Abstract:
Proposed are a highly reliable storage system and its control method capable of accelerating the processing speed of the copy processing seen from the host device. 
     With the storage system and its control method which stores a command issued from a host device in a command queue and executes the command stored in the command queue in the order that the command was stored in command queue, a copy queue is set for temporarily retaining a copy command among the commands issued from the host device in the memory, the copy command among the commands is moved from the host device stored in the command queue to the copy queue and an execution completion reply of copy processing according to the command is sent to the host device as a sender of the command, and the copy command that was moved to the copy queue is executed in the background in the order that the copy command was stored in the copy queue.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a storage system and its control method which is appropriate, for example, for the storage system in which the copy function of copying data from an arbitrary logical volume in the system to the same or another logical volume in the same system (hereinafter referred to as the internal copy function) is installed. 
       BACKGROUND ART 
       [0002]    Conventionally, the copy processing whose copy source and copy destination are specified by addresses is performed by reading data from the copy source address and writing the data to the copy destination address. 
         [0003]    One of the methods for shortening the processing time for this type of copy processing is using caches accessible in a short time and performing copy in the caches. By this method, when the relevant copy processing is completed, even before writing the data to the copy destination disk, the data can be accessed from the host device. 
         [0004]    As another technology related to the internal copy function, the technology of, when performing the copy processing in units of logical volumes, reporting the completion of the relevant copy processing to the host device before completing the copy processing and then performing the copy processing in the background exists. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    The shortening of time for the copy processing in which the copy source and the copy destination are specified by addresses is effective if the amount of data as the copy target is small. However, since multiple copy commands for copying a large amount of data to the same address are sequentially issued from the host device to the storage system, it may be possible that, in the storage system, the subsequent copy commands are made to wait until the processing for the preceding copy command is completed, which may cause time-out in the host device. 
         [0006]    Furthermore, if the copy processing is performed in units of logical volumes, the logical volume with the same capacity as the copy source must be prepared as the copy destination in advance, and the storage area not storing data is also included in the copy range. Therefore, the problem arises in that the copy time becomes longer accordingly. 
         [0007]    This invention is intended, in view of the above-mentioned problems, for proposing the highly-reliable storage system and its control method capable of accelerating the processing speed of the copy processing seen from the host device. 
       Solution to Problem 
       [0008]    For solving the above-mentioned problem, this invention proposes a storage system comprising a memory set with a command queue for temporarily retaining a command from a host device, and a control unit for storing the command issued from the host device in the command queue, and executing the command stored in the command queue in the order that the command was stored in the command queue, wherein the control unit sets a copy queue for temporarily retaining a copy command among the commands issued from the host device in the memory, moves the copy command among the commands from the host device stored in the command queue to the copy queue and sends an execution completion reply of copy processing according to the command to the host device as a sender of the command, and executes the copy command that was moved to the copy queue in the background in the order that the copy command was stored in the copy queue. 
         [0009]    Furthermore, this invention additionally proposes a control method of a storage system which stores a command issued from a host device in a command queue and executes the command stored in the command queue in the order that the command was stored in command queue comprising a first step of setting a copy queue for temporarily retaining a copy command among the commands issued from the host device in the memory, a second step of moving the copy command among the commands from the host device stored in the command queue to the copy queue and sending an execution completion reply of copy processing according to the command to the host device as a sender of the command, and a third step of executing the copy command that was moved to the copy queue in the background in the order that the copy command was stored in the copy queue. 
       Temporarily Retaining 
     Advantageous Effects of Invention 
       [0010]    According to this invention, if the host device sequentially issues multiple copy commands for copying a large amount of data to the same address, the host device is not required to wait for the completion of each copy command, which prevents the occurrence of time-out. Therefore, the highly-reliable storage system and its control method capable of accelerating the processing speed of the copy processing seen from the host device can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a block diagram showing the entire configuration of the computing system by this embodiment. 
           [0012]      FIG. 2  is a conceptual diagram explaining the internal processing overview of the storage system receiving copy commands. 
           [0013]      FIG. 3  is a table explaining copy queues. 
           [0014]      FIG. 4  is a table explaining copy command status. 
           [0015]      FIG. 5  is a conceptual diagram explaining a hash table. 
           [0016]      FIG. 6  is a conceptual diagram explaining a basic pattern of read/write operations. 
           [0017]      FIG. 7  is a table explaining a basic pattern of read/write operations. 
           [0018]      FIG. 8  is a conceptual diagram explaining a basic pattern of read/write operations. 
           [0019]      FIG. 9  is a conceptual diagram explaining a basic pattern of read/write operations. 
           [0020]      FIG. 10  is a conceptual diagram explaining a basic pattern of read/write operations. 
           [0021]      FIG. 11  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0022]      FIG. 12  is a table explaining an applied pattern of read/write operations. 
           [0023]      FIG. 13  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0024]      FIG. 14  is a table explaining an applied pattern of read/write operations. 
           [0025]      FIG. 15  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0026]      FIG. 16  is a table explaining an applied pattern of read/write operations. 
           [0027]      FIG. 17  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0028]      FIG. 18  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0029]      FIG. 19  is a conceptual diagram explaining an applied pattern of read/write operations. 
           [0030]      FIG. 20  is a flowchart showing the processing routine of the read processing. 
           [0031]      FIG. 21  is a flowchart showing the processing routine of the hash search processing. 
           [0032]      FIG. 22  is a conceptual diagram explaining the hash search processing. 
           [0033]      FIG. 23  is a flowchart showing the processing routine of the data read processing. 
           [0034]      FIG. 24  is a flowchart showing the processing routine of the write processing. 
           [0035]      FIG. 25A  is a flowchart showing the processing routine of the data write processing. 
           [0036]      FIG. 25B  is a flowchart showing the processing routine of the data write processing. 
           [0037]      FIG. 26  is a flowchart showing the processing routine of the copy command processing. 
           [0038]      FIG. 27  is a flowchart showing the processing routine of the hash table registration processing. 
       
    
    
     REFERENCE SIGNS LIST 
       [0039]      1 : a computing system,  2 : a host device,  4 : a storage system,  10 : a disk device,  12 A,  12 B: controllers,  22 A,  22 B: CPUs,  23 A,  23 B: cache memories,  30 : a command queue,  31 : a copy queue,  32 : a difference bitmap,  40 : a hash table,  41 : an entry,  42 : a bitmap, COM 1 , COM 10 , COM 11 , COM 20 , COM 21 , COM 30 , COM 31 , COM 40 , COM 41 , COM 50 , COM 51 , COM 60 , COM 61 : copy commands, AR 1 , AR 10 , AR 12 , AR 20 , AR 22 , AR 30 , AR 32 , AR 40 , AR 42 , AR 50 , AR 52 , AR 60 , AR 62 : copy source areas, AR 2 , AR 11 , AR 13 , AR 21 , AR 23 , AR 31 , AR 33 , AR 41 , AR 43 , AR 51 , AR 53 , AR 61 , AR 63 : copy destination areas. 
       DESCRIPTION OF EMBODIMENTS 
       [0040]    An embodiment of this invention is fully described below with reference to the attached figures. 
       (1) The Configuration of the Computing System by this Embodiment 
       [0041]    In  FIG. 1 ,  1  shows the entire computing system by this embodiment. This computing system  1  is configured by multiple host devices  2  connected to the storage system  4  via the network  3 . 
         [0042]    The host device  2  is a computing device including information processing resources such as a CPU (Central Processing Unit), memories and others configured of, for example, a personal computer, a work station, a mainframe, and others. The host device  2  also includes information input devices such as a keyboard switch, a pointing device, a microphone, and others (not shown in the figure), and information output devices such as a monitor display, a speaker, and others (not shown in the figure). 
         [0043]    The network  3  is, for example, configured of LAN (Local Area Network), SAN (Storage Area Network), Internet, a private line, a public line, and others. The communication between the host device  2  and the storage system  4  is performed in accordance with the iSCSI (internet Small Computer System Interface) protocol or the TCP/IP (Transmission Control Protocol/Internet Protocol) protocol if the network  3  is LAN, or performed in accordance with the Fibre Channel protocol if the network  3  is SAN. 
         [0044]    The storage system  4  is configured of a storage unit  11  configured of multiple disk devices  10 , and the controllers  12 A,  12 B of the 0 series and the 1 series respectively controlling data input/output for these multiple disk devices  10 . 
         [0045]    The disk devices  10  are, for example, configured of expensive disks such as SCSI disks and inexpensive disks such as SATA (Serial AT Attachment) disks and optical disks. One RAID group  13  is configured of one or more disk devices  10 , one or more logical volumes are set in the physical storage areas provided by each disk device  10  configuring one RAID group  13 . The data from the client is stored in these logical volumes, in units of blocks (hereinafter referred to as logical blocks) of a specified size. 
         [0046]    To each logical volume, a unique volume number is allocated. In the case of this embodiment, data input/output is performed by making the combination of this volume number and the block number of the logical block allocated to each logical block (LBA: Logical Block Address) an address and specifying the relevant address. 
         [0047]    The controllers  12 A,  12 B respectively include higher-level interfaces  20 A,  20 B, local memories  21 A,  21 B, CPUs  22 A,  22 B, cache memories  23 A,  23 B, and drive interfaces  24 A,  24 B. 
