Abstract:
A disk array system including a plurality of disk drives, including: a plurality of first-type disk drives being used to form a first-type logical unit having a plurality of a first-type of chunks; a plurality of second-type disk drives being used to form a second-type logical unit having a plurality of a second-type of chunks; and a storage controller, if the storage controller copies data stored in a source chunk to a destination chunk, selecting the destination chunk from the first-type of chunks or the second-type of chunks.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This is a continuation of U.S. application Ser. No. 11/328,103, filed Jan. 10, 2006 (now U.S. Pat. No. 7,430,572), which is a continuation of U.S. application Ser. No. 10/880,503, filed Jul. 1, 2004 (now U.S. Pat. No. 7,035,883). This application relates to and claims priority from Japanese Patent Application No. 2004-116069, filed on Apr. 9, 2004. The entirety of the contents and subject matter of all of the above is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a storage control system and method for controlling the storage of data to the storage device. 
     2. Description of the Related Art 
     In a system handling large amount of data, such as database system in a data center, data is managed using a storage control system that is built separately from the host computer. This storage control system is a disk array system, such as a RAID (Redundant Array of Independent/Inexpensive Disks), where many storage devices are enclosed in an array. 
     In such a storage control system, backup for copying data recorded in the storage device to another storage device is performed so that the data can be recovered even if the data recorded in the storage device become damaged. In this case, consistency of data must be guaranteed during the copying operation, because if the copied data is updated during the copying operation, a mismatch of data occurs, and the backup becomes meaningless. 
     A method for guaranteeing consistency of the backup data is to stop the program, besides the backup program, that access the data. But in case of a system for which high availability is required, programs cannot be stopped for a long time. Therefore it is necessary to provide a system that creates a stored image of the data at the point of the start of backup without interrupting the programs from updating data during backup. Hereafter the stored image of the data at a certain point in time is called “volume copy”, and the method which allows the system to update the data while having the volume copy of a specified point in time is called “volume copy management method”. Creating of a volume copy is called “volume copy acquisition”, and the data which is the target of volume copy acquisition is called “original data”. Stopping the status where the volume copy exists is called “volume copy deletion”. 
     One of the volume copy management methods is by duplicating data. 
     According to this method, for example, from a normal state where a volume copy is not acquired, a program on the storage control system creates all data on two storage areas (that is, data is duplicated). And the storage control system separates the two storage areas into individual storage areas, provides data existing on one storage area as original data, and provides the data existing on the other storage area as volume copy. 
     While the volume copy is acquired and duplication is being stopped (non-mirroring state), the storage control system enables update for the storage area of original data, and records the location of data update if a data update is generated. In the case of volume copy deletion, the storage control system restarts the duplication of the data, and copies the update data, of which content does not match between the two storage areas, from the storage area of the original data to the storage area provided as volume copy. This copying is called “mirror resynchronization”. In the case of volume copy deletion, the update data may be copied from the storage area of the volume copy to the storage area of the original data, which is the opposite of the above procedure, and such copying is called “reverse mirror resynchronization”. 
     The method of duplicating data by a program on a computer is shown in U.S. Pat. No. 5,051,887, for example. 
     As the storage device, a disk device (e.g. hard disk drive), magnetic tape storage device, or any other form of physical storage device can be used. Disk devices include high reliability high performance disk devices which are expensive but which have high reliability and performance, and low reliability low performance disk devices which are inexpensive but which have a lower reliability and performance than high reliability high performance disk devices. An example of a high reliability high performance device is a disk device having a fiber channel (hereafter “FC disk device”) interface, for which is being standardized by ANSI T11. An example of a low reliability low performance disk device is a disk device having an interface which is different from the interface of a high reliability high performance disk device, such as a disk device having an ATA (AT Attachment) interface (hereafter “ATA disk device), for which is being standardized by ANSI T13. 
     In the case of volume copy acquisition, the user may wish to store the original data in the storage area of the FC disk device (hereafter “FC storage area”), but may also wish to store the volume copy on the storage area of the ATA disk device (hereafter “ATA storage area”) because of the cost difference. 
     If the original data on the FC storage area and the volume copy on the ATA storage area are duplicated and used (that is, the FC storage area and the ATA storage area are in a paired status) to meet the above demand, the I/O processing performance (e.g. read or write speed of data) between the storage control system and the host device will drop, for example, when an I/O request (input/output request) is received from the host device to the storage control system and then processed, because the I/O processing performance of the FC disk device becomes equivalent to the ATA disk device. Also the performance of the ATA disk device becomes a bottleneck when the volume copy is acquired or when mirror resynchronization is performed. 
