote copy controllers  13 A,  13 B execute various types of control processing concerning remote copy as described later by coordinating with the remote copy controllers  13 B,  13 A in the storage apparatuses  2 B,  2 A connected via the remote copy path  3 . 
     The command controllers  14 A,  14 B control the various types of command processing according to commands from the hosts  4 A,  4 B received via the channel controllers  12 A,  12 B, and the transfer of data between the channel controllers  12 A,  12 B and the cache memories  15 A,  15 B, and between the remote copy controllers  13 A,  13 B and the cache memories  15 A,  15 B. 
     The cache memories  15 A,  15 B, for example, are configured from a volatile memory such as an SDRAM (Synchronous Dynamic Random Access Memory), and are primarily used for temporarily storing the write-target data or read-target data to be sent and received between the channel controllers  12 A,  12 B or remote copy controllers  13 A,  13 B and the disk controllers  18 A,  18 B. 
     The shared memories  16 A,  16 B store various control programs for controlling the operation of the storage apparatuses  2 A,  2 B, and various types of management information concerning the storage apparatuses  2 A,  2 B. The shared memories  16 A,  16 B also store the differential bitmaps  20 A,  20 B and the update bitmap management tables  21 A,  21 B described later. 
     The configuration controllers  17 A,  17 B have a function for reading and writing control programs and various types of management information from and into the shared memories  16 A,  16 B, and also have the function for partially creating management information and storing it in the shared memories  16 A,  16 B. 
     The disk controllers  18 A,  18 B act as an interface for sending and receiving data to and from the disk devices  10 A,  10 B. The disk controllers  18 A,  18 B, according to commands from the command controllers  14 A,  14 B, perform processing for writing write-target data in the disk devices  10 A,  10 B and reading read-target data from the disk devices  10 A,  10 B. When inputting and outputting data to and from the disk devices  10 A,  10 B, the disk controllers  18 A,  18 B convert the logical address designated by the hosts  4 A,  4 B into a physical address. Further, the disk controllers  18 A,  18 B perform data access according to the set RAID level when the disk devices  10 A,  10 B are managed based on a RAID system. 
     The remote copy path  3 , for instance, is configured from an FC cable, a LAN, a SAN or a dedicated line. Communication between the storage apparatuses  2 A,  2 B via the remote copy path  3 , for instance, is conducted according to a fibre channel protocol when the remote copy path  3  is an FC cable or a SAN, and conducted according to a TCP/IP (Transmission Control Protocol/Internet Protocol) protocol when the remote copy path  3  is a LAN. 
     The hosts  4 A,  4 B are computer devices comprising a CPU, a memory and so on. Various functions are realized as a result of the CPU provided to the hosts  4 A,  4 B executing the various programs stored in the memory. The hosts  4 A,  4 B, for example, are configured from a personal computer, a workstation, a mainframe computer or the like. 
     (2) Remote Copy Function of Present Embodiment 
     (2-1) Outline 
     The remote copy function loaded in the storage apparatuses  2 A,  2 B is now explained. 
     In the case of this embodiment, the storage apparatuses  2 A,  2 B are equipped with valid data copy, in addition to conventional copy, as the copy method of initial copy to be performed prior to the remote copy between the storage apparatuses  2 A,  2 B. Incidentally, in the ensuing explanation, the storage apparatus  2 A shall be the primary side of remote copy and the storage apparatus  2 B shall be the secondary side of remote copy, and the storage apparatus  2 A will be referred to as a primary storage apparatus  2 A and the storage apparatus  2 B will be referred to as a secondary storage apparatus  2 B. Moreover, in the ensuing explanation, the logical volume VOL to become the primary side of remote copy in the primary storage apparatus  2 A will be referred to as a primary volume PVOL, and the logical volume VOL to become the secondary side of remote copy in the secondary storage apparatus  2 B will be referred to as a secondary volume SVOL. 
     Conventional copy, as with conventional initial copy, is a copy method of copying all data in the primary volume PVOL to the secondary volume SVOL. Further, valid data copy is a copy method of copying only data (this is hereinafter referred to as “valid data”) in the primary volume PVOL written by the host  4 A to the secondary volume SVOL (this is hereinafter referred to as “valid data copy”). 
     In order to accommodate the initial copy based on such valid data copy, as shown in  FIG. 2 , the primary storage apparatus  2 A and the secondary storage apparatus  2 B set update bitmaps  22 A,  22 B, which are for managing the areas in the logical volume VOL to which valid data was written by the hosts  4 A,  4 B, in the logical volume VOL upon creating a logical volume VOL in the self storage apparatus regarding the logical volume designated by a user. When data from the hosts  4 A,  4 B is written in any of the areas in the logical volume VOL, the primary storage apparatus  2 A and the secondary storage apparatus  2 B record the fact that data has been written in such area in the update bitmaps  22 A,  22 B (in other words, they turn on the bit in the update bitmaps  22 A,  22 B corresponding to that area). 
     Meanwhile, when a command (hereinafter referred to as the “copy pair formation command”) is thereafter given from the host  4 A to the primary storage apparatus  2 A that designates the primary volume PVOL and the secondary volume SVOL for forming a copy pair, the primary storage apparatus  2 A and the secondary storage apparatus  2 B create differential bitmaps  20 A,  20 B respectively merged with the update bitmap  22 A of the primary volume PVOL in the respective shared memories  16 A,  16 B of the primary storage apparatus  2 A and the secondary storage apparatus  2 B. 
     Subsequently, the primary storage apparatus  2 A and the secondary storage apparatus  2 B execute initial copy for sequentially copying only the valid data in the primary volume PVOL to the secondary volume SVOL based on the differential bitmaps  20 A,  20 B. Thereupon, the primary storage apparatus  2 A and the secondary storage apparatus  2 B respectively manage the areas that were subject to initial copy in the primary volume PVOL or the secondary volume SVOL based on the differential bitmaps  20 A,  20 B. Specifically, the primary storage apparatus  2 A and the secondary storage apparatus  2 B respectively turn off the bit associated with an area in the differential bitmaps  20 A,  20 B each time they copy the valid data from the primary volume PVOL to such corresponding area in the secondary volume SVOL. Further, when there is an update bitmap  22 B of the secondary volume SVOL, the secondary storage apparatus  2 B turns on the bit associated with the area in such update bitmap  22 B in addition to updating the differential bitmap  20 B. 