         [0048]    The higher-level interfaces  20 A,  20 B are the interfaces for the network  3 , configured of NICs (Network Interface Cards) and others. These higher-level interfaces  20 A,  20 B send and receive write data, read data, various types of commands and others to/from the host device  2  via the network  3 . 
         [0049]    The local memories  21 A,  21 B are used for storing control programs read from the specified disk devices  10  when starting up the storage system  4  and also used as the work memories of the CPUs  22 A,  22 B. 
         [0050]    The CPUs  22 A,  22 B are the processors for responding to the read commands, the write commands, the copy commands and others sent from the host device  2  and controlling the data input/output processing for the disk devices  10 , and perform the specified control processing in accordance with the above-mentioned control programs stored in the local memories  21 A,  21 B. 
         [0051]    The cache memories  23 A,  23 B are used mainly for temporarily storing the data transferred between the higher-level interfaces  20 A,  20 B and the drive interfaces  24 A,  24 B. Furthermore, part of the cache memories  23 A,  23 B is used as the shared memories with the two controllers  12 A,  12 B of the 0 series and the 1 series. By this method, if a failure occurs in one of the controllers  12 A,  12 B, the other normal controller  12 A,  12 B is able to take over the information. 
         [0052]    The drive interfaces  24 A,  24 B are the interfaces for the disk devices  10 . These drive interfaces  24 A,  24 B, by controlling the corresponding disk devices  10  in accordance with the write commands and read commands from the host device  2  issued via the higher-level interfaces  20 A,  20 B, write/read the write data/read data in the address positions specified by the relevant write commands or read commands in the logical volumes specified by the write commands or read commands. 
       (2) Various Types of Processing in the Storage System 
       [0053]    Next, a sequence of processing performed in the storage system  4  receiving the copy commands from the host device  2  is described below. 
         [0054]    In the case of this embodiment, the storage system  4 , as shown in  FIG. 2 , includes a command queue  30  for temporarily storing the commands sent from the host device  2  in the cache memories  23 A,  23 B. The storage system  4  (CPUs  22 A,  22 B, to be more exact), stores the commands sent from the host device  2  in this command queue  30 , and performs the stored commands in order of storage in the relevant command queue  30 . 
         [0055]    Furthermore, the storage system  4 , in addition to the above-mentioned command queue  30 , includes a copy queue  31  for temporarily retaining copy commands only, in the cache memories  23 A,  23 B. 
         [0056]    This copy queue  31  is, as shown in  FIG. 3 , configured of the ID field  31 A, the copy source field  31 B, the copy destination field  31 C, and the status field  31 D. The ID field  31 A stores the IDs allocated to the corresponding copy commands (hereinafter referred to as copy command IDs). 
         [0057]    The copy source field  31 B is configured of the LUN field  31 BA, the starting address field  31 BB, and the ending address field  31 BC, the LUN field  31 BA stores the volume numbers of the logical volumes specified as the copy sources by the corresponding copy commands, and the starting address field  31 BB and the ending address field  31 BC respectively store the starting addresses and the ending addresses of the storage areas (hereinafter referred to as copy source areas) specified as the copy sources of the logical volumes. 
         [0058]    Similarly, the copy destination field  31 C is configured of the LUN field  31 CA, the starting address field  31 CB, and the ending address field  31 CC. The LUN field  31 CA stores the volume numbers of the logical volumes specified as the copy destinations by the corresponding copy commands, and the starting address field  31 CB and the ending address field  31 CC respectively store the starting addresses and the ending addresses of the storage areas (hereinafter referred to as copy destination areas) specified as the copy destinations of the logical volumes. 
         [0059]    Furthermore, the status field  31 D stores the copy status of the copy from the copy source areas to the copy destination areas. Note that the copy status is, as shown in  FIG. 4 , the “initial status” that the copy is not performed, the “background copy status” that the background copy is being performed, the “error status” that the copy cannot be performed due to the occurrence of a failure, and the “busy status” that the copy cannot be performed as the target area is used by the other processing. 
         [0060]    The storage system  4 , when receiving a command from the host device  2 , as shown in  FIG. 2 , stores this command in the command queue  30  (SP 1 ), then determines the type of this command and, if the command is a copy command, moves the relevant copy command to the copy queue  31 , and deletes the copy command from the command queue  30  (SP 2 ). 
         [0061]    Subsequently, the storage system  4  sends a response that the copy processing corresponding with the copy command is completed (hereinafter referred to as a performance completion response) to the host device  2  (SP 3 ). In this case, as the host device  2 , when receiving this copy performance completion response, recognizes that the copy corresponding with the copy command is completed, the subsequent commands for the same address are not made to wait in the relevant host device  2 . 
         [0062]    Furthermore, subsequently, the storage system  4 , in parallel with the processing in accordance with the commands stored in the command queue  30 , performs the copy processing in accordance with the copy commands stored in the copy queue  31  in order of storage in the copy queue  31  (SP 4 ). 
         [0063]    Note that the storage system  4 , when moving the copy commands to the copy queue at the above-mentioned step SP 2 , creates difference bitmaps  32  showing data difference between the copy source areas and the copy destination areas specified by the copy commands in the cache memories  23 A,  23 B. 
         [0064]    These difference bitmaps  23 A,  23 B are the respective difference bits made to correspond with the respective unit areas in the copy source areas. Therefore, for example, if the unit area is 64 kilobytes and the copy source area is 256 megabytes, a difference bitmap  32  becomes 512 bytes size. 
         [0065]    Furthermore, in the difference bitmap  32 , immediately after the copy commands are moved to the copy queue  31 , as the data respectively stored in the copy source areas and in the copy destination areas do not coincide, and therefore, all the difference bits are set to ON (“1”). 
         [0066]    The storage system  4 , each time it completes the background copy of a unit area of data in the copy source area to the copy destination area, sets the corresponding difference bit to OFF (“0”). Furthermore, the storage system  4 , when completing the copy processing in the background in accordance with one copy command stored in the copy queue  31 , deletes the entry of the copy command from the copy queue  31  and also deletes the difference bitmap  32  made to correspond with the relevant copy command from the cache memories  23 A,  23 B. 
         [0067]    Meanwhile, the host device  2 , after issuing a copy command as mentioned above, at the time when it receives the corresponding copy performance completion response, recognizes that the copy processing in accordance with the copy command is completed. Therefore, subsequently, before the copy processing in accordance with the copy command is completed, the host device  2  is able to issue read commands targeted for the copy destination areas of the copy command and write commands intending to rewrite the data stored in the copy source areas of the copy command. 
         [0068]    Therefore, for installing the above-mentioned background copy function in the storage system  4 , the storage system  4  must be configured to be able to send the data which should be stored in the copy destination areas as read data to the host device  2  after completing the copy command, even in cases where the read command is received before the completion of the copy processing in accordance with the copy command. Similarly, in cases the write command is received before the completion of the copy processing in accordance with the copy command, the storage system  4  must be configured to be able to save data stored in the copy destination areas of the copy command before performing the write command. 
         [0069]    As one of the methods for satisfying the former requirement, the method in which the storage system  4 , if receiving a read command from the host device  2 , searches the entire copy queue  31  and, if any unperformed copy command whose copy destination area partially or entirely overlaps with the storage area (hereinafter referred to as a read area) specified as the area whose data should be read by the read command exists, performs the read command after performing the copy command can be considered. 
         [0070]    Meanwhile, as one of the methods for satisfying the latter requirement, the method in which the storage system  4 , if receiving a write command from the host device  2 , searches the entire copy queue  31  and, if any unperformed copy command whose copy area partially or entirely overlaps with the storage area (hereinafter referred to as a write area) specified as the area whose data should be written by the write command exists, performs the write command after performing the copy command can be considered. 
         [0071]    However, as the above-mentioned processing for searching the entire copy queue  31  takes a fair amount of time, if such processing for searching the entire copy queue  31  is to be performed each time [the storage system  4 ] receives a read command or a write command, the response performance of the storage system  4  for read commands and write commands will deteriorate. 
         [0072]    Therefore, in the case of this embodiment, in the storage system  4 , as the means for speeding up the search processing, the hash table  40  as shown in  FIG. 5  is provided. 
         [0073]    This hash table  40  is configured of multiple slots  40 A to  40 L and, to each of these slots  40 A to  40 L, the numeral from “0” to “slot number −1” is made to correspond sequentially. 
         [0074]    The storage system  4 , when receiving a copy command, for each of the copy source areas and the copy destination areas of the copy command (hereinafter appropriately and collectively referred to as copy command target areas), an entry  41  of the hash table  40  is created as shown in  FIG. 5 . 
         [0075]    This entry  41  is created, for example, in units of areas delimited by each 1-megabyte border. Therefore, for example, if 1 megabyte is specified to be 1024 kilobytes and 1 address to be 1 kilobyte and the addresses of the target areas specified by the copy command are “1000” to “1200,” one entry  41  is created for the target areas whose addresses are “1000” to “1023,” and another entry  41  is created for the target areas whose addresses are “1024” to “1200.” Hereinafter, sequentially delimiting the target areas specified by the copy command by each 1-megabyte border of the address as mentioned above is referred to as “rounding off the target areas by each 1-megabyte border” 
         [0076]    An entry  41  is, as clearly shown in  FIG. 5 , configured of the copy command ID field  41 A, the bitmap field  41 B, the address attribute field  41 C, and the pointer field  41 D. 