     Such problems could occur when the attribute levels of the data write destination storage devices, such as performance or reliability, are apart. 
     SUMMARY OF THE INVENTION 
     With the foregoing in view, it is an object of the present invention to minimize the influence of a storage device with a low attribute level when storage devices with a different attribute level are used. Specifically, to minimize the drop of processing speed when data is synchronized among a high performance storage device and a low performance storage device, for example. 
     Other objects of the present invention will be clarified by the description herein below. 
     A storage control system according to a first aspect of the present invention comprises a plurality of physical storage devices with a different attribute level, a plurality of logical units provided on the plurality of physical storage devices, and a control device for writing data received from an external device to a logical unit selected from the plurality of logical units. The plurality of logical units further comprises one or more high level logical units provided on a physical storage device with a high attribute level and one or more low level logical units provided on a physical storage device with an attribute level lower than the high attribute level. Each of the plurality of logical units further comprises two or more chunks. The plurality of chunks constituting the plurality of logical units further include a plurality of high level chunks constituting the one or more high level logical units and a plurality of low level chunks constituting the one or more low level logical units. At least one of the plurality of high level chunks and at least one of the plurality of low level chunks are pool chunks that can be used dynamically. When data in a first chunk selected from the plurality of chunks is written to a second chunk, the control device selects either a high level chunk or a low level chunk from the plurality of pool chunks based on the status of use of the plurality of chunks, and writes the data in the first chunk to the selected chunk. 
     The first embodiment of the storage control system according to the first aspect of the present invention further comprises a primary volume further comprising one or more of the high level logical units selected from the plurality of logical units, a secondary volume further comprising one or more of the high level logical units and one or more of the low level logical units selected from the plurality of logical units, and a memory. The plurality of chunks include a plurality of pool chunks and a plurality of allocated chunks. The plurality of allocated chunks include a plurality of primary chunks constituting the primary volume, and a plurality of secondary chunks constituting the secondary volume. The plurality of secondary chunks include a plurality of high level secondary chunks constituting the high level logical unit in the secondary volume, and a plurality of low level secondary chunks constituting the low level logical unit in the secondary volume. The memory stores the correspondence of the primary chunk of the plurality of primary chunks to the high level secondary chunk or the low level secondary chunk. The control device corresponds a new secondary chunk having an attribute level different from the secondary chunk, selected from the plurality of pool chunks, to an original secondary chunk that is corresponded to a primary chunk selected from the plurality of primary chunks, and writes the data in the original secondary chunk to the new secondary chunk, based on the status of use of the plurality of chunks and the information stored in the memory. 
     The second embodiment of the storage control system according to the first aspect of the present invention is the above mentioned first embodiment, wherein the control device cancels the correspondence of the original secondary chunk and the selected primary chunk after writing the data in the original secondary chunk to the new secondary chunk, and corresponds the new secondary chunk to the selected primary chunk in the memory. 
     The third embodiment of the storage control system according to the first aspect of the present invention is the above mentioned first embodiment, wherein the operating condition of the plurality of chunks is a ratio of the high level secondary volume or the low level secondary volume in the storage capacity provided by the secondary volume. 
     The fourth embodiment of the storage control system according to the first aspect of the present invention is the above mentioned third embodiment, wherein when a threshold of a high level ratio, that is a ratio that the plurality of high level secondary chunks occupy in the storage capacity provided by the secondary volume, is recorded in the memory, if the high level ratio is more than the threshold of the high level ratio, the control device preferentially corresponds the low level secondary chunk to the selected original secondary chunk. Or when a threshold of a low level ratio, that is a ratio that the plurality of low level secondary chunks occupy in the storage capacity provided by the secondary volume, is recorded in the memory, if the low level ratio is more than the threshold of the low level ratio, the control device preferentially corresponds the high level secondary chunk to the selected original secondary chunk. 
     The fifth embodiment of the storage control system according to the first aspect of the present invention is the above mentioned fourth embodiment, wherein when the data update frequency is recorded in the memory for each of the plurality of primary chunks and the low level secondary chunk is preferentially corresponded to the selected primary, the control device preferentially selects the primary chunk of which the data update frequency recorded in the memory is lower, and when the high level secondary chunk is preferentially corresponded to the selected primary chunk, the control device preferentially selects the primary chunk of which the data update frequency is higher. 
