Abstract:
A apparatus for controlling a first storage and a second storage, has a controller for receiving a write command and a read command sent out from a host and for sending out the write command and the read command to the first storage and the second storage, a determining unit for sending out a request corresponding to the write command to the first storage and the second storage, for receiving a first response corresponding to the request from the first storage and a second response corresponding to the request from the second storage, and for determining one of the storages on the basis of each of response times, a first writing unit for writing data into the determined storage, and a second writing unit for writing the data written in the determined storage into the other storage after writing the data into the determined storage by the first writing unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-151527, filed on Jun. 25, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present art relates to an apparatus, a computer-readable recording medium and a storage system. 
       BACKGROUND 
       [0003]    Various types of systems, in which memory resources are virtualized and used, have been proposed so as to effectively utilize the memory resources in external virtual FC switches such as storages. For example, in the case in which a storage area network (SAN) which is a high-speed network used for connecting a host and a storage is established, there is a virtual FC switch which includes a function for inclusively virtualizing storage resources and causing the inclusively-virtualized storage resources to appear to be a new storage configuration, in addition to a function as a fibre channel (FC) switch which functions as a relay device. 
         [0004]    The virtual FC switch has a function for configuring an arbitrary area of a connected physical storage device as a virtual disk and showing the virtual disk to the host through a virtual target (VT). In addition, the virtual FC switch has a function for copying data to another virtual disk configured in the same way. 
         [0005]    [patent document 1] Japanese Laid-open Patent Publication No. 2006-127028 
       SUMMARY 
       [0006]    According to an aspect of an embodiment, an apparatus for controlling a first storage for storing data and a second storage for storing data, has a controller for receiving a write command sent out from a host and for sending out the write command to the first storage and the second storage, a determining unit for sending out a request corresponding to the write command to the first storage and the second storage, for receiving a first response corresponding to the request from the first storage and a second response corresponding to the request from the second storage, and for determining one of the storages on the basis of a first and second response times, the first response time and the second response time being time until receiving the response from the first and second storage after sending out the request to the first and second storage, a first writing unit for writing data into the determined storage, and a second writing unit for writing the data written in the determined storage into the other storage after writing the data into the determined storage by the first writing unit. 
         [0007]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a diagram for illustrating a data relay device according to an embodiment 1; 
           [0010]      FIG. 2  is a diagram illustrating a configuration of a virtual FC switch according to an embodiment 2; 
           [0011]      FIG. 3  is a diagram for illustrating an operation performed by the virtual FC switch according to the embodiment 2; 
           [0012]      FIG. 4  is a diagram for illustrating an operation performed by the virtual FC switch according to the embodiment 2; 
           [0013]      FIG. 5  is a diagram for illustrating the virtual FC switch according to the embodiment 2; 
           [0014]      FIG. 6  is a diagram illustrating a configuration of a virtual FC switch according to an embodiment 3; 
           [0015]      FIG. 7  is a diagram for illustrating an operation performed by the virtual FC switch according to the embodiment 3; 
           [0016]      FIG. 8  is a diagram for illustrating the related art; 
           [0017]      FIG. 9  is a diagram for illustrating the related art; 
           [0018]      FIG. 10  is a diagram for illustrating the related art; 
           [0019]      FIG. 11  is a diagram for illustrating the related art; 
           [0020]      FIG. 12  is a diagram for illustrating the related art; and 
           [0021]      FIG. 13  is a diagram for illustrating the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]      FIG. 8  is a diagram for illustrating the related art. As shown in  FIG. 8 , a virtual FC switch of the related art creates a virtual disk  1   b  from a storage  1   c  and connects the virtual disk  1   b  to a host through a virtual target VT  1   a.  In the same way, the virtual FC switch of the related art creates a virtual disk  2   b  from a storage  2   c  and connects the virtual disk  2   b  to the host through a virtual target VT  2   a.  When the virtual FC switch copies data from the virtual disk  1   b  to the virtual disk  2   b,  the virtual FC switch reads out data from the virtual disk  1   b  which is a copying source. Then, from the initial logical block address (LBA) of the virtual disk  1   b  to the final LBA thereof, the virtual FC switch repeatedly performs an operation to write data into the virtual disk  2   b  which is a copying destination. 