     When the secondary storage apparatus  2 B completes the initial copy based on valid data copy, it sends the self storage apparatus-side differential bitmap  20 B to the primary storage apparatus  2 A. Further, the primary storage apparatus  2 A verifies whether there is valid data that has not been copied to the secondary volume SVOL based on the secondary storage apparatus  2 B-side differential bitmap  20 B. When the foregoing valid data exists, the primary storage apparatus  2 A re-copies the valid data to the secondary volume SVOL. 
     Moreover, when erase processing is set in advance regarding such primary volume PVOL, the primary storage apparatus  2 A notifies this to the secondary storage apparatus  2 B. When the secondary storage apparatus  2 B receives this notice, it executes erase processing of initializing the unupdated area to which the valid data in the secondary volume SVOL was not copied. 
     (2-2) Configuration of Update Bitmap Management Table 
     As a means for realizing the foregoing valid data copy, as shown in  FIG. 1 , each shared memory  16 A,  16 B of the primary storage apparatus  2 A and the secondary storage apparatus  2 B stores update bitmap management tables  21 A,  21 B, respectively. 
     The update bitmap management tables  21 A,  21 B are tables for uniformly managing the update bitmaps  22 A,  22 B of the respective logical volumes VOL formed in the primary storage apparatus  2 A or the secondary storage apparatus  2 B, and, as shown in  FIG. 3 , are configured from a “logical unit number” column  23 A, an “update bitmap setting” column  23 B, an “update bitmap status” column  23 C, a “threshold area top position” column  23 D, and a “threshold area end position” column  23 E. 
     Among the above, the “logical unit number” column  23 A stores the identification number (volume number) of each logical volume VOL registered in the update bitmap management tables  21 A,  21 B. In the case of this embodiment, since all logical volumes VOL formed in the primary storage apparatus  2 A and the secondary storage apparatus  2 B are registered in the update bitmap management tables  21 A,  21 B, in effect, the identification number of all logical volumes VOL formed in the primary storage apparatus  2 A and the secondary storage apparatus  2 B will be stored in the “logical unit number” column  23 A. 
     The “update bitmap status” column  23 B stores information (“YES” or “NO”) regarding whether the update bitmaps  22 A,  22 B are set in the corresponding logical volume VOL. The “update bitmap status” column  23 C stores information regarding whether the update bitmaps  22 A,  22 B of the logical volume VOL are set as “valid” or “invalid.” When the update bitmaps  22 A,  22 B are set as “valid,” the corresponding bit of the update bitmaps  22 A,  22 B is turned on when data is written in the corresponding logical volume VOL. When the update bitmaps  22 A,  22 B are set as “invalid,” the corresponding bit of the update bitmap  22 A,  22 B is not turned on even when data is written in the logical volume VOL. 
     Further, the “threshold area top position” column  23 D and the “threshold area end position” column  23 E respectively store the top address and end address of the area (this is hereinafter referred to as the “threshold area”) that is set for determining whether the volume of written data in relation to the volume size of the corresponding logical volume VOL has exceeded a given ratio. In the case of this embodiment, giving consideration to the fact that a logical volume VOL is usually used from an area with a small address, an area of a given capacity (for instance, 100 cylinders (1500 tracks) worth) from the area in Which the address of the logical volume VOL is largest (end area of the logical volume VOL) is automatically allocated as the threshold area. Then, simultaneously with the entry of a new logical volume VOL being registered in the update bitmap management tables  21 A,  21 B, the top address and end address of the threshold area are stored in the “threshold area top position” column  23 D and the “threshold area end position” column  23 E. 
     (2-3) Creation of New Logical Volume 
       FIG. 4A  and  FIG. 4B  show the volume creation screen  30  that can be displayed on a management terminal (not shown) of the primary storage apparatus  2 A and the secondary storage apparatus  2 B according to prescribed operations in the storage system  1 . The volume creation screen  30  is a GUI (Graphical User Interface) screen for creating a new logical volume VOL in the primary storage apparatus  2 A and the secondary storage apparatus  2 B, and comprises a volume creation tag  31 A and a copy option tag  31 B. 
     In the volume creation tag  31 A, the user is able to make various settings regarding the configuration of the logical volume VOL to be newly created, and in the copy option tag  31 B, the user is able to set various options concerning the initial copy of remote copy with the logical volume VOL being the primary volume PVOL. 
     Practically, in the volume creation tag  31 A, by clicking a pulldown menu button  33  displayed on the right side of the volume type designation column  32 , a pulldown menu (not shown) listing the selectable volume types can be displayed, and, by selecting a desired volume type among the volume types displayed on the pulldown menu, the volume type of the logical volume VOL to be newly created can be designated. The designated volume type will be displayed on the volume type designation column  32 . 
     Further, in the volume creation tag  31 A, a desired volume size as the volume size of the logical volume VOL to be newly created can be designated by directly inputting such volume size in cylinder units into the volume size designation column  34 , and the number of volumes to be created can be designated by directly inputting such number of volumes into the number of volumes designation column  35 . 
     Further, in the volume creation tag  31 A, whether to set the update bitmaps  22 A,  22 B regarding the logical volume VOL to be newly created can be designated by clicking one of the radio buttons  36 A,  36 B respectively provided in correspondence to the option of setting (“YES”) or the option of no setting (“NO”). Nevertheless, when a volume size that is smaller than a predetermined size (this is hereinafter referred to as the “volume threshold size”) is designated as the volume size of the logical volume VOL to be newly created, the radio button  36 B corresponding to the option of setting the update bitmaps  22 A,  22 B will be inactively displayed, and only the option of not setting the update bitmap  22 A,  22 B can be selected. 
     Meanwhile, in the copy option tag  31 B, either valid data copy or conventional copy can be designated as the copy type (copy method) of initial copy in the remote copy where the logical volume VOL to be newly created will be the primary volume by clicking one of the radio buttons  37 A,  37 B respectively provided in correspondence to the foregoing copy types. 