     In the sixth embodiment of the storage control system according to the first aspect of the present invention, when the threshold of the data update frequency of the first chunk is recorded in the memory, the control device corresponds a high level chunk to the first chunk if the data update frequency is more than the threshold of the data update frequency. 
     The seventh embodiment of the storage control system according to the first aspect of the present invention is the above mentioned first embodiment, wherein when the threshold of the data update frequency of the primary chunk is recorded in the memory, the control device records the data update frequency in the memory for each of the plurality of primary chunks, and corresponds a high level chunk to the low level secondary chunk if the data update frequency of the selected primary chunk is more than the threshold of the data update frequency, and if a low level secondary chunk is corresponded to the selected primary chunk. 
     In the eighth embodiment of the storage control system according to the first aspect of the present invention, when the threshold of the data update frequency of the first chunk is recorded in the memory, the control device corresponds a low level chunk to the first chunk if the data update frequency is less than the threshold of the data update frequency. 
     The ninth embodiment of the storage control system according to the first aspect of the present invention is the above mentioned first embodiment, wherein when the threshold of the data update frequency of the primary chunk is recorded in the memory, the control device records the data update frequency in the memory for each of the plurality of primary chunks, and corresponds a high level chunk to the low level secondary chunk if the data update frequency of the selected primary chunk is more than the threshold of the data update frequency, and if a low level secondary chunk is corresponded to the selected primary chunk. 
     The tenth embodiment of the storage control system according to the first aspect of the present invention is the above mentioned first embodiment, wherein when a new primary volume, which is the same as the primary volume, is generated, the control device corresponds a high level chunk selected from the plurality of pool chunks to the low level secondary chunk if it is judged from the memory that the low level secondary chunk is corresponded to at least one of the plurality of primary chunks, writes the data in the low level secondary chunk to the corresponded high level chunk, and sets each of a plurality of high level chunks, comprised of the corresponded high level chunk and one or more high level secondary chunks corresponded to one or more primary chunks of the plurality of primary chunks, in the memory as primary chunks. 
     A storage control method according to a second aspect of the present invention is a storage control method for a storage control system comprising a plurality of physical storage devices with a different attribute level, and a plurality of logical units provided on the plurality of physical storage devices. The plurality of logical units further comprises one or more high level logical units provided on a physical storage device with a high attribute level, and one or more low level logical units provided on a physical storage device with an attribute level lower than the high attribute level. Each of the plurality of logical units further comprises two or more chunks. The plurality of chunks constituting the plurality of logical units further comprise a plurality of high level chunks constituting the one or more high level logical units, and a plurality of low level chunks constituting the one or more low level logical units. At least one of the plurality of high level chunks and at least one of the plurality of low level chunks are pool chunks that can be dynamically corresponded. The storage control method comprises steps of selecting either a high level chunk or a low level chunk from the plurality of pool chunks based on the operating condition of the plurality of chunks when data in a first chunk selected from the plurality of chunks is written to a second chunk, and Writing the data in the first chunk to the selected chunk. 
     The first embodiment of the storage control method according to the second aspect of the present invention is the storage control system further comprising a primary volume further comprising one or more of the high level logical units selected from the plurality of logical units, and a secondary volume further comprising one or more of the high level logical units and one or more of the low level logical units selected from the plurality of logical units. The plurality of chunks include a plurality of pool chunks and a plurality of allocated chunks. The plurality of allocated chunks include a plurality of primary chunks constituting the primary volume, and a plurality of secondary chunks constituting the secondary volume. The plurality of secondary chunks include a plurality of high level secondary chunks constituting the high level logical unit in the secondary volume, and a plurality of low level secondary chunks constituting the low level logical unit in the secondary volume. The storage control method comprises steps of corresponding a new secondary chunk having an attribute level different from the secondary chunk, selected from the plurality of pool chunks, to an original secondary chunk that is corresponded to a primary chunk selected from the plurality of primary chunks, based on a memory that stores the correspondence of a primary chunk of the plurality of primary chunks and the high level secondary chunk or the low level secondary chunk, and the operating condition of the plurality of chunks, and writing the data in the original secondary chunk to the new secondary chunk. 