         [0023]    In addition, even if the virtual FC switch receives “Write I/O” from the host during a copy operation, the virtual FC switch creates a mirror disk on the basis of both the copying source virtual disk and the copying destination virtual disk so as to maintain equivalence between data of the virtual disk  1   b  and the virtual disk  2   b.  Then, the virtual FC switch performs a synchronous write operation for writing “Write I/O”, received from the host, to both virtual disks. 
         [0024]    The synchronous write operation performed in the virtual FC switch will be described simply. As shown in  FIG. 9 , when the virtual FC switch receives “Write Command”, the virtual FC switch transfers the write command to the storages is and  2   c.  When the virtual FC switch receives “Ready” responses (for example, Ready ( 1 ) and Ready ( 2 )) to the write command from the storages  1   c  and  2   c,  the virtual FC switch transfers the “Ready” responses to the host. When the virtual FC switch receives data corresponding to the write command from the host, the virtual FC switch writes the data received from the host into the storages is and  2   c.  When the virtual FC switch receives statuses “Status-Good”, which indicate write completion, from the storages is and  2   c,  the virtual FC switch transfers the “Status-Good” to the host and completes the synchronous write operation. 
         [0025]    In addition, regarding “Read I/O” from the host, the virtual FC switch only reads out data from the virtual disk  1   b  which is a copying source, and transfers the data to the host. 
         [0026]    The read-out operation performed in the virtual FC switch will be described simply. As shown in  FIG. 10 , when the virtual FC switch receives “Read Command” (described hereinafter as read command), the virtual FC switch transfers the read command to the storage  1   c.  When the virtual FC switch receives a “Ready” response to the read command from the storage  1   c,  the virtual FC switch transfers the “Ready” response to the host. Subsequently, when the virtual FC switch receives data to be read (Data) from the storage  1   c,  the virtual FC switch transfers the data to the host. Then, when the virtual FC switch receives a status, “Status-Good”, which indicates read completion, from the storage  1   c,  the virtual FC switch transfers the status, “Status-Good”, which indicates read completion, to the host and completes the read operation. 
         [0027]    A virtual FC switch of the related art causes the following problem in the synchronous write operation described with reference to  FIG. 9 . Namely, as shown in  FIG. 11 , when the performance of the storage  2   c  which configures a copying destination virtual disk is low, a response (Ready) to a write command, which is transferred from the virtual disk  2   b  to the virtual FC switch, is delayed. 
         [0028]    Even though the virtual FC switch has already received a response from the storage  1   c  which is a copying source, the virtual FC switch obtains data to be written from the host, after a response to the write command arrives from the storage  2   c.  In this way, as a result, the I/O performance of the virtual FC switch is restrained by the storage  2   c  which has lower performance. Accordingly, the processing performance for I/O from the host (performance of a processing operation in which the write command and the read command from the host are processed) becomes worse. 
         [0029]    In addition, as shown in  FIG. 13 , when the performance of the storage  1   c  which configures a copying source virtual disk is low, contrary to the case shown in  FIG. 11 , the performance of the storage  2   b  which is a copying destination is restrained by the performance of the storage  1   b  which is a copying source and, as a result, the processing performance for I/O from the host becomes worse. 
         [0030]    In addition, in the case in which, by using a multiprotocol router, a virtual disk is configured by a storage which is remotely situated through a communication line and a copy operation is performed, for example, as shown in  FIG. 12 , a delay in a response from a storage  3   c  which is a copying destination may occur. Accordingly, in the same ways as the cases shown in  FIGS. 11 and 13 , the processing performance for I/O from the host may become worse. In addition,  FIGS. 11 to 13  are diagrams for illustrating problems of the related art. 
         [0031]    Taking the above-mentioned circumstances into account, a technique disclosed in the specification is provided. Then, it is an object to provide a data relay device and a storage system which are capable of preventing the processing performance for I/O from the host from becoming worse in a storage system including a plurality of storages which have individually different data input-output performance. 