     Further, in the copy option tag  31 B, either data guarantee copy of verifying whether copy was performed properly after the initial copy or conventional copy that does not conduct such verification can be designated as an alternate initial copy method when it is not possible to actually perform valid data copy (for instance, when the update bitmaps  22 A,  22  are not set regarding the logical volume VOL to be newly created or the update bitmaps  22 A,  22 B are invalidated) even though valid data copy was selected as the initial copy method of remote copy by clicking one of the radio buttons  38 A,  38 B respectively provided in correspondence to the foregoing copy types. 
     Moreover, in the copy option tag  31 B, whether to execute the foregoing erase processing after the completion of initial copy can be designated by clicking one of the radio button  39 A,  39 B respectively provided in correspondence to the option of execution (“YES”) and the option of no execution (“NO”). 
     After designating the desired conditions and the like regarding the logical volume VOL to be newly created in the volume creation tag  31 A and the copy option tag  31 B, the user clicks the OK button  40  displayed at the lower right part of the volume creation screen  30 . Thereby, the contents designated by the user for the various items on the volume creation screen  30  are incorporated into the management terminal as volume creation information, and given to the configuration controllers  17 A,  17 B of the primary storage apparatus  2 A and the secondary storage apparatus  2 B. 
     The configuration controllers  17 A,  17 B that received the volume creation information create a logical volume VOL according to the various designated contents contained in the volume creation information, and register the necessary information concerning the logical volume VOL in the update bitmap management tables  21 A,  21 B. Further, the configuration controllers  17 A,  17 B also store information, among the various types of information contained in the volume creation information, which is not registered in the update bitmap management tables  21 A,  21 B in the shared memories  16 A,  15 B. 
     Moreover, when it is designated in the volume creation information that the update bitmaps  22 A,  22 B should be set, the configuration controllers  17 A,  17 B set the update bitmaps  22 A,  22 B of the created logical volume VOL in such logical volume VOL. Incidentally, the specific processing contents of the configuration controllers  17 A,  17 B concerning the creation of a new logical volume VOL will be described later. 
     (2-4) Data Write Reflection Processing 
     When a write command and write-target data to be written in the created logical volume VOL are provided from the hosts  4 A,  4 B, as shown in  FIG. 5 , the primary storage apparatus  2 A and the secondary storage apparatus  2 B store such write command and write-target data in the cache memories  15 A,  15 B (SP 1 ) and, at the stage when the storage of such write command and write-target data in the cache memories  15 A,  15 B is complete, send a completion notice indicating that the writing of data is complete to the hosts  4 A,  4 B (SP 2 ). 
     The primary storage apparatus  2 A and the secondary storage apparatus  2 B thereafter write the data stored in the cache memories  15 A,  15 B into a corresponding area of the logical volume VOL designated in the write command. Nevertheless, when the update bitmaps  22 A,  22 B are set in the logical volume VOL and such update bitmaps  22 A,  22 B are valid, the primary storage apparatus  2 A and the secondary storage apparatus  2 B reflect the writing of data in the update bitmaps  22 A,  22 B simultaneously with such writing of data (SP 3 ). 
       FIG. 6  shows the specific processing contents of the disk controllers  18 A,  18 B in the primary storage apparatus  2 A and the secondary storage apparatus  2 B concerning the data write reflection processing for reflecting the writing of data in the primary volume PVOL in the update bitmaps  22 A,  22 B. 
     The disk controllers  18 A,  18 B are periodically referring to the cache memories  15 A,  15 B, and, when they discover that a command from the hosts  4 A,  4 B has been stored in the cache memories  15 A,  15 B, the disk controllers  18 A,  18 B determine whether that command is a write command (SP 10 ). 
     When the disk controllers  18 A,  18 B obtain a negative result in this determination, they end this processing. Contrarily, when the disk controllers  18 A,  18 B obtain a positive result, they sequentially determine whether the update bitmap  22 A,  22 B are set in the logical volume VOL to be written with data designated in the write command, and, if the update bitmaps  22 A,  22 B are set, determine whether “valid” is set in the update bitmaps  22 A,  22 B (SP 11 , SP 12 ). 
     When the disk controllers  18 A,  18 B obtain a negative result in the determination at either step SP 11  or step SP 12 , they end this processing. Contrarily, when the disk controllers  18 A,  18 B obtain a positive result at both step SP 11  and step SP 12 , they reflect the writing of data in the update bitmaps  22 A,  22 B set in the logical volume VOL (SP 13 ). The disk controllers  18 A,  18 B thereafter end this data write reflection processing. 
     (2-5) Management of Update Bitmap 
     The initial copy based on the foregoing valid data copy is performed by referring to the update bitmaps  22 A,  22 B when the logical volume VOL in which the update bitmaps  22 A,  22 B are “valid” is to become the primary volume PVOL. 
     In the foregoing case, for instance, when performing initial copy with a logical volume VOL originally having a small volume size or a logical volume in which a majority of the logical blocks have been updated (data was written from the hosts  4 A,  4 B) being the primary volume PVOL, there will not be much difference in the time required to perform full copy of copying all data in the primary volume PVOL to the secondary volume SVOL, and the time required to perform valid data copy. 
     Thus, in the present embodiment, when the volume size of the logical volume VOL at the time such logical volume VOL was created is smaller than the volume threshold size described with reference to  FIG. 4  (first non-reflection condition), or when the unupdated area to which valid data in the logical volume VOL was not written becomes smaller than the threshold area size described with reference to  FIG. 3  (second non-reflection condition), the writing of data from the hosts  4 A,  4 B into the logical volume VOL is not reflected in the update bitmaps  22 A,  22 B. 
     Incidentally, the trigger for ceasing to reflect the writing of data in the update bitmaps  22 A,  22 B will be at the time the logical volume VOL is created in a case where the volume size of the logical volume VOL is smaller than the volume threshold size, and at the time when the unupdated area in the logical volume VOL becomes smaller than the threshold area size in such a case. 
     The specific processing contents concerning the former (first non-reflection condition) are now explained. This processing is performed by the configuration controllers  17 A,  17 B of the primary storage apparatus  2 A and the secondary storage apparatus  2 B according to the processing routine shown in  FIG. 7 . 