     The second embodiment of the storage control method according to the second aspect of the present invention is the above mentioned first embodiment, wherein the storage control method further comprises steps of canceling the correspondence of the original secondary chunk and the selected primary chunk after writing the data in the original secondary chunk to the new secondary chunk, and corresponding the new secondary chunk to the selected primary chunk in the memory. 
     The third embodiment of the storage control method according to the second aspect of the present invention is the above mentioned first embodiment, wherein the operating condition of the plurality of chunks is a ratio of the high level secondary volume or the low level secondary volume in the storage capacity provided by the secondary volume. 
     The fourth embodiment of the storage control method according to the second aspect of the present invention is the storage control method further comprising a step of corresponding a high level chunk to the first chunk if the data update frequency is more than the threshold of the data update frequency when the threshold of the data update frequency of the first chunk is recorded in the memory. 
     The fifth embodiment of the storage control method according to the second aspect of the present invention is the above mentioned first embodiment, wherein the storage control method further comprises steps of recording the data update frequency in the memory for each of the plurality of primary chunks when the threshold of the data update frequency of the primary chunk is recorded in the memory, and corresponding a high level chunk to the low level secondary chunk if the data update frequency of the selected primary chunk is more than the threshold of the data update frequency, and if a low level secondary chunk is corresponded to the selected primary chunk. 
     The sixth embodiment of the storage control method according to the second aspect of the present invention is the storage control method further comprising a step of corresponding a low level chunk to the first chunk if the data update frequency is less than the threshold of the data update frequency when the threshold of the data update frequency of the first chunk is recorded in the memory. 
     The seventh embodiment of the storage control method according to the second aspect of the present invention is the above mentioned first embodiment, wherein the storage control method further comprises steps of recording the data update frequency to each of a plurality of primary chunks when the threshold of the data update frequency of the primary chunk is recorded in the memory, and corresponding a high level chunk to the low level secondary chunk if the data update frequency of the selected primary chunk is more than the threshold of the data update frequency, and if a low level secondary chunk is corresponded to the selected primary chunk. 
     The eighth embodiment of the storage control method according to the second aspect of the present invention is the storage method further comprising steps of deciding that the low level secondary chunk is corresponded to at least one of the plurality of primary chunks when a new primary volume, which is the same as the above mentioned primary volume, is generated, corresponding a high level chunk selected from the plurality of pool chunks to the low level secondary chunk, writing data in the low level secondary chunk to the corresponded high level chunk, and setting each of the plurality of high level chunks, comprised of the corresponded high level chunk and one or more high level secondary chunks which are corresponded to each of one or more primary chunks out of the plurality of primary chunks, in the memory as a primary chunk. 
     The above mentioned storage control method can be implemented in a single storage control system, for example, or on a network where a plurality of computers are connected. The above mentioned memory may be a single memory, for example, or a plurality of memories. In the case of a plurality of memories, the memories may be distributed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram depicting the concept of an embodiment of the present invention; 
         FIG. 2  shows a configuration example of a storage control system according to an embodiment of the present invention; 
         FIG. 3  shows a configuration example of the volume copy LU registration table  309 ; 
         FIG. 4  shows a configuration example of a volume copy management table; 
         FIG. 5  shows a configuration example of a setup value table  307 ; 
         FIG. 6  shows a configuration example of a failure handling volume copy management table; 
         FIG. 7  shows the processing flow to be executed by the volume copy acquisition program  302 ; 
         FIG. 8  shows the processing flow to be executed by the update frequency threshold swap program  303 ; 
         FIG. 9  shows the processing flow to be executed by the disk usage ratio swap program  304 ; 
         FIG. 10  shows the processing flow to be executed by the failure swap program  305 ; 
         FIG. 11  is a diagram depicting the pair setting of the volume copy which is executed in S 1  in  FIG. 7 ; 
         FIG. 12  are tables for describing the flow of information to be registered in the volume copy management table  308  in the processing flow shown in  FIG. 7 ; 
         FIG. 13  are tables showing an example of the failure handling volume copy management table  306  before update and after update; and 
         FIG. 14  shows a configuration example of the LU management table  911 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 1  is a diagram depicting the concept of the present embodiment. The concept of the present embodiment will be described first with reference to  FIG. 1 . 