         [0032]    Hereinafter, an embodiment according to a data relay device and a storage system disclosed in the present application will be described in detail with reference to figures. In addition, the technique disclosed in the present application is not limited to an example, described hereinafter, of the embodiment according to the data relay device and the storage system. 
       Embodiment 1  
       [0033]      FIG. 1  is a diagram for illustrating a data relay device according to an embodiment 1. As shown in  FIG. 1 , the data relay device according to the embodiment 1 includes a first write processing section, a write location storage section, and a second write processing section. 
         [0034]    When a write command is transferred from a higher-level device during a copy session used for copying data from a first virtual storage section to a second virtual storage section, the first write processing section executes the write command only on one virtual storage section having higher input-output performance. 
         [0035]    The write location storage section stores a write location of the virtual storage section on which the write command is executed by the first write processing section. 
         [0036]    The second write processing section writes data stored in the virtual storage section into the other virtual storage section on which the write command has not been executed yet, after the write processing performed by the first write processing section is terminated, the data being fixed by the write location stored in the write location storage section. 
         [0037]    In this way, the data relay device according to the embodiment 1 performs a data processing operation by using, at full value, the performance of the storage device having higher performance and prevents the processing performance for write I/O from a host from becoming worse. 
       Embodiment 2  
       [0038]    A virtual FC switch which is an example of an embodiment of the data relay device disclosed in the present application will be described hereinafter. 
         [0039]    Regarding the summary of operations performed by the data relay device according to an embodiment 2, when the data relay device receives write I/O (write command) from a host during a data copy operation performed between virtual disks configured with arbitrary areas of a plurality of connected physical storage devices, the data relay device performs, through a mirror disk, a synchronous write operation on individual virtual disks in synchronization with the write I/O from the host. 
         [0040]    The main feature of the data relay device according to the embodiment 2 exists in the following description. 
         [0041]    Namely, in the case in which the input-output performance of a first virtual storage section which is a copying source is higher than the input-output performance of a second virtual storage section which is a copying destination, a first write processing section, included in the virtual FC switch, executes a write command only on the first virtual storage section in parallel with the copy operation. A write location storage section, included in the virtual FC switch, stores a write location of the first virtual storage section on which the write command is executed by the first write processing section. After the write processing performed by the first write processing section is terminated, a second write processing section, included in the virtual FC switch, writes data stored in the first virtual storage section into the second virtual storage section, the data being fixed by the write location stored in the write location storage section, and thereby makes data in the virtual disks equivalent to each other. The virtual FC switch according to the embodiment 2 will be specifically described hereinafter. 
       Configuration of Virtual FC Switch (Embodiment 2)   
       [0042]      FIG. 2  is a diagram illustrating a configuration of the virtual FC switch according to the embodiment 2. In addition, in  FIG. 2 , only processing function sections necessary for explaining the virtual FC switch according to the embodiment 1 are illustrated. 
         [0043]    As shown in  FIG. 2 , the virtual FC switch  200  according to the embodiment 1 is connected in a state in which the virtual FC switch  200  can send and receive data to and from a host computer  100  and storage devices  300  and  400  which are physical storage resources. The virtual FC switch  200  includes an input-output control section  210 , VT&#39;s  220  and  230 , a control section  240 , a mirror disk  250 , and virtual disks  260  and  270 . 
         [0044]    The input-output control section  210  controls input and output of various types of information sent and received to and from the host computer  100  or the like. For example, the input-output control section  210  controls input of write I/O or read I/O or the like, sent from the host computer  100 . 
         [0045]    The virtual targets (VT)  220  and  230  cause the virtual disks  260  and  270  corresponding to the virtual targets (VT)  220  and  230  respectively to be identified by the host  100 . Namely, the host  100  realizes access to the virtual disks  260  and  270  by transmitting an access request to a virtual target (VT) set in the virtual FC switch  200 . 