     In other words, when the configuration controllers  17 A,  17 B receive the foregoing volume creation information from a management terminal not shown, they foremost define the logical volume VOL according to the volume type and volume size designated by the user in a quantity designated by the user (SP 20 ). Further, the configuration controllers  17 A,  17 B thereafter determine whether the update bitmaps  22 A,  22 B are set in the created logical volume VOL based on the volume creation information (SP 21 ). 
     When the configuration controllers  17 A,  17 B obtain a negative result in this determination, they register the entry regarding the logical volume VOL defined at step SP 20  in the update bitmap management tables  21 A,  21 B ( FIG. 3 ) indicating that the update bitmaps  22 A,  22 B are not set (SP 22 ). Accordingly, since the update bitmaps  22 A,  22 B are not set in this logical volume VOL, valid data copy cannot be used as the initial copy method of remote copy. 
     Contrarily, when the configuration controllers  17 A,  17 B obtain a positive result in the determination at step SP 21 , they determine whether the volume size of the logical volume VOL defined at step SP 20  is greater than the volume threshold size (SP 23 ). 
     When the configuration controllers  17 A,  17 B obtain a negative result in this determination, they register the entry regarding the logical volume VOL defined at step SP 20  in the update bitmap management tables  21 A,  21 B indicating that the update bitmaps  22 A,  22 B are set (setting of the update bitmaps  22 A,  22 B is “YES”), but the update bitmaps  22 A,  22 B are invalid (status of the update bitmaps  22 A,  22 B is “invalid”) (SP 24 ). Therefore, even when data is written from the hosts  4 A,  4 B in this logical volume VOL, since such writing of data is not reflected in the update bitmaps  22 A,  22 B, valid data copy cannot be used as the initial copy method of remote copy. 
     When the configuration controllers  17 A,  17 B obtain a positive result in the determination at step SP 23 , they register the entry regarding the logical volume VOL defined at step SP 20  in the update bitmap management tables  21 A,  21 B indicating that the update bitmaps  22 A,  22 B are set (setting of the update bitmaps  22 A,  22 B is “YES”), and the update bitmaps  22 A,  22 B are valid (status of the update bitmaps  22 A,  22 B is “valid”) (SP 25 ). Therefore, with this logical volume VOL, when data is written from the hosts  4 A,  4 B in this logical volume VOL, since such writing of data is reflected in the update bitmaps  22 A,  22 B, valid data copy can be used as the initial copy method of remote copy based on the update bitmaps  22 A,  22 B. 
     The specific processing contents concerning the latter (second non-reflection condition) are now explained. As described above, the disk controllers  18 A,  18 B of the primary storage apparatus  2 A and the secondary storage apparatus  2 B are periodically referring to the cache memories  15 A,  15 B. When the disk controllers  18 A,  18 B discover that a write command is stored in the cache memories  15 A,  15 B, they store the write-target data stored in the cache memories  15 A,  15 B in an area designated in the logical volume (this is hereinafter referred to as the “designated logical volume”) VOL designated in the write command. Concurrently with this, the disk controllers  18 A,  18 B execute the write data volume determination processing illustrated in  FIG. 8 . 
     In other words, when the disk controllers  18 A,  18 B detect that a write command is stored in the cache memories  15 A,  15 B, they refer to the corresponding “threshold area top position” column  23 D ( FIG. 3 ) and “threshold area end position” column  23 E ( FIG. 3 ) in the update bitmap management tables  21 A,  21 B, and determine whether the threshold area in the designated logical volume VOL is contained in the write destination of the write-target data designated in the write command (SP 30 ). When the disk controllers  18 A,  18 B obtain a negative result in this determination, they end this write data volume determination processing. 
     Contrarily, when the disk controllers  18 A,  18 B obtain a positive result in the determination at step SP 30 , they read the update bitmaps  22 A,  22 B of the designated logical volume VOL from the designated logical volume VOL into the cache memories  15 A,  15 B (SP 31 ), and determine whether the number of bits that are turned off in the update bitmaps  22 A,  22 B is less than the number of bits associated with any one of the areas in the threshold area (SP 32 ). 
     When the disk controllers  18 A,  18 B obtain a negative result in this determination, they change the location of the threshold area in the designated logical volume VOL. Specifically, the disk controllers  18 A,  18 B conduct a search from the terminal of the designated logical volume VOL. When there are areas that are consecutively unupdated in a size that is the same as the threshold area, the disk controllers  18 A,  18 B register such areas as the new threshold area in the update bitmap management tables  21 A,  21 B. Further, when there are no areas that are consecutively unupdated in a size that is the same as the threshold area, the disk controllers  18 A,  18 B register the unupdated area of the maximum size among the consecutive unupdated areas in the designated logical volume VOL as the new threshold area in the update bitmap management tables  21 A,  21 B (SP 33 ). 
     Contrarily, when the disk controllers  18 A,  18 B obtain a positive result in the determination at step SP 32 , they change the setting of the entry of the designated logical volume VOL in the update bitmap management tables  21 A,  21 B to “YES” for the setting of the update bitmaps  22 A,  22 B, and to “invalid” for the status of the update bitmaps  22 A,  22 B. Thereby, even when data from the hosts  4 A,  4 B is thereafter written in the designated logical volume VOL, such writing of data will not be reflected in the update bitmaps  22 A,  22 B (refer to step SP 12  of  FIG. 6 ). 
     (2-6) Initial Copy Processing 
     (2-6-1) Initial Copy Method 
     As described above, the storage system  1  has valid data copy and conventional copy as the initial copy method of remote copy. 
     Here, for valid data copy, upon referring to the differential bitmap  20 A copied with the update bitmap  22 A in the primary volume PVOL, only valid data (data written from the host  4 A) among the data in the primary volume PVOL is copied to the secondary volume SVOL. 
     Therefore, even when valid data copy is designated as the initial copy method of remote copy regarding a certain primary volume PVOL, valid data copy cannot be performed when the update bitmap  22 A of that primary volume PVOL is not set or the update bitmap  22 A is invalid. 
     In the foregoing case, initial copy based on the copy method (data guarantee copy or conventional copy) designated as the alternate copy method in the copy option tag  31 B of the volume creation screen  30  described with reference to  FIG. 4  will be performed. 