         [0077]    The copy command field  41 A stores the copy command ID corresponding with the copy command, and the bitmap field  41 B stores the bitmap  42  configured of multiple difference bits respectively made to correspond with respective delimited areas created by delimiting the corresponding 1-megabyte target area in units of the above-mentioned unit areas (e.g. 64-kilobyte areas). 
         [0078]    Meanwhile, the address attribute field  41 C stores the attribute information showing whether the target area made to correspond with the entry  41  is the copy source area or the copy destination area, and the pointer field  41 D, if the next entry  41  for the same copy command exists, stores the pointer to the entry  41  (the starting address of the area storing the data of the entry  41  in the cache memories  23 A,  23 B, hereinafter referred to as the hash pointer). 
         [0079]    The storage system  4  creates one or more entries  41  corresponding with the target areas (the copy source area and the copy destination area) of the received copy command for each target area and, for each of these target areas, divides each starting address by the number of slots in the hash table  40 , and makes the correspondence of the slots made to correspond with the remainder value (hereinafter referred to as the target slot)  40 A to  40 K or  40 L with the entries  41  of the copy command. 
         [0080]    As more specifically described, the storage system  4 , if no entries of the other copy command are made to correspond with the target slots  40 A to  40 K or  40 L in accordance with the target areas of the copy command, stores the hash pointer for the first entry  41  of the relevant target area in the target slots  40 A to  40 K or  40 L or, if the correspondence is already made, stores the hash pointer for the first entry  41  of the relevant target area in a pointer field  41 D of the last entry  41  corresponding with the target slots  40 A to  40 K or  40 L. 
         [0081]    The storage system  4 , for the bitmap  42  stored in the bitmap field  41 B of the first entry  41 , sets each difference bit corresponding with the above-mentioned delimited area included in the target area of the copy command to ON (“1”). 
         [0082]    Furthermore, the storage system  4 , as for next and subsequent entries  41  of the target areas, stores the hash pointer of each entry  41  in the pointer field  41 D in the prior entry  41  and also, as for the bitmaps  42  stored in the bitmap field  41 B of the next and subsequent entries  41 , sets each difference bit corresponding with the delimited area included in the target area to ON (“1”). 
         [0083]    The above-mentioned correspondence of the entries  41  in the copy commands with the slots  40 A to  40 L in the hash table  40  is made when registering the copy commands to the copy queue  31 . Then, the respective entries  41  made to correspond with the slots  40 A to  40 L are deleted after the copy in accordance with the corresponding copy command is completed. 
         [0084]    As mentioned above, the storage system  4 , when receiving a read command or write command from the host device  2 , firstly divides the starting address of the read area specified by the read command or the write area specified by the write command by the number of slots in the hash table  40 , refers to the bitmaps  42  respectively stored in the bitmap field  42  of each of the entry  41  made to correspond with the slots  40 A to  40 L which were made to correspond with the remainder value of that time, and determines whether there are any copy commands whose copy destination areas are, as mentioned above, part of or the entire read area or write area specified by the read command or write command received at that time and which are not yet performed. 
       (3) Flow of Read Processing or Write Processing in the Storage System 
     (3-1) Basic Operation Patterns 
       [0085]    Next, as shown in  FIG. 6 , an operation pattern of the storage system  4  in the case, if a copy command COM 1  for copying data from the copy source area AR 1  in a logical volume VOL 1  to the copy destination area AR 2  in another logical volume VOL 2  is stored in the copy queue  31 , the storage system  4  performs the read command or write command whose read area or write area partially or entirely overlaps with the copy source area AR 1  or the copy destination area AR 2  of the copy command COM 1  is described below. 
         [0086]      FIG. 7  shows the concrete contents of the read processing and the write processing, under the above-mentioned condition for  FIG. 6 , performed by the storage system  4  for, among the read area or the write area specified by the read command or the write command, the areas overlapping with the copy source area AR 1  and the copy destination area AR 2  of the copy command COM 1 . Note that, among the read areas or the write areas, for the areas not overlapping with the copy source area AR 1  or the copy destination area AR 2 , the normal read processing or write processing is performed. 
         [0087]    As shown in  FIG. 7 , the storage system  4 , if it is going to perform a read command and the read area specified by the read command partially or entirely overlaps with the copy source area AR 1  of the copy command COM 1 , regardless of whether the copy in accordance with the copy command COM 1  is not yet performed or already performed for the overlapping area of the read area, reads the data normally (refer to the “Copy source” field in the row whose “#” is “1” or “2” in  FIG. 7 ). The same processing is to be performed if the read area specified by the read command partially or entirely overlaps with the copy destination area AR 2  of the copy command COM 1  (refer to the “Copy destination” field in the row whose “#” is “2” in  FIG. 7 ). 
         [0088]    Meanwhile, the storage system  4 , if it is going to perform a read command and the read area specified by the read command partially or entirely overlaps with the copy destination area AR 2  of the copy command COM 1  and the copy in accordance with the copy command COM 1  is not performed for the overlapping area, for the overlapping area of the read area, after performing the copy in accordance with the copy command COM 1 , reads the data from the relevant overlapping area (refer to the “Copy destination” field in the row whose “#” is “1” in  FIG. 7 ). 
         [0089]    As more specifically described, the storage system  4 , as shown in  FIG. 8 , firstly reads the data stored in the area overlapping with the read area of the copy source area AR 1  of the copy command COM 1  to the read sides  23 AR,  23 BR of the cache memories  23 A,  23 B (SP 10 ). The storage system  4 , subsequently, copies this data to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B (SP 11 ). 
         [0090]    Next, the storage system  4  sends the data copied to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B as read data to the host device  2  (SP 12 ), writes the relevant read data to the copy destination area AR 2  of the copy command COM 1  and, in accordance with this, changes the difference bits corresponding with the areas which performed the copy corresponding with the copy command COM 1  in the difference bitmap  32  ( FIG. 2 ) from ON to OFF (SP 13 ). 
         [0091]    Meanwhile, the storage system  4 , if it is going to perform a write command and the write area specified by the relevant write command partially or entirely overlaps with the copy source area AR 1  of the copy command COM 1  and the copy in accordance with the copy command COM 1  is not performed for the overlapping area, for the overlapping area of the write area, saves the data stored in the relevant overlapping area to the corresponding copy destination area AR 2  specified by the copy command COM 1 , and then writes the write data (refer to the “Copy source” field in the row whose “#” is “3” in  FIG. 7 ). 
         [0092]    As more specifically described, the storage system  4 , as shown in  FIG. 9 , firstly reads the data stored in the area overlapping with the write area of the copy source area AR 1  of the copy command COM 1  to the read sides  23 AR,  23 BR of the cache memories  23 A,  23 B (SP 20 ). The storage system  4 , subsequently, copies this data to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B (SP 21 ), and copies the data copied to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B to the copy destination area AR 2  of the copy command COM 1  (SP 22 ). 
         [0093]    Next, the storage system  4  receives the write data provided by the host device  2  and writes it to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B (SP 23 ). The storage system  4  then reads this write data from the cache memories  23 A,  23 B, and writes it to the write area specified by the write command. The storage system  4 , for the difference bitmap  32  ( FIG. 2 ) corresponding with the copy command COM 1 , changes the difference bits corresponding with the areas to which the data was copied at step SP 22  from ON to OFF (SP 24 ). 
         [0094]    Meanwhile, the storage system  4 , if it is going to perform a write command and the write area specified by the write command partially or entirely overlaps with the copy destination area AR 2  of the copy command COM 1  and the copy in accordance with the copy command COM 1  is not performed for the overlapping area, for the overlapping area of the write area, copies the data stored in the relevant overlapping area to the copy destination area AR 2  of the relevant copy command, and then writes the write data (refer to the “Copy destination” field in the row whose “#” is “3” in  FIG. 7 ). 
         [0095]    As more specifically described, the storage system  4 , as shown in  FIG. 10 , firstly reads the data from the area overlapping with the write area of the copy source area AR 1  of the copy command COM 1  to the read sides  23 AR,  23 BR of the cache memories  23 A,  23 B (SP 30 ), and copies this data to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B (SP 31 ). 
         [0096]    The storage system  4  then overwrites the above-mentioned data copied to the write sides  23 AW,  23 BW of the cache memories  23 A,  23 B with the write data received from the host device  2  (SP 32 ). Subsequently, the storage system  4  writes the data overwritten with the write data to the copy destination area AR 2  of the copy command COM 1 , and changes the difference bits corresponding with the write area overlapping with the copy source area AR 1  of the relevant copy command COM 1  in the difference bitmap  32  ( FIG. 2 ) corresponding with the copy command COM 1  from ON to OFF (SP 33 ). 
         [0000]    (3-2) Operation Pattern when Two Related Copy Commands Exist 
         [0097]    Next, the operation pattern of the storage system  4  in the case, if two copy commands whose copy source areas and/or copy destination areas overlap are stored in the copy queue  31 , the storage system  4  performs the read command or write command is described below. 
         [0098]    Note that, in the description below, among the read areas or the write areas specified by the read commands or write commands, for the areas not overlapping with these two copy source areas or copy destination areas, the normal read processing or write processing is performed. 