         [0046]    The virtual disk  260  corresponds to the storage device  300  having higher data input-output performance than the storage device  400  and functions as a copying source. The virtual disk  270  corresponds to the storage device  400  and functions as a copying destination. In addition, the input-output performance is the response performance of a storage device, which corresponds to the time taken to enter a standby state in the condition in which the storage device can deal with I/O from the host. For example, in the case shown in  FIG. 9 , the time from transmission of a write command from the host to the storage device and reception of a response from the storage device corresponds to the data input-output performance. 
         [0047]    The control section  240  controls processing operations for write I/O and read I/O and a data copy operation performed between the virtual disks  260  and the virtual disk  270 . Then, as shown in  FIG. 2 , the control section  240  includes a write control section  241 , a bit setting section  242 , and a copy control section  243 . 
         [0048]    When the write command is transferred from the host  100  during a copy session used for copying data from the virtual disk  260  to the virtual disk  270 , the write control section  241  only executes a write command on a virtual disk having higher input-output performance. 
         [0049]    Specifically, when the write command is transferred from the host  100  during the copy session, the write control section  241  only writes data into the virtual disk  260  which corresponds to the storage  300  having higher input-output performance. In addition, the write control section  241  performs a write operation in synchronization with a copy operation during the copy session performed by the copy control section  243  described hereinafter. A specific operation, performed by the write control section  241 , or the like will be described hereinafter with reference to  FIGS. 3 and 4 . 
         [0050]    The bit setting section  242  stores a write location of the virtual disk  260  on which the write control section  241  executes the write command during the copy session. Specifically, the bit setting section  242  stores the write location by setting a bit in a bit map to “ON”, the bit corresponding to an area in the virtual disk  260  in which the write control section  241  stores data from the host  100 . In addition, while the embodiment is not limited to the case in which the bit map is used, various types of information may be used as long as the copy control section  243  described hereinafter can identify the write location of the virtual disk  260  on which the write command from the host  100  is executed. A specific operation, performed by the bit setting section  242 , or the like will be described hereinafter with reference to  FIGS. 3 and 4 . 
         [0051]    The copy control section  243  controls a copy operation performed during a copy session used for copying data from the virtual disk  260  to the virtual disk  270 , the data being written in the virtual disk  260  in response to the write I/O from the host. Specifically, from the initial logical block address (LBA) of the virtual disk  260  to the final LBA thereof, the copy control section  243  repeats an operation to write data into the virtual disk  270  which is a copying destination. 
         [0052]    In addition, the copy control section  243  writes data stored in the virtual disk  260  into the virtual disk  270 , after the write processing performed by the write control section  241  is terminated, the data being fixed by the write location stored in the bit map. Specifically, after the write processing on the virtual disk  260  is terminated, the copy control section  243  scans the bit map and thereby writes data, which is written in areas in the virtual disk  260 , into the virtual disk  270 , the areas corresponding to bits set to “ON” in the bit map. In addition, a specific operation, performed by the copy control section  243 , or the like will be described hereinafter with reference to  FIGS. 3 and 4 . 
       Operation Performed By Virtual FC Switch (Embodiment 2)   
       [0053]      FIGS. 3 and 4  are diagrams for illustrating an operation performed by the virtual FC switch according to the embodiment  2 . An operation, performed when write I/O is received from the host  100  during a copy operation for copying data written in the virtual disk  260  to the virtual disk  270 , will be described hereinafter. In addition, it is assumed that an operation performed by the virtual FC switch  200 , described hereinafter, is performed in no association with and in synchronization with the copy operation. 
         [0054]    As shown in  FIG. 3 , the write control section  241  in the virtual FC switch  200  sends write I/O received from the host  100  to individual storage devices (storage devices  300  and  400 ) (Step S 1 ). Then, the write control section  241  sends a “Ready” response to the host  100  after reception of “Ready” responses from individual storage devices is completed (Step S 2 ). 
         [0055]    When the write control section  241  receives data from the host  100 , the write control section  241  generates a mirror disk from the virtual disk  260  and the virtual disk  270  and only writes the data, received from the host  100 , into the virtual disk  260  through the mirror disk (Step S 3 ). 