     As  FIG. 1  (A) shows, in the present embodiment, a plurality of storage devices, where the attribute level of at least one of reliability and performance is different, coexist. In the plurality of storage devices, one or more fiber channel disk devices (hereafter “FC disk device”)  201 , which are high reliability high performance disk devices, for example, are included as storage devices with a high attribute level. Also as a storage device with a low attribute level, one or more serial ATA disk devices (hereafter “SATA disk device”)  203 , which have a low reliability low performance but which are less expensive than the FC disk device  201 , are included. In the present embodiment, “reliability” refers to durability which can hold data without damage and to probability of failure which may occur, and specifically to MTBF (Mean Time Between Failure). “Performance” refers to the value of the data transfer rate and the speed of response. 
     A plurality of logical units (hereafter LU) are disposed on one or more FC disk devices  201  and on one or more SATA disk devices  203 . Each LU is comprised of a plurality of same sized sub-LUs (hereafter called chunks). And hereafter the LU  205  disposed on the FC disk device  201  is called “FC-LU”, and the LU  207  disposed on the SATA disk device  203  is called “SATA-LU”. The chunk constituting the FC-LU  205  is called “FC-chunk”, and the chunk constituting the FC-LU  206  is called “SATA-chunk”. In  FIG. 1 , the FC-chunk is indicated by a blank frame, and the SATA-chunk is indicated by a hatched frame. 
     In the present embodiment, one virtual LU is comprised of one or more LUs. A virtual LU is also called a “logical volume”. In the present embodiment, a virtual LU is either a primary volume (hereafter PVOL)  204  or a secondary volume (hereafter SVOL)  206 , for example. The PVOL  204  is comprised of one or more FC-LUs  205 . The SVOL  206 , on the other hand, may be comprised of only one or more FC-LUs  205  or only one or more SATA-LUs  207 , or a combination of FC-LU  205  and SATA-LU  207 . Hereafter the FC-LU  205  constituting the PVOL  204  is called the “PVOL-FC-LU  205 ”, and the FC-chunk constituting the PVOL  204  is called the “PVOL-FC-chunk”. The FC-LU  205  constituting the SVOL  206  is called the “SVOL-FC-LU  205 ”, the SATA-LU  207  constituting the SVOL  206  is called the “SVOL-SATA-LU- 207 ”, the FC-chunk constituting the SVOL  206  is called the “SVOL-FC-chunk”, and the SATA-chunk constituting the SVOL  206  is called the “SVOL-SATA-chunk”. The SVOL-FC-chunk and the SVOL-SATA-chunk may commonly be called the “SVOL-chunk”. 
     In the present embodiment, the storage control program can perform management by duplicating of PVOL  204  and SVOL  206 , and in this case, when the data from the host device is written to the PVOL  204 , for example, the same data can be written to the SVOL  206 . Specifically, in the storage control system, the PVOL-FC-chunk and the SVOL-FC-chunk or the SVOL-SATA-chunk are duplicated and managed, and when sub-data (one composing element of data) is written to a PVOL-FC-chunk, the same sub-data is also written to the duplicated SVOL-FC-chunk or SVOL-SATA-chunk. Hereafter storing the same data to the PVOL  204  and the SVOL  206  is called “mirroring”, a pair of PVOL  204  and SVOL  206  is called a “volume pair”, and a pair of PVOL-FC-chunk and SVOL-FC-chunk or SVOL-SATA-chunk is called a “chunk pair”. 
     In the present embodiment, other than the PVOL-FC-chunk, SVOL-FC-chunk and SVOL-SATA-chunk, a pool chunk group  208 , comprised of a plurality of pool chunks which belong to neither PVOL  204  nor SVOL  206 , exists. The plurality of pool chunks constituting the pool chunk group  208  includes a “pool-FC-chunk” which is an FC-chunk, and a “pool-SATA-chunk” which is an SATA-chunk. The storage control system selects a pool chunk from the pool chunk group  208 , allocates it to an SVOL-chunk, copies the sub-data in the SVOL-chunk to the selected pool chunk, sets the pool chunk as SVOL-chunk, and sets the SVOL-chunk, which is the copy source of the sub-data, as a pool chunk, so as to exchange the pool chunk and SVOL-chunk. Allocation of pool chunks to the SVOL-chunk can be determined depending on variety of policy, for example, the update frequency of sub-data in the PVOL-chunk which is chunk pair partner of the SVOL-chunk. 