       (3-2-1) First Applied Operation Pattern 
       [0099]      FIG. 11  shows the case where the first and second copy commands COM 10  and COM 11  whose copy source areas AR 10  and AR 12  overlap are registered in the copy queue  31 . This case assumes that they were registered to the copy queue  31  in order of the first copy command COM 10  and the second copy command COM 11 , and that these first and second copy commands COM 10  and COM 11  are performed in this order. 
         [0100]      FIG. 12  shows the concrete contents of the read processing and the write processing performed by the storage system  4  in the above-mentioned case where the storage system  4  performs a read command or write command whose read area or write area partially or entirely overlaps with the copy source areas AR 10 , AR 12  or the copy destination areas AR 11 , AR 13  of the first and/or second copy commands COM 10 , COM 11 , for the areas among the read area or write area overlapping with the copy source areas AR 10 , AR 12  or the copy destination areas AR 11 , AR 13 . 
         [0101]    In  FIG. 12 , the “Copy source (with overlapping)” column shows the contents of the processing by the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time partially or entirely overlaps with the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11 . 
         [0102]    Meanwhile, the “Copy source (without overlapping)” column shows the contents of the processing by the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time does not overlap with the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  (but overlaps with the copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  other than the relevant overlapping area). 
         [0103]    Furthermore, the “Copy destination” column shows the contents of the processing by the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time overlaps with the copy destination areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11 . 
         [0104]    The storage system  4 , if it performs a read command and the read area specified by the relevant read command partially or entirely overlaps with the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11 , for the area of the read area overlapping with the relevant overlapping part, regardless of whether the copy in accordance with the copy commands COM 10 , COM 11  is not yet performed or already performed, reads the data normally (refer to the “Copy source (with overlapping)” field in the row whose “#” is “1” in  FIG. 12 ). 
         [0105]    The storage system  4 , if the read area specified by the read command does not overlap with the overlapping parts of the copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11 , for the area of the read area overlapping with the copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11 , regardless of whether the copy in accordance with the copy commands COM 10 , COM 11  is not yet performed or already performed, reads the data normally (refer to the “Copy source (without overlapping)” field in the row whose “#” is “1” and “2” in  FIG. 12 ). 
         [0106]    Meanwhile, the storage system  4 , if the read area specified by the read command partially or entirely overlaps with either one of the copy destination areas AR 11 , AR 13  of the first or second copy commands COM 10 , COM 11  and the copy in accordance with the corresponding first or second copy command COM 10 , COM 11  is not performed for the overlapping area, as explained with reference to  FIG. 8 , for the area overlapping with the copy destination areas AR 11 , AR 13  of the first or second copy command COM 10 , COM 11  of the read area, after performing the copy in accordance with the relevant first or second copy command COM 10 , COM 11 , reads the data. Furthermore, the storage system  4 , along with this processing, changes the difference bits corresponding with the areas to which the data was copied as mentioned above in the difference bitmap  32  ( FIG. 2 ) corresponding with the first or second copy command COM 10 , COM 11  from ON to OFF (refer to the “Copy destination” field in the row whose “#” is “1” in  FIG. 12 ). 
         [0107]    The storage system  4 , if the read area specified by the read command partially or entirely overlaps with the copy destination areas AR 11 , AR 13  of either one of the first and second copy commands COM 10 , COM 11  and the corresponding first or second copy command COM 10 , COM 11  for the overlapping area is already performed, for the relevant overlapping area, reads the data normally (refer to the “Copy destination” field in the row whose “#” is “2” in  FIG. 12 ). 
         [0108]    Meanwhile, the storage system  4 , if it performs a write command and the write area specified by the relevant write command partially or entirely overlaps with the overlapping parts of the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  and the copy for the overlapping area (for the area of the read area overlapping with the relevant overlapping part) in accordance with the first and second copy commands COM 10 , COM 11  is not yet performed, for the relevant overlapping area, saves the data to the copy destination areas AR 11 , AR 13  of the respectively corresponding first or second copy command COM 10 , COM 11 . 
         [0109]    Furthermore, the storage system  4 , along with this processing, changes the difference bits corresponding with the areas where data was saved at that time in the difference bitmap  32  respectively corresponding with the first and second copy commands COM 10 , COM 11  from ON to OFF, and then writes data to the write area specified by the write command (refer to the “Copy source (with overlapping)” field in the row whose “#” is “3” in  FIG. 12 ). 
         [0110]    Furthermore, the storage system  4 , if the write area specified by the write command partially or entirely overlaps with the overlapping parts in the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  and the copy in accordance with the first and second copy commands COM 10 , COM 11  are already performed for the overlapping area (the area overlapping with the overlapping part in the read area), for the relevant overlapping area, writes the write data normally (refer to the “Copy source (with overlapping)” field in the row whose “#” is “4” in  FIG. 12 ). 
         [0111]    Furthermore, the storage system  4 , if the write area specified by the write command does not overlap with the overlapping parts of these two copy source areas AR 10 , AR 12  of the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  (but partly overlaps with the copy source areas AR 10 , AR 12  of the first or second copy commands COM 10 , COM 11  other than the relevant overlapping part) and the copy in accordance with the corresponding first and second copy commands COM 10 , COM 11  is not yet performed for the areas overlapping with the copy source areas AR 10 , AR 12  of the write area, for the overlapping areas, as explained with reference to  FIG. 9 , saves the data stored in the copy source areas AR 10 , AR 12  of the first or second copy command COM 10 , COM 11  to the copy destination areas of the relevant first or second copy command COM 10 , COM 11 , and then writes the write data (refer to the “Copy source (without overlapping)” field in the row whose “#” is “3” in  FIG. 12 ). 
         [0112]    The storage system  4 , if the write area specified by the write command does not overlap with the overlapping parts of these two copy source areas AR 10 , AR 12  of the respective copy source areas AR 10 , AR 12  of the first and second copy commands COM 10 , COM 11  (but partly overlaps with the copy source areas AR 10 , AR 12  of the first or second copy command COM 10 , COM 11  other than the relevant overlapping part) and the copy in accordance with the corresponding first or second copy command COM 10 , COM 11  is already performed for the areas overlapping with the copy source areas AR 10 , AR 12  of the write area, for the overlapping areas, writes the write data normally (refer to the “Copy source (without overlapping)” field in the row whose “#” is “4” in  FIG. 12 ). 
         [0113]    Meanwhile, the storage system  4 , if the write area specified by the write command partially or entirely overlaps with the copy destination areas AR 11 , AR 13  of the first or second copy command COM 10 , COM 11  and the copy for the overlapping area in accordance with the corresponding first or second copy command COM 10 , COM 11  is not yet performed, for the overlapping area, copies the data stored in the relevant overlapping areas to the copy destination areas AR 11 , AR 13  of the relevant first or second copy command COM 10 , COM 11 , and then writes the write data (refer to the “Copy destination” field in the row whose “#” is “3” in  FIG. 12 ). 
         [0114]    Furthermore, the storage system  4 , if the write area specified by the write command partially or entirely overlaps with the copy destination areas AR 11 , AR 13  of the first or second copy command COM 10 , COM 11  and the copy for the overlapping area in accordance with the corresponding first or second copy command COM 10 , COM 11  is already performed, for the relevant overlapping area, writes the write data normally (refer to the “Copy destination” field in the row whose “#” is “4” in  FIG. 12 ). 
       (3-2-2) Second Applied Operation Pattern 
       [0115]    Next, as shown in  FIG. 13 , the case where the first and second copy commands COM 20 , COM 21  (“Fast1” and “Fast2”) whose copy destination areas AR 21 , AR 23  partially or entirely overlap are registered to the copy queue  31  is described below. 
         [0116]    Note that the storage system  4 , when performing the first copy command COM 20 , searches whether there is any area in the copy destination area AR 21  of the first copy command COM 20  overlapping with the copy destination area AR 23  of the second copy command COM 23  and, if there is, after performing the first copy command COM 20 , sets the difference bit for the relevant overlapping part in the difference bitmap  32  corresponding with the second copy command COM 21  to OFF. 
         [0117]      FIG. 14  shows the concrete contents of the read processing and the write processing performed by the storage system  4  in the above-mentioned case where the storage system  4  receives a read command or write command whose read area or write area partially or entirely overlaps with the copy source areas AR 20 , AR 22  or the copy destination areas AR 21 , AR 23  of the first and/or second copy command COM 20 , COM 21 . 
         [0118]    In  FIG. 14 , the “Copy source” column shows the contents of the operation of the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time partially or entirely overlaps with the copy destination areas AR 20 , AR 21  of the first or second copy command COM 20 . 
         [0119]    Meanwhile, the “Copy destination (with overlapping)” column in  FIG. 14  shows the contents of the operation of the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time partially or entirely overlaps with the overlapping parts of the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21 . 
         [0120]    Furthermore, the “Copy destination (without overlapping)” column in  FIG. 14  shows the contents of the operation of the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time does not overlap with the overlapping parts of the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  (but overlaps with the copy source areas AR 20 , AR 22  of the first or second copy command COM 20 , COM 21  other than the relevant overlapping part). 