         [0056]    When the write control section  241  completes an operation for writing the data in the virtual disk  260 , the bit setting section  242  in the virtual FC switch sets bits in the bit map to “ON”, the bits corresponding to areas in which the data is written in the virtual disk  260  (Step S 4 ). 
         [0057]    When the bit setting section  242  completes the bit setting operation for the bit map, the bit setting section  242  sends a status indicating completion of write I/O to the host  100  (Step S 5 ) and the virtual FC switch  200  completes the operation. 
         [0058]    Next, with reference to  FIG. 4 , an asynchronous write operation for writing the data, written in the virtual disk  260 , in the virtual disk  270  will be described, the asynchronous write operation performed, for example regularly, in the virtual FC switch  200 . 
         [0059]    As shown in  FIG. 4 , the copy control section  243  in the virtual FC switch  200  scans the bit map included in the bit setting section  242  (Step S 1 ) and determines whether or not a location at which a bit is set to “ON” exists (Step S 2 ). 
         [0060]    When, as a result of the determination, the location at which the bit is set to “ON” exists in the bit map (Step S 2  YES), data, written in an area in the virtual disk  260 , is written in the virtual disk  270  (Step S 3 ), the area corresponding to the bit setting section in which the bit is set to “ON”. Then, the virtual FC switch  200  completes the operation. 
         [0061]    On the other hand, when the location at which the bit is set to “ON” does not exist in the bit map (Step S 2  NO), the virtual FC switch  200  completes the asynchronous write operation without change. 
       Advantage Due To Embodiment 2   
       [0062]    As mentioned above, according to the embodiment 2, when write I/O is received from the host  100  during a copy operation for copying data written in the virtual disk  260  to the virtual disk  270 , the virtual FC switch  200  only writes data from the host  100  into the virtual disk  260  which is a copying source (refer to ( 1 ) in  FIG. 5 ). 
         [0063]    Next, the virtual FC switch  200  sets a bit in the bit map to “ON”, the bit corresponding to an area in which the data is written in the virtual disk  260  (refer to ( 2 ) in  FIG. 5 ). Subsequently, at another time which is different from a time when the data from the host  100  is written in the virtual disk  260 , namely, asynchronously, the virtual FC switch  200  writes data written in the virtual disk  260  into the virtual disk  270 . Accordingly, the virtual FC switch  200  makes the virtual disk  260  and the virtual disk  270  equivalent to each other (refer to ( 3 ) in  FIG. 5 ).  FIG. 5  is a diagram for illustrating the virtual FC switch according to the embodiment 2. 
         [0064]    According to the embodiment 2, the virtual FC switch does not perform a synchronous write operation for writing data from the host  100  in both the virtual disk  260  and the virtual disk  270  in synchronization with write I/O from the host  100  in the same way as the related art. In addition, the virtual FC switch only writes data into the virtual disk  260  which corresponds to the storage device  300  having higher data input-output performance than the storage device  400 . 
         [0065]    Accordingly, it is necessary for the virtual FC switch to wait for a response from the virtual disk  270  which corresponds to the storage device  400  having lower data input-output performance than the storage device  300 , the response indicating a state in which the virtual disk  270  can deal with I/O from the host (Ready response, for example, refer to  FIG. 9 ). In addition, the virtual FC switch only writes data into the virtual disk  260 , which is in a state where the virtual disk  260  can deal with I/O from the host, and responds to the host  100  with completion of writing. Namely, the performance of the storage device  300  is not restrained by the performance of the storage device  400 , nor becomes the performance of the virtual FC switch  200  dealing with I/O from the host worse. 
         [0066]    In this way, in the storage system including a plurality of storages which have individually different data input-output performance, the virtual FC switch  200  according to the embodiment  2  performs a data processing operation by using, at full value, the performance of the storage device having higher performance and prevents the processing performance for write I/O from the host from becoming worse. 
       Embodiment 3  
       [0067]    In the above-mentioned embodiment 2, a synchronous write operation for writing data from the host  100  in both the virtual disk  260  and the virtual disk  270  in synchronization with write I/O from the host  100  and an asynchronous write operation for writing data, written in the virtual disk  260  in response to write I/O from the host  100 , in the virtual disk  270  in asynchronous timing with the write I/O may be switched. 