       FIG. 9  is a diagram showing the comparison of the copy range and the status of data guarantee processing when adopting valid data copy, data guarantee copy or conventional copy as the initial copy method. 
     As evident from  FIG. 9 , with valid data copy, only valid data is copied as the copy range, and whether copy was properly performed is verified after the initial copy is complete (data guarantee processing). Further, with data guarantee copy, all areas of the primary volume PVOL are copied as the copy range, and data guarantee processing is performed after the copy is complete. Moreover, with conventional copy, all areas of the primary volume PVOL are copied as the copy range, and data guarantee processing is not performed after the copy is complete. 
     Incidentally,  FIG. 10  shows which copy method will be used for the initial copy according to the respective settings of the copy type and alternate copy method, and the status (valid/invalid) of the update bitmap  22 A at such time. As evident from  FIG. 10 , when valid data copy is set as the copy type and the update bitmap  22 A is set regarding the primary volume PVOL, regardless of whether the data guarantee copy or the conventional copy is set as the alternate copy method, valid data copy will be performed if the update bitmap  22 A is valid, and data guarantee copy will be performed if the update bitmap  22 A is invalid. 
     Further, when valid data copy is set as the copy type and the update bitmap  22 A is not set regarding the primary volume PVOL, data guarantee copy is performed when data guarantee copy is set as the alternate copy method, and conventional copy is performed when conventional copy is set as the alternate copy method. Moreover, when conventional copy is set as the copy type regarding the primary volume PVOL, conventional copy is performed as per normal. 
     The selection and decision of the copy method of initial copy, as described above, is conducted by the remote copy controller  13 A based on configuration information regarding the primary volume PVOL input by the user with a management terminal (not shown) in the primary storage apparatus  2 A upon creating the primary volume PVOL. 
     (2-6-2) Initial Copy Prestart Processing 
       FIG. 11  shows the processing contents of the remote copy controller of the primary storage apparatus  2 A (this is hereinafter referred to as the “primary-side remote copy controller”)  13 A ( FIG. 1 ) prior to starting the initial copy. When the primary-side remote copy controller  13 A receives a copy pair formation command designating the primary volume PVOL and the secondary volume SVOL sent from the host  4 A according to the user&#39;s operation via the channel controller  12 A ( FIG. 1 ), it starts the first initial copy prestart processing illustrated in  FIG. 11 , and foremost determines whether the copy type of initial copy set regarding the primary volume PVOL designated in the copy pair formation command is valid data copy (SP 40 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it communicates with the remote copy controller of the secondary storage apparatus  2 B (this is hereinafter referred to as the “secondary-side remote copy controller”)  13 B ( FIG. 1 ), and conducts a preliminary check on whether it is possible to form a copy pair with the primary volume PVOL and the secondary volume SVOL designated in the copy pair formation command. Further, in addition to this, the primary-side remote copy controller  13 A notifies the copy method of initial copy (conventional copy in this case) to be executed subsequently to the secondary-side remote copy controller  13 B. When the primary-side remote copy controller  13 A receives a notice to the effect that a copy pair can be formed from the secondary-side remote copy controller  13 B (refer to step SP 65  of  FIG. 12 ), it sets the primary volume PVOL and the secondary volume SVOL as a copy pair (SP 41 ). The primary-side remote copy controller  13 A thereafter proceeds to step SP 50 . 
     Contrarily, when the primary-side remote copy controller  13 A obtains a positive result in the determination at step SP 40 , it refers to the update bitmap management table  21 A ( FIG. 3 ) and determines whether the update bitmap  22 A is set in the primary volume PVOL (SP 42 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it determines whether the alternate copy method designated in the volume creation information regarding the primary volume PVOL is data guarantee copy (SP 43 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it proceeds to step SP 41 . Contrarily, when the primary-side remote copy controller  13 A obtains a positive result in this determination, it communicates with the secondary-side remote copy controller  13 B, and conducts a preliminary check on whether a copy pair can be formed with the primary volume PVOL and the secondary volume SVOL designated in the copy pair formation command. Further, in addition to this, the primary-side remote copy controller  13 A notifies the copy method of initial copy (data guarantee copy in this case) to be executed subsequently to the secondary-side remote copy controller  13 B. When the primary-side remote copy controller  13 A receives a notice to the effect that a copy pair can be formed from the secondary-side remote copy controller  13 B (refer to step SP 65  of  FIG. 12 ), it sets the primary volume PVOL and the secondary volume SVOL as a copy pair (SP 45 ). 
     The primary-side remote copy controller  13 A thereafter creates a differential bitmap  20 A ( FIG. 1 ) with all bits turned on in the shared memory  16 A (FIG.  1 ) (SP 46 ), and then proceeds to step SP 50 . 
     Meanwhile, when the primary-side remote copy controller  13 A obtains a positive result in the determination at step SP 42 , it refers to the update bitmap management table  21 A and determines whether the update bitmap  22 A of the primary volume PVOL is valid (SP 44 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it proceeds to step SP 45 . Contrarily, when the primary-side remote copy controller  13 A obtains a positive result in this determination, it communicates with the secondary-side remote copy controller  13 B, and conducts a preliminary check on whether a copy pair can be formed with the primary volume PVOL and the secondary volume SVOL designated in the copy pair formation command. Further, in addition to this, the primary-side remote copy controller  13 A notifies the copy method of initial copy (valid data copy in this case) to be executed subsequently to the secondary-side remote copy controller  13 B. When the primary-side remote copy controller  13 A receives a notice to the effect that a copy pair can be formed from the secondary-side remote copy controller  13 B (refer to step SP 65  of  FIG. 12 ), it sets the primary volume PVOL and the secondary volume SVOL as a copy pair (SP 47 ). 
     Subsequently, the primary-side remote copy controller  13 A creates a differential bitmap  20 A for valid data copy reflecting the current update bitmap  22 A in the shared memory  16 A (SP 48 ), and sends the created differential bitmap  20 A to the secondary-side remote copy controller  13 B (SP 49 ). 
     The primary-side remote copy controller  13 A thereafter starts the initial copy with the copy method notified to the secondary-side remote copy controller  13 B at step SP 41 , step SP 45  or step SP 47  (SP 50 ). 