         [0121]    The storage system  4 , if it is going to perform a read command and the read area specified by the relevant read command partially or entirely overlaps with either one of the copy source areas AR 20 , AR 22  of the first and second copy commands COM 20 , COM 21 , regardless of whether the copy in accordance with the first and second copy commands COM 20 , COM 21  is not yet performed or already performed for the overlapping area, for the relevant overlapping area, reads the data normally (refer to the “Copy source” field in the row whose “#” is “1” and “2” in  FIG. 12 ). 
         [0122]    The storage system  4 , if the read area specified by the read command partially or entirely overlaps with the overlapping parts in the copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  and the copy in accordance with the copy commands COM 20 , COM 21  is not yet performed or already performed for the overlapping area (the area overlapping with the overlapping part of the read area), for the relevant overlapping area, completes the copy processing with reference to the second copy command COM 21 , and then reads the data. Furthermore, the storage system  4 , along with this processing, changes the difference bit for the area where the copy was performed as mentioned above in the difference bitmap  32  corresponding with the first and second copy commands COM 20 , COM 21  from ON to OFF (refer to the “Copy destination (with overlapping)” field in the row whose “#” is “1” in  FIG. 14 ). 
         [0123]    Furthermore, the storage system  4 , if the read area specified by the read command partially or entirely overlaps with the overlapping parts in the copy source areas AR 20 , AR 21  of the first and second copy commands COM 20 , COM 21  and the copy in accordance with the first and second copy commands COM 20 , COM 21  is already performed for the overlapping part (the area overlapping with the corresponding overlapping part of the read area), for the relevant overlapping area, reads the data normally (refer to the “Copy destination (with overlapping)” field in the row whose “#” is “2” in  FIG. 14 ). 
         [0124]    Meanwhile, the storage system  4 , if the read area specified by the read command does not overlap with the overlapping parts of the copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  (but partly overlaps with the copy source areas AR 20 , AR 22  of the first or second copy command COM 20 , COM 21  other than the relevant overlapping part) and the copy in accordance with the corresponding first or second copy command COM 20 , COM 21  is not yet performed for the areas overlapping with the copy destination areas AR 21 , AR 23  of the read area, for the overlapping areas, as explained with reference to  FIG. 8 , performs the first and second copy commands COM 20 , COM 21 , and then reads the data. 
         [0125]    Furthermore, the storage system  4 , from the difference bitmap  32  ( FIG. 2 ) corresponding with the first or second copy command COM 20 , COM 21 , changes the difference bit corresponding with the area where the copy was performed at that time from ON to OFF (refer to the “Copy destination (without overlapping)” field in the row whose “#” is “1” in  FIG. 14 ). 
         [0126]    Meanwhile, the storage system  4 , if the read area specified by the read command does not overlap with the overlapping parts of the copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20  (but partly overlaps with the copy source areas AR 20 , AR 22  of the first or second copy command COM 20 , COM 21  other than the relevant overlapping part) and the copy in accordance with the corresponding first and second copy commands COM 20 , COM 21  is already performed for the areas overlapping with the copy destination areas AR 21 , AR 23  of the read area, for the overlapping areas, reads the data normally (refer to the “Copy destination (without overlapping)” field in the row whose “#” is “2” in  FIG. 14 ). 
         [0127]    Meanwhile, the storage system  4 , if it performs a write command and the write area specified by the relevant write command partially or entirely overlaps with the copy source areas AR 20 , AR 22  of the first or second copy command COM 20 , COM 21  and the copy in accordance with the first or second copy command COM 20 , COM 21  is not yet performed for the overlapping area, as explained with reference to  FIG. 9 , for the overlapping area, saves the data stored in the relevant overlapping area to the copy destination areas AR 21 , AR 23  of the first or second copy command COM 20 , COM 21 , and then writes the write data normally (refer to the “Copy source” field in the row whose “#” is “3” in  FIG. 14 ). 
         [0128]    Meanwhile, the storage system  4 , even if the write area specified by the write command partially or entirely overlaps with either one of the copy source areas AR 20 , AR 21  of the first and second copy commands COM 20 , COM 21 , if the copy in accordance with the first or second copy command COM 20 , COM 21  is already performed for the overlapping area, for the overlapping area, writes the write data normally (refer to the “Copy source” field in the row whose “#” is “4” in  FIG. 14 ). 
         [0129]    Furthermore, the storage system  4 , if the write area specified by the write command partially or entirely overlaps with the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  and the copy in accordance with the first and second copy commands COM 20 , COM 21  is not yet performed for the overlapping area (the area overlapping with the overlapping part of the write area), for the overlapping area, copies the data stored in the copy source area AR 22  of the second copy command COM 21 , and writes the write data. Furthermore, the storage system  4  sets each difference bit corresponding with the area where the relevant copy was performed in the difference bitmap  32  corresponding with each of the first and second copy commands COM 20 , COM 21  to OFF (refer to the “Copy destination (with overlapping)” field in the row whose “#” is “3” in  FIG. 14 ). 
         [0130]    Meanwhile, the storage system  4 , if the write area specified by the write command partially or entirely overlaps with the overlapping parts of the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  and the copy in accordance with the first and second copy commands COM 20 , COM 21  for the overlapping area (the area overlapping with the overlapping part of the write area) is already performed, for the overlapping area, writes the write data normally (refer to the “Copy destination (with overlapping)” field in the row whose “#” is “4” in  FIG. 14 ). 
         [0131]    Furthermore, the storage system  4 , if the write area specified by the write command does not overlap with the overlapping parts of the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  (but overlaps with the copy destination areas AR 21 , AR 23  of the first or second copy command COM 20 , COM 21  other than the relevant overlapping part) and the copy in accordance with the corresponding first or second copy command COM 20 , COM 21  is not yet performed for the areas overlapping with the copy destination areas AR 21 , AR 23  of the write area, for the overlapping areas, as explained with reference to  FIG. 10 , copies the data stored in the copy source areas AR 20 , AR 22  of the first or second copy command COM 20 , COM 21 , and then writes the write data (refer to the “Copy destination (without overlapping)” field in the row whose “#” is “3” in  FIG. 14 ). 
         [0132]    Meanwhile, the storage system  4 , if the write area specified by the write command from the host device  2  does not overlap with the overlapping parts of the copy destination areas AR 21 , AR 23  of the respective copy destination areas AR 21 , AR 23  of the first and second copy commands COM 20 , COM 21  (but overlaps with the copy destination areas AR 21 , AR 23  of the first or second copy command COM 20 , COM 21  other than the relevant overlapping part) and the corresponding first or second copy command COM 20 , COM 21  is already performed for the area overlapping with the copy destination areas AR 21 , AR 23  of the write area, for the overlapping area, writes the write data normally (refer to the “Copy destination (without overlapping)” field in the row whose “#” is “4” in  FIG. 14 ). 
       (3-2-3) Third Operation Pattern 
       [0133]    Next, as shown in  FIG. 15 , the case where the first and second copy commands COM 30 , COM 31  (“Fast1” and “Fast2”) whose copy source areas AR 30 , AR 32  partially or entirely overlap with the copy destination areas AR 31 , AR 33  are registered to the copy queue  31  is described below. 
         [0134]    In  FIG. 15 , the copy source area AR 30  of the first copy command COM 30  is the area of addresses “100” to “150” in the first logical volume VOL 30 , the copy destination area AR 31  of the relevant first copy command COM 30  is the area of addresses “10” to “60” in the second logical volume VOL 31 , the copy source area AR 32  of the second copy command COM 31  is the area of addresses “120” to “220” in the first logical volume VOL 30 , and the copy destination area AR 33  of the relevant second copy command COM 31  is the area of addresses “30” to “130” in the second logical volume VOL 31 . This case also assumes that the first and the second copy commands COM 30  and COM 31  were registered to the copy queue  31  in this order. 
         [0135]      FIG. 16  shows the concrete contents of the read processing and the write processing performed by the relevant storage system  4  in the above-mentioned case where the storage system  4  receives a read command or write command whose read area or write area partially or entirely overlaps with the copy source areas AR 30 , AR 32  or the copy destination areas AR 31 , AR 33  of the first and/or second copy command COM 30 , COM 31 . 
         [0136]    In  FIG. 16 , the “Copy source (with overlapping)” column and the “Copy destination (with overlapping)” column show the contents of the operation of the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time includes the overlapping parts of the copy source areas AR 30 , AR 32  or the copy destination areas AR 31 , AR 33  of the first and second copy commands COM 30 , COM 31 . 
         [0137]    Meanwhile, in  FIG. 16 , the “Copy source (without overlapping)” column and the “Copy destination (without overlapping)” column show the contents of the operation of the storage system  4  in the case the read area or write area specified by the read command or write command to be performed at that time does not include any overlapping parts of the copy source areas AR 30 , AR 32  of the first and second copy commands COM 30 , COM 31  or any overlapping parts of the copy destination areas AR 30 , AR 32  of the first and second copy commands COM 30 , COM 31 . 
         [0138]    As clearly shown in  FIG. 16 , if the storage system  4  performs a read command or write command and the read area or write area specified by the relevant read command or write command partially or entirely overlaps with the overlapping parts of the respective copy source areas AR 30 , AR 32  of the first and second copy commands COM 30 , COM 31 , performs the same processing as mentioned above with reference to each of the “Copy source (with overlapping)” fields whose “#” are “1” to “4” in  FIG. 12  (refer to the “Copy source (with overlapping)” fields in the rows whose “#” are “1” to “4” in  FIG. 16 ). 