       Configuration of Virtual FC Switch (Embodiment 3)   
       [0068]      FIG. 6  is a diagram illustrating a configuration of a virtual FC switch according to an embodiment 3. The virtual FC switch  200  according to the embodiment 3 is different from the virtual FC switch  200  according to the embodiment 2 by the presence of a mode switching control section  244 . 
         [0069]    Specifically, when reception of “Ready” responses from individual storage devices is completed in the write control section  241 , the mode switching control section  244  causes the write control section  241  to halt a write operation temporarily. Then, the mode switching control section  244  calculates a reception-time difference between “Ready” responses from the storage devices, compares the calculated reception-time difference with a predetermined threshold value, and determines an input-output performance difference between the storage devices on the basis of the comparison result. 
         [0070]    When, as a result of the comparison of the reception-time difference between “Ready” responses from the individual storage devices with the predetermined threshold value, the reception-time difference exceeds the predetermined threshold value, the mode switching control section  244  switches, for example, from a synchronous write mode which is temporarily set in the write control section  241  at the present moment to an asynchronous write mode. Then, the mode switching control section  244  causes the write control section  241  to resume the write operation. 
         [0071]    The write control section  241  resumes the write operation, sends a “Ready” response to the host  100 , and only writes data, received from the host  100 , into the virtual disk  260  according to the asynchronous write mode set by the mode switching control section  244 . Subsequent operations are the same as the above-mentioned embodiment 2 (for example, refer to operations, performed after Step S 2 , in  FIG. 3  and operations in  FIG. 4 ). 
         [0072]    In addition, When, as a result of the comparison of the reception-time difference between “Ready” responses from the individual storage devices with the predetermined threshold value, the reception-time difference is less than or equal to the predetermined threshold value, the mode switching control section  244  maintains, for example, the synchronous write mode which is temporarily set in the write control section  241  at the present moment. Then, the mode switching control section  244  causes the write control section  241  to resume the write operation. 
         [0073]    The write control section  241  resumes the write operation, sends a “Ready” response to the host  100 , and writes data, received from the host  100 , into both the virtual disk  260  and the virtual disk  270  according to the synchronous write mode set by the mode switching control section  244 . When the data write operation is completed, the write control section  241  sends a status indicating completion of write I/O to the host  100  in the same way as in the asynchronous write mode. 
       Operation Performed By Virtual FC Switch (Embodiment 3)   
       [0074]      FIG. 7  is a diagram for illustrating an operation performed by the virtual FC switch according to the embodiment 3. The operation performed by the virtual FC switch according to the embodiment 3 is different from the operation performed by the virtual FC switch according to the embodiment 2 by the point described hereinafter. In addition, an operation performed when a data write mode in the write control section  241  is set to the synchronous write mode at the present moment will be described hereinafter. 
         [0075]    Namely, the mode switching control section  244  calculates a reception-time difference between “Ready” responses from the individual storage devices, compares the calculated reception-time difference with the predetermined threshold value, and determines an input-output performance difference between the storage devices on the basis of the comparison result (Step S 3 ). 
         [0076]    When, as a result of the comparison of the reception-time difference between “Ready” responses from the individual storage devices with the predetermined threshold value, the reception-time difference exceeds the predetermined threshold value (Step S 3  affirmative), the mode switching control section  244  switches, for example, from a synchronous write mode which is temporarily set in the write control section  241  at the present moment to an asynchronous write mode (Step S 4 ). Then, the mode switching control section  244  causes the write control section  241  to resume the write operation. 
         [0077]    The write control section  241  sends a “Ready” response to the host  100 , and only writes data, received from the host  100 , into the virtual disk  260  according to the asynchronous write mode set by the mode switching control section  244  (Step S 5 ). Subsequent operations are the same as the above-mentioned embodiment 2 (for example, refer to operations performed after Step S 2  in  FIG. 4 ). Then, the write control section  241  sends a status indicating completion of write I/O to the host  100  (Step S 7 ) and completes the data write operation. 