     Meanwhile,  FIG. 12  shows the processing contents of the secondary-side remote copy controller  13 B notified of the copy method of initial copy to be subsequently executed from the primary-side remote copy controller  13 A at step SP 41 , step SP 45  or step SP 47  of the first initial copy prestart processing. 
     When the secondary-side remote copy controller  13 B is notified of the copy method, it starts the second initial copy prestart processing illustrated in  FIG. 12 , and foremost determines whether a copy pair can be formed with the logical volume VOL to become the primary volume PVOL and the logical volume VOL to become the secondary volume SVOL notified by the primary-side remote copy controller  13 A during the preliminary check at step SP 41 , step SP 45  or step SP 47  of the first initial copy prestart processing described with reference to  FIG. 11  (SP 60 ). 
     In the foregoing case, for example, if the volume size of the logical volume VOL to become the secondary volume SVOL is smaller than the volume size of the logical volume VOL to become the primary volume PVOL, a copy pair cannot be formed with these logical volumes VOL. Thereby, the secondary-side remote copy controller  13 B executes prescribed error processing such as sending an error message to such effect to the primary-side remote copy controller  13 A (SP 61 ). 
     Contrarily, when the secondary-side remote copy controller  13 B obtains a positive result in the determination at step SP 60 , it determines whether the copy method notified from the primary-side remote copy controller  13 A at step SP 41 , step SP 45  or step SP 47  of the first initial copy prestart processing described with reference to  FIG. 11  is data guarantee copy, valid data copy or conventional copy (SP 62 ). 
     When the copy method is valid data copy, the secondary-side remote copy controller  13 B secures an area for creating the differential bitmap  20 A in the shared memory  16 B (SP 63 ), and thereafter proceeds to step SP 65 . Accordingly, when the secondary-side remote copy controller  13 B receives the differential bitmap  20 A sent from the primary-side remote copy controller  13 A (refer to step SP 49  of  FIG. 11 ), it creates the secondary storage apparatus  2 B-side differential bitmap  20 B by copying the differential bitmap  20 A to the foregoing area. 
     Further, when the copy method is data guarantee copy, the secondary-side remote copy controller  13 B creates a differential bitmap  20 B with all bits turned on (SP 64 ), thereafter notifies the primary-side remote copy controller  13 A that a pair can be formed (SP 65 ), and then ends this second initial copy prestart processing. 
     Here,  FIG. 13  shows the specific processing contents of the primary-side remote copy controller  13 A at step SP 48  of the first initial copy prestart processing described with reference to  FIG. 11 . 
     When the primary-side remote copy controller  13 A proceeds to step SP 48 , it starts the differential bitmap creation processing illustrated in  FIG. 13 , and foremost creates a differential bitmap  20 A with all bits turned off in the shared memory  16 A (SP 70 ). Subsequently, the primary-side remote copy controller  13 A determines whether write-target data which has not yet been written in the primary volume PVOL exists in the cache memory  15 A (SP 71 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it proceeds to step SP 73 . Contrarily, when the primary-side remote copy controller  13 A obtains a positive result in this determination, it turns on the bit in the differential bitmap  20 A corresponding to the area in the primary volume PVOL to which the write-target data is to be written (SP 72 ). 
     Subsequently, the primary-side remote copy controller  13 A reads the update bitmap  22 A of the primary volume PVOL from the primary volume PVOL into the cache memory  15 A via the disk controller  18 A ( FIG. 1 ) (SP 73 ). 
     The primary-side remote copy controller  13 A thereafter merges the update bitmap  22 A with the differential bitmap  20 A created at step SP 70  (SP 74 ), and then ends this differential bitmap creation processing. 
     Incidentally, when the primary-side remote copy controller  13 A creates a differential bitmap  20 A, since the subsequent writing of data from the host  4 A in the primary volume PVOL will be reflected in the differential bitmap  20 A, the writing of data from the host  4 A will never be unreflected in the differential bitmap  20 A. 
     (2-6-3) Initial Copy Processing 
     Initial copy is performed by the primary-side remote copy controller  13 A coordinating with the secondary-side remote copy controller  13 B. 
     Specifically, when performing the initial copy based on valid data copy, the primary-side remote copy controller  13 A foremost sequentially reads data from the respective areas where all bits are on in the differential bitmap  20 A of the primary volume PVOL via the disk controller  18 A ( FIG. 1 ) while referring to the differential bitmap  20 A created in the shared memory  16 A. Subsequently, the primary-side remote copy controller  13 A sends the read data to the secondary-side remote copy controller  13 B, and additionally turns off the corresponding bits in the differential bitmap  20 A. 
     Further, the secondary-side remote copy controller  13 B sequentially writes the data sent from the primary-side remote copy controller  13 A in the corresponding area of the secondary volume SVOL via the disk controller  18 B ( FIG. 1 ). Subsequently, the secondary-side remote copy controller  13 B additionally turns off the corresponding bit of the differential bitmap  20 B created in the shared memory  16 B. 
     Thus, with the initial copy based on valid data copy, each time valid data stored in the primary volume PVOL is copied to the secondary volume SVOL, the corresponding bits in the primary storage apparatus  2 A-side differential bitmap  20 A and the corresponding bits in the secondary storage apparatus  2 B-side differential bitmap  20 B are turned off. 
     The primary-side remote copy controller  13 A and the secondary-side remote copy controller  13 B repeat the foregoing processing until all bits in the primary storage apparatus  2 A-side differential bitmap  20 A are turned off, and then end the initial copy when all bits in the differential bitmap  20 A are eventually turned off. 
     The primary-side remote copy controller  13 A and the secondary-side remote copy controller  13 B perform initial copy according to the same procedures as in the case of valid data copy when performing the initial copy based on data guarantee copy. Nevertheless, in the foregoing case, since all bits of both the primary storage apparatus  2 A-side differential bitmap  20 A and the secondary storage apparatus  2 B-side differential bitmap  20 A are turned on, the difference with the valid data copy is in that all data stored in each area of the primary volume PVOL are copied to the secondary volume SVOL. 