         [0139]    Meanwhile, the storage system  4 , for the cases other than those described above, performs the same processing as mentioned above with reference to  FIG. 14  (refer to each of the “Copy source (with overlapping)” fields, each of the “Copy destination (with overlapping)” fields, and each of the “Copy destination (without overlapping)” fields in the rows whose “#” are “1” to “4” in  FIG. 16 ). 
       (3-2-4) Fourth Operation Pattern 
       [0140]    As shown in  FIG. 17 , in the status the first copy command COM 40  is registered to the copy queue  31 , if the second copy command COM 41  whose copy source area AR 42  partially or entirely overlaps with the copy destination area AR 41  of the first copy command  40  and whose copy destination area AR 43  partially or entirely overlaps with the copy source area AR 40  of the first copy command COM 40  is provided from the host device  2 , the storage system  4  rejects the second copy command COM 41 . 
         [0141]    As mentioned above, the storage system  4 , by rejecting the second copy command COM 41 , prevents the copy processing from entering the loop status. 
       (3-2-5) Fifth Operation Pattern 
       [0142]    As shown in  FIG. 18 , in the status the first copy command COM 50  is registered to the copy queue  31 , if the second copy command COM 51  whose copy source area AR 52  partially or entirely overlaps with the copy destination area AR 51  of the first copy command COM 50  is provided from the host device  2 , the storage system  4  rejects the second copy command COM 51 . 
         [0143]    As mentioned above, the storage system  4 , by rejecting the second copy command COM 51 , prevents the multistage processing. 
       (3-2-6) Sixth Operation Pattern 
       [0144]    As shown in  FIG. 19 , in the status the first copy command COM 60  is registered to the copy queue  31 , if the second copy command COM 61  whose copy destination area AR 63  partially or entirely overlaps with the copy source area AR 60  of the first copy command COM 60  is provided from the host device  2 , the storage system  4  also rejects the second copy command COM 61 . 
         [0145]    As mentioned above, the storage system  4 , by rejecting the second copy command COM 61 , prevents the copy processing from entering the loop status. 
       (4) Concrete Contents of Processing by CPU of Storage System 
       [0146]    Next, the concrete contents of the read processing and the write processing performed by the CPUs  22 A,  22 B ( FIG. 1 ) of the relevant storage system  4  for the storage system  4  to perform the above-mentioned first to sixth patterns of processing are described below. 
       (4-1) Various Types of Processing by CPU Related to Read Processing 
     (4-1-1) Read Processing 
       [0147]      FIG. 20  shows the procedure of the read processing performed by the CPUs  22 A,  22 B of the storage system  4  when processing a read command stored in the command queue  30 . 
         [0148]    The CPUs  22 A,  22 B, when starting this read processing, firstly make the starting address of the read area specified by the read command and the length of the read area specified by the relevant read command into an argument, and determine whether the relevant read area partially or entirely overlaps with the copy source area or the copy destination area of any copy command registered to the copy queue  31  or not, by referring to the hash table  40  ( FIG. 5 ) (SP 40 ). 
         [0149]    Next, the CPUs  22 A,  22 B, with reference to the result of the determination obtained at step SP 40 , control the disk devices  10  via the drive interfaces  24 A,  24 B, read the data stored or which should be stored in the read area specified by the read command from the disk devices  10 , send the read data to the host device  2  (SP 41 ), and complete this read processing. 
       (4-1-2) Hash Search Processing 
       [0150]      FIG. 21  shows the concrete contents of the hash search processing performed by the CPUs  22 A,  22 B at step SP 40  of the read processing. 
         [0151]    The CPUs  22 A,  22 B, when proceeding to step SP 40  of the read processing, start this hash search processing, firstly by resetting (set to OFF) the copy source overlap flag and the copy destination overlap flag maintained in the specified positions of the cache memories  23 A,  23 B (SP 50 , SP 51 ). The copy source overlap flag shows that the read area specified by the read command partially or entirely overlaps with the copy source area of any copy command registered to the copy queue  31 , and the copy destination overlap flag shows that the read area partially or entirely overlaps with the copy destination area of any copy command registered to the copy queue  31 . These copy source overlap flag and the copy destination overlap flag are, as described later, used in the subsequent processing. 
         [0152]    Next, the CPUs  22 A,  22 B round off the starting address of the read area specified by the read command by the 1-megabyte border and, by the remainder from the division of the result value by the number of slots in the hash table  40  ( FIG. 5 ), detect the corresponding slots  40 A to  40 L ( FIG. 5 ) in the hash table  40  (SP 52 ). 
         [0153]    Next, the CPUs  22 A,  22 B determine whether “Null” showing that there is no pointer for the next entry is stored in the slots  40 A to  40 L or not (SP 53 ). 
         [0154]    At this point, obtaining the affirmative result to this determination means that the copy command whose copy source area or copy destination area partially or entirely overlaps with the read area specified by the read command is not stored in the copy queue  31 . Therefore, at this point, the CPUs  22 A,  22 B complete this hash search processing and return to the read processing. 
         [0155]    Meanwhile, obtaining the negative result to this determination means that the copy command whose copy source area or copy destination area partially or entirely overlaps with the read area specified by the read command is stored in the copy queue  31 . Therefore, at this point, the CPUs  22 A,  22 B access the entry  41  ( FIG. 5 ) specified by the pointer stored in the slots  40 A to  40 L (SP 54 ). 
         [0156]    Next, the CPUs  22 A,  22 B obtain the bitmap  42  ( FIG. 5 ) stored in the bitmap field  41 B ( FIG. 5 ) of the entry  41  accessed at step SP 54  and, with reference to the obtained bitmap  42 , calculate the starting address and the ending address of the storage area specified as the copy source area or the copy destination area (target area) corresponding with the entry  41  in the copy command corresponding with the entry  41  (SP 55 ). 
         [0157]    Next, the CPUs  22 A,  22 B determine whether the starting address of the read area specified by the read command is smaller than the ending address of the target area of the copy command and, at the same time, whether the ending address of the read area is larger than the starting address of the target area of the copy command or not (SP 56 ). 
         [0158]    Obtaining the negative result to this determination means that, as shown in  FIG. 22(A)  or (D), the read area and the target area of the copy command do not overlap either partially or entirely. Therefore, at this point, the CPUs  22 A,  22 B proceed to step SP 61 . 
         [0159]    On the other hand, obtaining the affirmative result to this determination at step SP  56  means that, as shown in  FIG. 22(B)  or (C), the read area partially or entirely overlaps with the target area of the copy command. Therefore, at this point, the CPUs  22 A,  22 B obtain the command ID of the copy command corresponding with the relevant entry  41  from the copy command ID field  41 A ( FIG. 5 ) of the entry  41 , and stores it (SP 57 ). 
         [0160]    Furthermore, the CPUs  22 A,  22 B refer to the address attribute field  41 C ( FIG. 5 ) of the entry  41  and determine whether the address attribute stored in the relevant address attribute field  41 C is “Copy source” or not (whether the entry  41  corresponds with the copy source area of the copy command or not) (SP 58 ). 
         [0161]    The CPUs  22 A,  22 B, if obtaining the affirmative result to this determination, set the copy source overlap flag reset at step SP 50  to ON (SP 59 ), and then proceed to step SP 61 . On the other hand, the CPUs  22 A,  22 B, if obtaining the negative result to this determination, set the copy destination overlap flag reset at step SP 51  to ON (SP 60 ), and then proceed to step SP 61 . 
         [0162]    The CPUs  22 A,  22 B proceed to step SP 61 , refers to the pointer field  41 D ( FIG. 5 ) of the entry  41 , and determine whether “Null” is stored or not (SP 61 ). 
         [0163]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, return to step SP 54 , and then perform the same processing as mentioned above. On the other hand, the CPUs  22 A,  22 B, if obtaining the affirmative result to this determination, complete this hash search processing and return to the read processing. 
       (4-1-3) Data Read Processing 
       [0164]    On the other hand.  FIG. 23  shows the concrete contents of the data read processing performed by the CPUs  22 A,  22 B at step SP 41  of the read processing explained with reference to  FIG. 20 . 
         [0165]    The CPUs  22 A,  22 B proceed to step SP 41  of the read processing and start this data read processing. Firstly, by controlling the corresponding disk devices  10  via the drive interfaces  24 A,  24 B, [the CPUs  22 A,  22 B] read data from the read area specified by the read command, and store the read data in the read sides of the cache memories  23 A,  23 B (SP 70 ). 
         [0166]    Next, the CPUs  22 A,  22 B determine whether the copy destination overlap flag mentioned above for step SP 51  of  FIG. 21  is set to ON or not (SP 71 ) and, if it obtains the negative result, proceed to step SP 85 . 
         [0167]    On the other hand, the CPUs  22 A,  22 B, if obtaining the affirmative result to the determination at step SP 71 , set a pointer (hereinafter referred to as a copy command pointer) for the copy command to be performed first of the copy commands registered to the copy queue  31  (SP 72 ), and then determine whether the copy command ID of the copy command for which the copy command pointer was set matches the copy command ID stored at step SP 57  in  FIG. 21  or not (SP 73 ). 