         [0078]    Here, returning to the description of Step S 3 , when, as a result of the comparison of the reception-time difference between “Ready” responses from the individual storage devices with the predetermined threshold value, the reception-time difference is less than or equal to the predetermined threshold value (Step S 3  negative), the mode switching control section  244  maintains the synchronous write mode which is set in the write control section  241  (Step S 8 ). Then, the mode switching control section  244  causes the write control section  241  to resume the write operation. 
         [0079]    The write control section  241  sends a “Ready” response to the host  100  and writes data, received from the host  100 , into both the virtual disk  260  and the virtual disk  270  according to the synchronous write mode set by the mode switching control section  244  (Step S 9 ). Then, the write control section  241  sends a status indicating completion of write I/O to the host  100  (Step S 7 ) and completes the data write operation. 
         [0080]    In the operation of the virtual FC switch, when an operation for entirely copying data from the virtual disk  260  to the virtual disk  270  is completed, the two virtual disks are separated from each other, so that the two virtual disks work independently from each other. 
         [0081]    In the case in which the write control section  241  is set in the asynchronous write mode, even when the operation for entirely copying data from the virtual disk  260  to the virtual disk  270  is completed, it takes a time to reflect write I/O from the host in the virtual disk  270  which is a copying destination. Accordingly, the separation operation is waited. Therefore, the case in which data from the host  100  is written in the two virtual disks synchronously while the write control section  241  is set in the synchronous write mode is more convenient. 
         [0082]    In the embodiment 3 described above, the mode switching control section  244  calculates a reception-time difference between “Ready” responses from the individual storage devices, compares the calculated reception-time difference with the predetermined threshold value, and determines an input-output performance difference between the storage devices on the basis of the comparison result. Then, when there is not so much of an input-output performance difference between the storage devices, the write control section  241  is set in the synchronous write mode and data from the host  100  is written in the two virtual disks synchronously. 
         [0083]    On the other hand, when there is an input-output performance difference between the storage devices, the write mode in the write control section  241  is switched to the asynchronous write mode and an write operation is performed so that the performance of a storage device which has higher performance is not restrained by the performance of a storage device  400  which has lower performance. 
         [0084]    In this way, the virtual FC switch  200  according to the embodiment  3  performs a data processing operation by using, at full value, the performance of the storage device having higher performance. Accordingly, the virtual FC switch  200  not only prevents the processing performance corresponding to write I/O from the host from becoming worse but also realizes switching to a write mode which is more suitable in the operation of the virtual FC switch. 
         [0085]    In addition, the determination operation for an input-output performance difference between the storage devices, performed on the basis of the calculated reception-time difference between “Ready” responses from the individual storage devices, may be performed regularly or in response to reception of write I/O from the host  100 . 
       Embodiment 4  
     (1) Operation Performed When Storage Device Corresponding To Copying Source Virtual Disk Has Lower Performance 
       [0086]    When the virtual FC switch  200  receives write I/O from the host  100 , the virtual FC switch  200  updates data in the virtual disk  260  which functions as a copying source during a data copy operation performed between the virtual disks. Furthermore, when the virtual FC switch  200  receives read I/O from the host  100 , the virtual FC switch  200  reads out data from the virtual disk  260  which functions as a copying source. Namely, the response performance of the virtual FC switch  200  for I/O from the host  100  depends on the input-output performance of the storage device corresponding to the virtual disk which functions as a copying source. 
         [0087]    Accordingly, when the storage device  300  corresponding to the virtual disk  260  which functions as a copying source has lower input-output performance than the storage device  400  corresponding to the virtual disk  270  which functions as a copying destination, the performance for responding to the host  100  becomes degraded. 
         [0088]    In the case in which the storage device  300  corresponding to the virtual disk  260  which is a copying source has lower input-output performance than the storage device  400  corresponding to the virtual disk  270  which is a copying destination, if data is written only in the virtual disk  260 , which has functioned as a copying source, when write I/O is received from the host  100  during a data copy operation performed between the virtual disks, as a result, the performance for responding to the host becomes more degraded. Therefore, in this case, a synchronous write operation is performed. 