     Meanwhile, in the initial copy based on conventional copy, the differential bitmaps  20 A,  20 B are neither created nor used on both the side of the primary storage apparatus  2 A and the side of the secondary storage apparatus  2 B. Here, the primary-side remote copy controller  13 A sequentially reads the data stored in each area of the primary volume PVOL via the disk controller  18 A, and sequentially sends such data to the secondary-side remote copy controller  13 B. Further, the secondary-side remote copy controller  13 B sequentially writes the data sent from the primary-side remote copy controller  13 A in the corresponding area of the secondary volume SVOL via the disk controller  18 B. 
     The primary-side remote copy controller  13 A and the secondary-side remote copy controller  13 B repeat the foregoing processing until all data stored in each area of the primary volume PVOL are all copied to the corresponding area in the secondary volume SVOL, and then ends the initial copy when the copy of all such data is complete. 
     (2-7) Initial Copy Post-Termination Processing 
     (2-7-1) Initial Copy Data Guarantee Processing 
     The data guarantee processing to be performed after the initial copy based on valid data copy or data guarantee copy is now explained. 
       FIG. 14  shows the specific processing contents of the data guarantee processing (this is hereinafter referred to as the “first initial copy data guarantee processing”) to be performed by the primary-side remote copy controller  13 A after the initial copy based on valid data copy or data guarantee copy. 
     When the initial copy is complete, the primary-side remote copy controller  13 A starts the first initial copy data guarantee processing, and foremost determines whether a bit that has been turned on exists in the differential bitmap  20 A used in the initial copy (SP 80 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it reads data from an area in the primary volume PVOL corresponding to the bit that is turned on, and transfers such data to the secondary-side remote copy controller  13 B (SP 81 ). The primary-side remote copy controller  13 A thereafter turns off such bit in the differential bitmap  20 A (SP 82 ), and then repeats the foregoing processing until no bits that have been turned on exist in the differential bitmap  20 A (SP 80  to SP 82 -SP 80 ). 
     When all bits in the differential bitmap  20 A are eventually turned off, the primary-side remote copy controller  13 A notifies the completion of data copy to the secondary-side remote copy controller  13 B (SP 83 ). Further, when the primary-side remote copy controller  13 A consequently receives information (this is hereinafter referred to as the “secondary-side differential bitmap status information”) concerning the current status of the secondary storage apparatus  2 B-side differential bitmap  20 B sent from the secondary-side remote copy controller  13 B as described above (SP 85 ), it determines whether a bit that has been turned on exists in the secondary storage apparatus  2 B-side differential bitmap  20 B based on this secondary-side differential bitmap status information (SP 85 ). 
     Here, to obtain a positive result in this determination implies that the data in the primary volume PVOL which is considered to have been copied by the primary storage apparatus  2 A-side differential bitmap  20 A has not actually been copied to the secondary volume SVOL. Thereby, the primary-side remote copy controller  13 A reads the secondary storage apparatus  2 B-side differential bitmap  20 B into the shared memory  16 A of the self storage apparatus via the secondary-side remote copy controller  13 B (SP 86 ). 
     By thereafter controlling the disk controller  18 A based on the secondary storage apparatus  2 B-side differential bitmap  20 B, the primary-side remote copy controller  13 A reads data from all areas corresponding to the bits that are turned on in the differential bitmap  20 B of the primary volume PVOL, and transfers such data to the secondary-side remote copy controller  13 B (SP 87 ). 
     The primary-side remote copy controller  13 A turns off the corresponding bits in the secondary storage apparatus  2 B-side differential bitmap  20 B (SP 88 ), thereafter returns to step SP 83 , and then repeats the foregoing processing (SP 83  to SP 88 -SP 83 ) until it obtains a positive result at step SP 85  (in other words, until all bits of the secondary storage apparatus  2 B-side differential bitmap  20 B are turned off). The primary-side remote copy controller  13 A ends this first initial copy data guarantee processing when all bits of the secondary storage apparatus  2 B-side differential bitmap  20 B are eventually turned off. 
     Meanwhile,  FIG. 15  shows the specific processing contents of the data guarantee processing (this is hereinafter referred to as the “second initial copy data guarantee processing”) to be performed by the secondary-side remote copy controller  13 B that received a data copy completion notice at step SP 83  of the first initial copy data guarantee processing ( FIG. 14 ). 
     When the secondary-side remote copy controller  13 B receives the foregoing notice, it starts the second initial copy data guarantee processing, and foremost refers to the differential bitmap  20 B stored in the shared memory  16 B and determines whether a bit that has been turned on in the differential bitmap  20 B exists (SP 90 ). 
     When the secondary-side remote copy controller  13 B obtains a positive result in this determination, it sends secondary-side differential bitmap status information indicating that a bit that has been turned on exists to the primary-side remote copy controller  13 A (SP 91 ). Contrarily, when the secondary-side remote copy controller  13 B obtains a negative result in the determination at step SP 90 , it sends secondary-side differential bitmap status information indicating that a bit that has been turned on does not exist to the primary-side remote copy controller  13 A (SP 92 ). The secondary-side remote copy controller  13 B thereafter ends this second initial copy data guarantee processing. 
     (2-7-2) Erase Processing 
     The erase processing to be performed by the primary-side remote copy controller  13 A and the secondary-side remote copy controller  13 B after the initial copy data guarantee processing is now explained. 
       FIG. 16  shows the specific processing contents of the first erase processing to be executed by the primary-side remote copy controller  13 A after the completion of the first initial copy data guarantee processing explained with reference to  FIG. 14 . 
     When the foregoing first initial copy data guarantee processing is complete, the primary-side remote copy controller  13 A starts the first erase processing, and foremost determines whether an erase mode is set in the primary volume PVOL based on the volume creation information regarding the primary volume PVOL (SP 100 ). 
     When the primary-side remote copy controller  13 A obtains a negative result in this determination, it proceeds to step SP 103 . Contrarily, when the primary-side remote copy controller  13 A obtains a positive result in this determination, it sends a status change (Dup(Erase)) command and a differential bitmap  20 A representing the status of the current primary volume PVOL to the secondary-side remote copy controller  13 B (SP 101 ). Incidentally, this differential bitmap  20 A is created according to the same processing as the processing explained with reference to  FIG. 13 . 