         [0168]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, proceed to step SP 83  or, on the other hand, if obtaining the affirmative result, determine whether the starting address of the read area specified by the read command is smaller than the starting address of the target area of the copy command for which the copy command pointer is set at that time or not (SP 74 ). 
         [0169]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, set the starting address of the read area specified by the read command as the starting address of the area (hereinafter referred to as an overlapping area starting address) where the read area specified by the read command overlaps with the target area of the copy command (SP 75 ) or, if obtaining the affirmative result, set the starting address of the target area of the copy command as the overlapping area starting address (SP 76 ). 
         [0170]    Next, the CPUs  22 A,  22 B determine whether the ending address of the read area specified by the read command is smaller than the ending address of the target area of the copy command or not (SP 77 ). 
         [0171]    The CPUs  22 A,  22 B, if obtaining the affirmative result to this determination, set the ending address of the read area specified by the read command as the ending address of the overlapping area (hereinafter referred to as an overlapping area ending address) (SP 78 ) or, if obtaining the negative result, set the ending address of the target area of the copy command as the overlapping area ending address (SP 79 ). 
         [0172]    Next, the CPUs  22 A,  22 B determine whether each difference bit corresponding with the overlapping area in the difference bitmap  32  ( FIG. 2 ) corresponding with the copy command for which the copy command pointer is set at that time is set to ON or not (SP 80 ). 
         [0173]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, proceed to step SP 83  or, if obtaining the affirmative result, read the data stored in the storage area from the overlapping area starting address set at step SP 75  or at step SP 76  in the copy source area of the copy command to the overlapping area ending address set at step SP 78  or at step SP 79 , and overwrite the corresponding data part of the data stored on the read sides of the cache memories  23 A,  23 B at step SP 70  with the read data. 
         [0174]    Next, the CPUs  22 A,  22 B set the difference bit corresponding with the data read at step SP 81  in the difference bitmap  32  ( FIG. 2 ) corresponding with the copy command to OFF (SP 82 ), and then proceed to step SP 83 . 
         [0175]    Next, the CPUs  22 A,  22 B proceed to step SP 83 , and move the copy command pointer mentioned above for step SP 72  to the copy command to be performed after the copy command for which the copy command pointer is set at that point of the copy commands registered to the copy queue  31  (SP 83 ). Then, the CPUs  22 A,  22 B determine whether the processing from step SP 73  to step SP 83  were performed for all the copy commands registered to the copy queue  31  (SP 84 ). 
         [0176]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, return to step SP 73  and repeat the processing from step SP 73  to step SP 83  until it obtains the affirmative result at step SP 84 . The CPUs  22 A,  22 B, if obtaining the affirmative result by completing the processing from step SP 73  to step SP 83  for all the copy commands registered to the copy queue  31  in due course at step SP 84 , send the performance completion response of the relevant read command and the read data stored in the read sides of the cache memories  23 A,  23 B at that time to the host device  2  as the sending source of the read command (SP 85 ), and then return to the read processing in  FIG. 20 . 
       (4-2) Various Types of Processing by CPU Related to Write Processing 
     (4-1-1) Write Processing 
       [0177]    On the other hand.  FIG. 24  shows the procedure of the write processing performed by the CPUs  22 A,  22 B of the storage system  4  when processing a write command stored in the command queue  30 . 
         [0178]    The CPUs  22 A,  22 B, when starting this write processing, firstly make the starting address of the write area specified by the write command and the length of the write area specified by the relevant write command into an argument, and determine whether the write area partially or entirely overlaps with the copy source area or the copy destination area of any copy command registered to the copy queue  31  or not, by referring to the hash table  40  ( FIG. 5 ) (SP 90 ). As more specifically described, at this step SP 90 , the CPUs  22 A,  22 B perform the hash search processing explained with reference to  FIG. 21 . 
         [0179]    Next, the CPUs  22 A,  22 B, with reference to the result of the determination obtained at step SP 90 , control the disk devices  10  ( FIG. 1 ) via the drive interfaces  24 A,  24 B ( FIG. 1 ), store the write data in the write area specified by the write command from the disk devices  10  (SP 91 ), and then complete this write processing. 
       (4-2-2) Data Write Processing 
       [0180]      FIG. 25A  and  FIG. 25B  show the concrete contents of the data write processing performed by the CPUs  22 A,  22 B at step SP 91  of the write processing. 
         [0181]    The CPUs  22 A,  22 B proceed to step SP 91  of the write processing and start this data write processing by determining whether the copy source overlap flag mentioned above for step SP 50  of  FIG. 21  is set to ON or not (SP 100 ). Then, the CPUs  22 A,  22 B, if obtaining the negative result to this determination, proceed to step SP 114 . 
         [0182]    On the other hand, the CPUs  22 A,  22 B, if obtaining the affirmative result to the determination at step SP 100 , set a copy command pointer for the copy command to be performed first of the copy commands registered to the copy queue  31  (SP 101 ), and then determine whether the copy command ID of the copy command for which the copy command pointer was set matches the copy command ID stored at step SP 57  in  FIG. 21  or not (SP 102 ). 
         [0183]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, proceed to step SP 113  or, on the other hand, if obtaining the affirmative result, determine whether the starting address of the write area specified by the write command is smaller than the starting address of the target area of the copy command for which the copy command pointer is set at that time or not (SP 103 ). 
         [0184]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, set the starting address of the read area specified by the write command as the starting address of the area (overlapping area starting address) where the write area specified by the write command overlaps with the target area of the copy command (SP 104 ) or, if obtaining the affirmative result, set the starting address of the target area of the copy command as the overlapping area starting address (SP 105 ). 
         [0185]    Next, the CPUs  22 A,  22 B determine whether the ending address of the write area specified by the write command is smaller than the ending address of the target area of the copy command (SP 106 ). 
         [0186]    The CPUs  22 A,  22 B, if obtaining the affirmative result to this determination, set the ending address of the write area specified by the write command as the ending address of the overlapping area (overlapping area ending address) (SP 107 ) or, if obtaining the negative result, set the ending address of the target area of the copy command as the overlapping area ending address (SP 108 ). 
         [0187]    Next, the CPUs  22 A,  22 B determine whether each difference bit corresponding with the overlapping area in the difference bitmap  32  corresponding with the copy command for which the copy command pointer is set at that time is set to ON or not (SP 109 ). 
         [0188]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, proceed to step SP 113  or, if obtaining the affirmative result, by controlling the corresponding disk devices  10 , read the data stored in the storage area from the overlapping area starting address set at step SP 104  or at step SP 105  in the copy source area of the copy command to the overlapping area ending address set at step SP 107  or at step SP 108 , and store the read data on the read sides of the cache memories  23 A,  23 B (SP 110 ). 
         [0189]    Furthermore, the CPUs  22 A,  22 B copy the data stored in the read sides of the cache memories  23 A,  23 B to the write sides of the cache memories  23 A,  23 B (SP 111 ), and then set the difference bit corresponding with the data read from the disk devices  10  at step SP 110  in the difference bitmap  32  corresponding with the copy command to OFF (SP 112 ), and proceed to step SP 113 . 
         [0190]    Next, the CPUs  22 A,  22 B proceed to step SP 113 , and move the copy command pointer mentioned above for step SP 101  to the copy command to be performed after the copy command for which the copy command pointer is set at that point (SP 113 ). 
         [0191]    Then, the CPUs  22 A,  22 B determine whether the processing from step SP 102  to step SP 113  were performed for all the copy commands registered to the copy queue  31  or not (SP 114 ). 
         [0192]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, return to step SP 102  and repeat the processing from step SP 102  to step SP 114  until obtaining the affirmative result at step SP 114 . The CPUs  22 A,  22 B, if obtaining the affirmative result by completing the processing from step SP 102  to step SP 113  for all the copy commands registered to the copy queue  31  in due course at step SP 114 , proceed to step SP 115 . 
         [0193]    The CPUs  22 A,  22 B, when proceeding to step SP 115 , determine whether the copy destination overlap flag mentioned above for step SP 51  of  FIG. 21  is set to ON or not (SP 115 ) and, if obtaining the negative result, proceed to step SP 128 . 
         [0194]    On the other hand, the CPUs  22 A,  22 B, if obtaining the affirmative result to the determination at step SP 115 , subsequently, perform the processing from step SP 116  to step SP 127  by the same method as the processing from step SP 101  to step SP 114 . Then, the CPUs  22 A,  22 B, when obtaining the affirmative result at step SP 127  in due course, proceed to step SP 128 . 
         [0195]    The CPUs  22 A,  22 B, when proceeding to step SP 128 , receive the write data sent along with the write command from the host device  2 , and store this write data on the write sides of the cache memories  23 A,  23 B (SP 128 ). 
         [0196]    In this case, the CPUs  22 A,  22 B, as explained with reference to  FIG. 10  for example, if the data copied on the write sides of the cache memories  23 A,  23 B at step SP 111  must be overwritten with the write data sent from the host device  2 , overwrite the relevant data with the write data. 
         [0197]    The CPUs  22 A,  22 B send the performance completion response of the relevant write command to the host device  2  as the sending source of the write command (SP 129 ), and return to the write processing in  FIG. 24 . 
       (4-3) Copy Command Reception Processing 
       [0198]    On the other hand,  FIG. 26  shows the concrete processing routine of the CPUs  22 A,  22 B of the storage system  4  in the case [the CPUs  22 A,  22 B] receive a copy command from the host device  2 . 