     Specifically, in the case when the PVOL-FC-chunk “# 2 ” and SVOL-FC-chunk “ 2 ” form a chunk pair, as shown in  FIG. 1  (A), for example, if it is detected that the update frequency of the sub-data in the PVOL-FC-chunk “# 2 ” is lower than a predetermined threshold, the storage control system selects a pool SATA-chunk (e.g. # 51 ”) from the pool chunk group  208 , and copies the sub-data B in the SVOL-FC-chunk “# 2 ” to the selected pool SATA-chunk “# 51 ”. And, as shown in  FIG. 1  (B), the storage control system sets the pool SATA-chunk “# 51 ” as SVOL-FC-chunk “# 51 ” instead of SVOL-FC chunk “# 2 ”, and sets the SVOL-FC-chunk “# 2 ” as the pool SATA-chunk “# 2 ” instead of the pool SATA-chunk “# 51 ”. In this way, if it is detected that the chunk pair partner of PVOL-FC-chunk, of which update frequency of the sub-data is lower than the predetermined threshold, is the SVOL-FC-chunk, then the chunk pair partner of the PVOL-FC-chunk is switched from the SVOL-FC-chunk to the pool SATA-chunk (after swap, the pool SATA-chunk becomes the SVOL-SATA-chunk). 
     Also, in the case when PVOL-FC-chunk “# 8 ” and SVOL-SATA-chunk “# 4 ” form a chunk pair, as shown in  FIG. 1  (A), for example, if it is detected that the update frequency of the sub-data in PVOL-FC-chunk “# 8 ” is higher than the predetermined threshold, the storage control system selects the pool FC-chunk (e.g. “# 53 ”) from the pool chunk group  208 , and copies the sub-data H in the SVOL-SATA-chunk “# 4 ” to the selected pool FC-chunk “# 53 ”. And the storage control system sets the pool FC-chunk “# 53 ” as SVOL-FC-chunk “# 53 ” instead of SVOL-SATA-chunk “# 4 ”, and sets the SVOL-SATA-chunk “# 4 ” as the pool SATA-chunk “# 4 ” instead of the pool FC-chunk “# 53 ”. In this way, when it is detected that the chunk pair partner of the PVOL-FC-chunk, of which the update frequency of the sub-data is higher than the predetermined threshold, is the SVOL-SATA chunk, then the chunk pair partner of the PVOL-FC-chunk is switched from the SVOL-SATA-chunk to the pool FC-chunk (after swap, the pool FC-chunk becomes the SVOL-FC-chunk). 
     The above is the description on the concept of the present embodiment. In the description herein below, it is assumed that one or more FC-LUs  205  constituting the PVOL  204  and one or more FC-LUs  205  and SATA-LU  207  constituting the SVOL  206  exist in a same storage control system, but these may be distributed in a plurality of systems. 
     Now the present embodiment will be described. 
       FIG. 2  shows a configuration example of the storage control system according to the present embodiment. 
     The storage control system  200  is comprised of one or more fiber channel interface devices (hereafter fiber I/F)  290 A and  290 B (this is not limited to an FC interface device, but may be another interface device). For example, a host device  100 , such as a personal computer, may be connected to the fiber I/F  290 A, and a backup server  400  having a tape device (e.g. magnetic tape recording device)  500  may be connected to the fiber I/F  290 B. The backup server  400  reads data in the SVOL  206  via the fiber I/F  290 B, and writes the data to the tape device  500 . If the data in the SVOL  206  is updated during backup, data consistency is lost, so the storage control system  200  does not allow backup by the backup server  400  during mirroring. A case of when backup is allowed, for example, is allowing a data update from the host device  100  to the PVOL  204 , but not to the SVOL  206 , which is a non-mirroring status. 
     The storage control system  200  comprises a management interface (hereafter management I/F)  240 , such as a LAN controller, for example. A management terminal  600 , for managing the storage control system  200 , is connected to the management I/F  240 . 
     The storage control system  200  is comprised of a plurality of disk devices  201  and  203 , a disk controller  250  for controlling the plurality of disk devices  201  and  203 , a cache memory  230  for temporarily storing data to be exchanged between an external device (e.g. host device  100 ) and the disk devices  201  and  203 , a CPU  210  for controlling operation of the storage control system  200 , and a control memory  220  for storing a computer program to be read by the CPU  210  and such control information as a table which is referred to by the CPU  210 . 