     Subsequently, the primary-side remote copy controller  13 A awaits the erase processing completion notice described later, which is a response of the processing result to the status change (Dup(Erase)) command, to be sent from the secondary-side remote copy controller  13 B (SP 102 ). 
     When the primary-side remote copy controller  13 A eventually receives the erase processing completion notice, it sends a status change (Duplex) command to the secondary-side remote copy controller  13 B for changing the pair status of the copy pair formed from the primary volume PVOL and the secondary volume SVOL to a Duplex status (status where the data contents of the primary volume PVOL and the secondary volume SVOL are the same) (SP 103 ), and then ends this first erase processing. 
     Meanwhile,  FIG. 17  shows the specific processing contents of the second erase processing to be executed by the secondary-side remote copy controller  13 B that received the status change (Dup(Erase)) command and the differential bitmap  20 A representing the status of the current primary volume PVOL sent from the primary-side remote copy controller  13 A. 
     When the secondary-side remote copy controller  13 B receives the status change (Dup(Erase)) command and the differential bitmap  20 A sent from the primary-side remote copy controller  13 A, it starts the second erase processing illustrated in  FIG. 17 , and foremost creates a secondary storage apparatus  2 B-side differential bitmap  20 B in the shared memory  16 B by copying the differential bitmap  20 A (SP 110 ). 
     Subsequently, the secondary-side remote copy controller  13 B determines whether a bit that has been turned off exists in the differential bitmap  20 B created at step SP 110  (SP 111 ). 
     Here, to obtain a positive result in this determination implies that an unupdated area to which data was not copied in the initial copy exists in the secondary volume SVOL. Thereby, the secondary-side remote copy controller  13 B implements erase processing for initializing the area corresponding to such bit in the secondary volume SVOL (SP 112 ), turns on the bit (bit corresponding to the area subject to erase processing) in the differential bitmap  20 B (SP 113 ), and then returns to step SP 111 . The secondary-side remote copy controller  13 B thereafter repeats the foregoing processing (SP 111  to SP 113 -SP 111 ) until it obtains a negative result at step SP 111 . 
     When the secondary-side remote copy controller  13 B eventually obtains a negative result at step SP 111  as a result of all bits in the differential bitmap  20 B being turned on, it sends the foregoing erase processing completion notice to the primary-side remote copy controller  13 A (refer to step SP 102  of  FIG. 16 ) (SP 114 ), and then ends this second erase processing. 
     (3) Effect of Present Embodiment 
     In the foregoing configuration, when the update bitmaps  22 A,  22 B are set regarding the logical volume VOL in the respective storage apparatuses  2 A,  2 B, the storage system  1  manages the areas in the logical volume VOL to which valid data was written by the hosts  4 A,  4 B using the update bitmaps  22 A,  22 B. Further, when valid data copy is set as the copy method of initial copy, during the initial copy, the storage system  1  creates a differential bitmap  20 A reflecting the contents of the update bitmap  22 A on the side of the primary storage apparatus  2 A, and further creates a differential bitmap  20 B merged with the differential bitmap  20 A on the side of the secondary storage apparatus  2 B, sends only the valid data to the secondary storage apparatus  2 B using the differential bitmap  20 A on the side of the primary storage apparatus  2 A, and manages the areas to which the valid data in the secondary volume SVOL was copied using the differential bitmap  20 B on the side of the secondary storage apparatus  2 B. 
     Accordingly, since the storage system  1  is able to determine the status of the valid data that has not been copied to the secondary volume SVOL based on the secondary storage apparatus  2 B-side differential bitmap  20 B, it is possible to effectively prevent the copy omission of valid data, and thereby realize a copy system and a copy method capable of performing initial copy in a short amount of time and with high reliability. 
     (4) Other Embodiments 
     Incidentally, in the foregoing embodiment, although a case was explained where three types of copy methods; namely, valid data copy, data guarantee copy and conventional copy could be used as the copy method of initial copy between the storage apparatuses  2 A,  2 B, the present invention is not limited thereto, and, if only the valid data copy is to be used as the copy method of initial copy, initial copy based on the other copy methods may or may not be performed. 
     Further, in the foregoing embodiment, although a case was explained where the primary storage apparatus  2 A creates the differential bitmap  20 A reflecting the contents of the update bitmap  22 A so as to enable differential copy with the secondary storage apparatus  2 B based on the differential bitmap  20 A, the present invention is not limited thereto, and, without creating the differential bitmap  20 A, differential copy may be performed with the secondary storage apparatus  2 B using the update bitmap  22 A as is. Nevertheless, as a result of creating the differential bitmap  20 A, since it is possible to recognize the area storing the valid data in the primary volume PVOL on the side of the first storage apparatus  2 A based on the update bitmap  22 A even after the initial copy is complete, there is an advantage in that it is possible to perform differential copy once again to another secondary volume SVOL. 
     Moreover, in the foregoing embodiment, although a case was explained where a plurality of components, such as the disk controllers  18 A,  18 B and the remote copy controllers  13 A,  13 B, are used to control the update processing of the update bitmap  22 A,  22 B, various types of processing before the initial copy and the initial copy processing, the present invention is not limited thereto, and a processor for collectively controlling such processing may be additionally provided. 
     Further, in the foregoing embodiment, although a case was explained where, upon initial copy, copying of the valid data from the primary volume PVOL to the secondary volume SVOL was primarily handled by the primary storage apparatus  2 A in order to send the valid data from the primary storage apparatus  2 A to the secondary storage apparatus  2 B, the present invention is not limited thereto, and copying of the valid data from the primary volume PVOL to the secondary volume SVOL can be primarily handled by the secondary storage apparatus  2 B such that the secondary storage apparatus  2 B reads the valid data from the primary storage apparatus  2 A. 
     Moreover, in the foregoing embodiment, although a case was explained where this invention was applied to the remote copy between the storage apparatuses  2 A,  2 B, the present invention is not limited thereto, and this invention may also be applied to the internal copy to be performed in a single storage apparatus  2 A,  2 B. 
     The present invention may be broadly applied to a copy system of various configurations that copies data written from a host in a primary-side first volume to a secondary-side second volume synchronously or asynchronously with the writing of such data.