         [0199]    The CPUs  22 A,  22 B, when receiving a copy command from the host device  2 , start the reception processing of this copy command. Firstly, [the CPUs  22 A,  22 B] make the starting address of the copy source area of the copy command and the length of the copy source area specified by the relevant copy command into an argument, and determine whether the copy source area overlaps with the copy destination area or the copy source area of any copy command registered to the copy queue  31  or not, by referring to the hash table  40  ( FIG. 5 ) (SP 130 ). As more specifically described, at this step SP 130 , the CPUs  22 A,  22 B perform the hash search processing explained with reference to  FIG. 21 . 
         [0200]    Next, the CPUs  22 A,  22 B determine whether the copy source overlap flag mentioned above for step SP 50  of  FIG. 21  is set to ON or not (SP 131 ). 
         [0201]    At this point, obtaining the affirmative result to this determination, as explained with reference to  FIG. 17  and  FIG. 19 , means that the copy source area of the copy command received at that time partially or entirely overlaps with the copy destination area of any of the copy commands already registered to the copy queue  31 . Therefore, at this point, the CPUs  22 A,  22 B send the error response that the relevant copy command cannot be performed to the host device  2  as the sending source of the copy command (SP 138 ), and then complete this copy command reception processing. 
         [0202]    Meanwhile, the CPUs  22 A,  22 B, if obtaining the negative result to this determination at step SP 131  make the starting address of the copy destination area of the copy command received at that time and the length of the copy destination area specified by the relevant copy command into an argument, and determine whether the copy destination area partially or entirely overlaps with the copy source area of any copy command registered to the copy queue  31  or not, by referring to the hash table  40  (SP 132 ). As more specifically described, at this step SP 132 , the CPUs  22 A,  22 B perform the hash search processing explained with reference to  FIG. 21 . 
         [0203]    Next, the CPUs  22 A,  22 B determine whether the copy destination overlap flag mentioned above for step SP 51  of  FIG. 21  is set to ON or not (SP 133 ). 
         [0204]    At this point, obtaining the affirmative result to this determination, as explained with reference to  FIG. 18 , means that the copy destination area of the copy command received at that time partially or entirely overlaps with the copy source area of any of the copy commands already registered to the copy queue  31 . Therefore, at this point, the CPUs  22 A,  22 B send the error response that the relevant copy command cannot be performed to the host device  2  as the sending source of the copy command (SP 138 ), and then complete this copy command reception processing. 
         [0205]    On the other hand, obtaining the negative result to this determination at step SP 133  means that the copy destination area of the copy command received at that time does not overlap with the copy source area of any of the copy commands already registered to the copy queue  31 . Therefore, at this point, the CPUs  22 A,  22 B register the copy source area and the copy destination area of the copy command to the hash table  40  ( FIG. 5 ) respectively (SP 134 , SP 135 ), and register the copy command to the copy queue  31  (SP 136 ). 
         [0206]    Next, the CPUs  22 A,  22 B send the response (copy performance completion response) that the copy processing in accordance with the relevant copy command is completed to the host device  2  as the sending source of the copy command (SP 137 ), and then complete this copy command reception processing. 
       (4-4) Hash Table Registration Processing 
       [0207]    On the other hand,  FIG. 27  shows the concrete contents of the hash table registration processing performed at step SP 134  and at step SP 135  of the copy command reception processing explained with reference to  FIG. 26 . 
         [0208]    The CPUs  22 A,  22 B, when proceeding to step SP  134  or step SP 135 , start this hash table registration processing. Firstly, the CPUs  22 A,  22 B reset the counter (hereinafter referred to as an address counter) not shown in the figure for counting addresses as described later (set the count value to “0”) (SP 140 ). 
         [0209]    Next, the CPUs  22 A,  22 B, by rounding off the address of the starting address of the copy source area specified by the copy command which is targeted at that time (in the case of step SP 134  in  FIG. 26 ) or the starting address of the copy destination area (in the case of step SP 135  in  FIG. 26 ) with the counter value of the address counter added to it by the 1-megabyte border, determine a part of the area of the target area (copy source area or copy destination area) to be registered to the hash table  40  ( FIG. 5 ) (SP 141 ). 
         [0210]    Next, the CPUs  22 A,  22 B create an entry  41  of the area determined at step SP 141 . As more specifically described, an entry  41  is created by storing the copy command ID allocated to the corresponding copy command in the copy command ID field  41 A ( FIG. 5 ), storing the bitmap  42  ( FIG. 5 ) whose bit corresponding with the area is set to ON in the bitmap field  41 B ( FIG. 5 ), and storing the address attribute (copy source area or copy destination area) in the address attribute field  41 C ( FIG. 5 ). Then, the CPUs  22 A,  22 B store the entry  41  created as mentioned above in the cache memories  23 A,  23 B (SP 142 ). 
         [0211]    Next, the CPUs  22 A,  22 B set a pointer for the entry  41  created at step SP 142  (hash pointer) in the corresponding slots  40 A to  40 L in the hash table  40  or in the pointer field  41 D ( FIG. 5 ) of the last entry  41  of the entries  41  made to correspond with the slots  40 A to  40 L (SP 143 ). 
         [0212]    Next, the CPUs  22 A,  22 B, with reference to the value smaller than or equal to 1 megabyte among the value of the target area starting address with the count value of the address counter added to it, set the bit corresponding with the 64-kilobyte unit area which is targeted at that time in the bitmap  42  stored in the bitmap field  41 B of the entry  41  to ON (SP 144 ). 
         [0213]    Furthermore, the CPUs  22 A,  22 B add a 64-kilobyte numeral value to the count value of the address counter (SP 145 ), and then determine whether the value smaller than or equal to 1 megabyte among the value of the target area starting address with the count value of the address counter added to it has become “0” or not (SP 146 ). 
         [0214]    At this point, obtaining the negative result to this determination means that the 64-kilobyte unit area which is targeted at that time is not the last unit area of the 1-megabyte border. Therefore, at this time, the CPUs  22 A,  22 B determine whether the count value of the address counter is smaller than the length of the target area of the copy command which is targeted at that time or not (SP 147 ). 
         [0215]    The CPUs  22 A,  22 B, if obtaining the negative result to this determination, return to step SP 144 , and repeat the processing from step SP 144  to step SP 147  until obtaining the affirmative result at step SP 146  or step SP 147 . By this processing, the CPUs  22 A,  22 B, for each of the unit areas in the 1-megabyte area which is targeted at that time, sequentially set the corresponding bits in the bitmap  42  stored in the bitmap field  41 B of the entry  41  to ON. 
         [0216]    The CPUs  22 A,  22 B, if obtaining the affirmative result at step SP 146  by completing the same processing in due course for all the unit areas in the 1-megabyte area which is targeted at that time, return to step SP 141 , and then repeat the same processing for the next 1-megabyte area (SP 141  to SP 147 ). 
         [0217]    The CPUs  22 A,  22 B, if obtaining the negative result at step SP 147  by completing the same processing in due course for all the 1-megabyte areas in the target areas of the copy command which is targeted at that time, set “Null” indicating that there is no more entry in the pointer field  41 D of the entry  41  (SP 148 ), and then complete this hash table registration processing. 
       (5) Advantageous Effects of this Embodiment 
       [0218]    As mentioned above, in the storage system  4  of this embodiment, the copy queue  31  is provided in addition to the command queue  30  and, if the command stored in the command queue  30  is a copy command, moves the command to the copy queue  31 , sends the copy performance completion response to the host device  2  as the sending source of the copy command, and then performs the copy processing in accordance with the copy command in the background. By this method, in the host device  2 , the subsequent commands to access the same address are not made to wait, which prevents the occurrence of time-out in advance. Therefore, the highly reliable storage system capable of accelerating the processing speed of the copy processing seen from the host device can be achieved. 
       (6) Other Embodiments 
       [0219]    Note that, though the above-mentioned embodiment discloses the case where this invention is applied to the storage system  4  configured as in  FIG. 1 , this invention is not limited to the case but may also be broadly applied to other various types of storage systems. 
         [0220]    Furthermore, though the above-mentioned embodiment discloses the case where the copy queue  31  ( FIG. 2 ) is set in the cache memories  23 A,  23 B, this invention is not limited to the case but it may also be permitted to set [the copy queue  31 ] in other than the cache memories  23 A,  23 B, e.g. in the local memories  21 A,  21 B. 
         [0221]    Furthermore, though the above-mentioned embodiment discloses the case where the number of slots in the hash table  40  ( FIG. 5 ) is 12, this invention is not limited to the case but the number of slots other than 12 may also be permitted. 
         [0222]    Furthermore, though the above-mentioned embodiment discloses the case where, as the control unit which stores the commands given from the host device  2  in the command queue  30  and also performs the commands stored in the command queue  30  in order of storage in the command queue  30 , the CPUs  22 A,  22 B managing the operation control of the entire storage system  4 , this invention is not limited to the case but it may also be permitted to provide the hardware with the function as the control unit separately from the CPUs  22 A,  22 B. 
       INDUSTRIAL APPLICABILITY 
       [0223]    This invention can be broadly applied to storage systems of various types of configurations in which the copy function is installed.