     The plurality of disk devices  201  and  203  include one or more FC disk devices  201  and one or more SATA disk device  203 . The one or more FC disk devices  201  has a plurality of FC-LUs  205 , as mentioned above. Each FC-LU  205  can be a composing element of the PVOL  204  or can be a composing element of the SVOL  206 . The one or more SATA disk devices  203 , on the other hand, has one or more SATA-LUs  207 , as mentioned above. The SATA-LU  207  can be a composing element of the SVOL  206 . In the illustrated example, one FC-LU  205  constitutes the PVOL  204 , and one FC-LU  205  and one SATA-LU  207  constitute the SVOL  206 . 
     In the control memory  220 , a basic control program  301 , volume copy acquisition program  302 , update frequency threshold swap program  303 , disk usage ratio swap program  304 , failure swap program  305 , volume copy LU registration table  309 , volume copy management table  308 , setup value table  307 , failure handling volume copy management table  306  and LU management table  911  are stored. 
     The basic control program  301  is a computer program for controlling the basic operation of the storage control system  200 . For example, the basic control program  301  reads data from the LU  205  and  206 A or  206 B according to the I/O request from the host device  100 , and sends it to the host device  100  via the cache memory  230 , or stores the data included in the I/O request in the first LU  205 . 
     The volume copy acquisition program  302  is a computer program for executing volume copy acquisition. 
     The update frequency threshold swap program  303  selects the type of chunk (in other words, either pool FC-chunk or pool SATA-chunk) to be the data swap partner of the SVOL-chunk corresponded to the PVOL-FC chunk, based on whether the update frequency of the PVOL-FC chunk is over the predetermined update frequency threshold. 
     The disk usage ratio swap program  304  selects the type of chunk (in other words, either pool FC-chunk or pool SATA-chunk) to be the copy destination of the sub-data of the SVOL-chunk, based on whether the ratio of the SVOL-FC-chunk (or SVOL-SATA-chunk) in the SVOL  206  is over the predetermined ratio. 
     The failure swap program  305  switches the SVOL-FC-chunk corresponding to the PVOL-FC-chunk to the PVOL-FC chunk when a failure occurs to the PVOL  204  (e.g. when the FC disk device having the PVOL-FC-LU is damaged). When failure occurs to the PVOL  204  and if the SVOL-SATA-chunk corresponding to the PVOL-FC-chunk exists in the SVOL  207 , the failure swap program  305  moves the sub-data in the SVOL-SATA-chunk to the pool FC-chunk selected from the pool chunk group  208 , and switches the pool FC-chunk to the PVOL-FC-chunk. In this way, when failure occurs to the PVOL  204 , the failure swap program  305  constructs a new PVOL by the SVOL-FC-chunk corresponding to the PVOL-FC-chunk and the pool FC-chunk allocated to the SVOL-SATA-chunk corresponding to the PVOL-FC-chunk. 
     Now each table  309 - 306  and  911 , which are stored in the control memory  220 , will be described with reference to  FIG. 3  to  FIG. 6 . 
       FIG. 3  shows a configuration example of the volume copy LU registration table  309 . 
     The volume copy LU registration table  309  is a table where the information on a plurality of LUs in the storage control system  200  is registered. Specifically, in the volume copy LU registration table  309 , a pair number, external LU number, internal LU number, LU capacity, disk type, PVOL/SVOL information and access attribute are registered for each of the plurality of LUs. 
     The pair number is an identification number of the volume pair. 
     The external LU number is an LU number received from an external device, such as a host device. When two or more LUs are provided to the external device as one logical volume, the external LU number becomes a same number for these two or more LUs. In the case of this example, if “ 2 ” is specified from the external device as an LU number, the LU with the internal LU number “ 2 ” and the LU with the internal LU number “ 3 ” are provided to the external device as one SVOL  206 . 
     The internal LU number is an LU number which the storage control system  200  recognizes and manages. In this embodiment, the LU with the internal LU number “ 1 ”, for example, is PVOL-FC-LU  205 , and the LU with the internal LU number “ 2 ” is SVOL-FC-LU  205 , and the LU with the internal LU number “ 3 ” is SVOL-SATA-LU  207 . 
     The LU capacity is a storage capacity of an LU. 
     The disk type is a type of disk device which has the corresponding LU (e.g. interface). Specifically, a disk type indicates either an FC disk device or an SATA disk device. 
     PVOL/SVOL information is information to indicate whether the corresponding LU constitutes a PVOL or an SVOL. 