         [0089]    After the data copy operation performed between the virtual disks is completed and data stored in the virtual disk  260  and data stored in the virtual disk  270  become equivalent to each other (after a copy operation between the virtual disks, performed by the copy control section  243 , is completed), the write control section  241  only writes data, which corresponds to write I/O from the host  100 , into the virtual disk  270  which is a copying destination. 
         [0090]    In addition, for example, by receiving completion notice of the copy operation from the copy control section  243 , the write control section  241  recognizes a time when data stored in the virtual disk  260  and data stored in the virtual disk  270  become equivalent to each other. 
         [0091]    When an operation for writing data in the virtual disk  270 , performed by the write control section  241 , is completed, namely, a data write operation after the time when data stored in the virtual disk  260  and data stored in the virtual disk  270  become equivalent to each other is completed, the bit setting section  242  sets bits in the bit map to “ON”, the bits corresponding to areas in which the data is written in the virtual disk  270 . 
         [0092]    Then, the copy control section  243  scans the bit map included in the bit setting section  242  and, by writing data, stored in the area in the virtual disk  270 , in the virtual disk  260  on the basis of a location at which a bit is set to “ON” on the bit map, makes the virtual disk  270  and the virtual disk  260  equivalent to each other. 
         [0093]    In this way, while equivalence between the virtual disks is maintained, the performance for responding to the host can be improved as far as possible. 
         [0094]    In addition, when a data write destination is selected between the virtual disks in response to write I/O from the host  100 , taking into account degradation of the input-output performance, which is due to temporal load growth in the storage, the data write destination may be selected on the basis of the reception-time difference between “Ready” responses from the individual storage devices, as described in the above-mentioned embodiment 3. 
       (2) Operation According To Update Count of Virtual Disk 
       [0095]    According to the practical example described in the embodiment mentioned above, while a data processing operation which uses, at full value, the performance of the storage device having higher performance is realized, the response performance for write I/O from the host  100  can be improved. 
         [0096]    However, in the case in which write I/O is frequently sent from the host  100  and update of the virtual disk is performed whenever the write I/O is received, a processing load, due to a data copy operation performed between the virtual disks, increases in the virtual FC switch  200 . In addition, when the virtual FC switch and the storage device are connected through a multiprotocol router, transmission and reception of data between the host  100  and the virtual FC switch  200  increases, the data being transmitted and received in accordance with a data copy operation performed between the virtual disks. Accordingly, the transmission and reception of data becomes a load on communication traffic. 
         [0097]    Therefore, the update count of each bit is also stored in the bit map included in the bit setting section  242 . Then, when the bit map is operated, the bit having a high frequency of update is prevented from being scanned each time. Accordingly, a processing load, due to a data copy operation performed between the virtual disks, can decrease in the virtual FC switch  200 . 
       (3) Device Configuration Etc. 
       [0098]    Individual constructional elements included in the virtual FC switch  200  shown in  FIG. 2  or  FIG. 6  are illustrated as functional concepts. Therefore, it is not necessary for the constructional elements to be physically configured to be as illustrated. Namely, the specific configuration of the virtual FC switch  200  which is integrated or distributed is not limited to the illustrated examples. Then, for example, the bit setting section  242  and the copy control section  243 , shown in  FIG. 2 , may be functionally or physically integrated. Alternatively, the write control section  241  and the mode switching control section  244 , shown in  FIG. 6 , may be functionally or physically integrated. 
         [0099]    In this way, all or part of the virtual FC switch  200  may be functionally or physically integrated or distributed in arbitrary units according to various loads and various statuses of use. Furthermore, the individual processing functions performed in the virtual FC switch  200  (refer to  FIGS. 2 and 4 ) may be in whole or in arbitrary part realized, for example, by a central processing unit (CPU) and a program analyzed and executed in the CPU, or as a hardware which uses hard-wired logic. 
         [0100]    As mentioned above, the present invention has been specifically described for better understanding of the embodiments thereof and the above description does not limit other aspects of the invention. Therefore, the present invention can be altered and modified in a variety of ways without departing from the gist and scope thereof. 
         [0101]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.