     The access attribute is information to indicate what kind of access is enabled to the corresponding LU. For example, “R/W enable” indicates that both read and write are enabled, “R only” indicates that read is enabled but that write is disabled, “W only” indicates that write is enabled but that read is disabled, and “R/W disable”, which is not shown in the drawing, indicates that both read and write are disabled. Various types can be used for the access attribute. 
       FIG. 14  shows a configuration example of the LU management table  911 . 
     In the LU management table  911 , a plurality of LU information items, corresponding to the plurality of LUs existing in the storage control system  200  respectively, is registered. The LU information includes such LU information elements as the internal LU # (number), LU capacity, disk type, selectability information and pool chunk #. Here the selectability information is information that indicates if the LU can be selected or not when a volume copy pair is being set (“volume copy” is a stored image of data at a certain point of time). The pool chunk # is a number assigned to a chunk for which the kind of chunk and how the chunk will be used is not defined, and which can be dynamically allocated (therefore it is a pool chunk). 
       FIG. 4  is a configuration example of a volume copy management table. 
     The volume copy management table  308  is a table for managing original data and information on volume copy. The volume copy management table  308  is largely divided into a left column, middle column and right column. 
     In the left column, information on PVOL  204 , which stores original data, is registered. Specifically, in the left column, the LU #, chunk # and update frequency, for example, are registered for each chunk constituting the PVOL  204 . The LU # is an internal LU number of an LU which has a corresponding chunk, and is registered as a PVOL. The chunk # is a serial number of the chunk assigned within an LU. For example, the minimum value of a chunk # is 1, and the maximum value of a chunk # is a value of the quotient obtained when the LU capacity of an LU having a corresponding chunk is divided by the chunk size (the quotient is rounded up if a remainder is generated). The update frequency is a number of times when the sub-data stored in the corresponding chunk is updated, and the initial value is 0. The update frequency is incremented or reset by the CPU  210 , for example. 
     In the middle column, information on an SVOL  206  for storing the volume copy is registered. Specifically, in the middle column, a disk type, LU # and chunk #, for example, are registered for each chunk constituting an SVOL  206 . The disk type is a type of disk device (e.g. interface type) which has an LU having a corresponding chunk. The LU # is an internal LU number of an LU which has a corresponding chunk, and is registered as an SVOL. The chunk # is a serial number of the chunk assigned within the LU. Each row in this middle column corresponds to each row of the left column. In other words, information on a PVOL-FC-chunk is registered in the rows of the left column, and in each row of the middle column, information on an SVOL-chunk (specifically, either an SVOL-FC-chunk or an SVOL-SATA-chunk) is registered. 
     In the right column, information on a swap partner chunk is registered. Here “swap partner chunk” is a chunk to be the swap destination (in other words the shift destination) of the sub-data in the corresponding SVOL-chunk. In this right column, the disk type, LU# and chunk # are registered for each swap partner chunk. As a swap partner chunk, a pool chunk, which has not yet been decided how to be used as a chunk, can be allocated. A blank means that an SVOL-chunk has no swap partner chunk. 
     The CPU  210  refers to this volume copy management table  308 , and can identify the following. For example, the CPU  210  can identify that an FC-chunk with LU # “ 2 ” and chunk # “ 1 ” is corresponded to the PVOL-FC-chunk with LU # “ 1 ” and chunk # “ 1 ” as an SVOL chunk. And the CPU  210  can also identify that a pool SATA-chunk with LU # “ 3 ” and chunk # “ 5 ” is corresponded to the SVOL-chunk as the swap partner chunk. 
     The above is a configuration example of the volume copy management table  308 . In the volume copy management table  308 , a plurality of SVOL-chunks may be corresponded to one PVOL-FC-chunk, or a plurality of swap partner chunks may be corresponded to one SVOL-chunk. 
       FIG. 5  shows a configuration example of the setup value table  307 . 
     In the setup value table  307 , information can be input and registered from the management terminal  600 , for example. The chunk size, swap period, update frequency threshold and disk ratio threshold, for example, are registered in the setup value table  307 . 
     The chunk size is a value for driving an LU into a certain number of byte units to be chunks. The swap period is a value for indicating the schedule to swap the data stored in an SVOL-chunk with a pool FC-chunk or pool SATA-chunk, based on the update frequency threshold or disk ratio threshold (e.g. “weekly” if this is to be done once every week). 
     The update frequency threshold is a threshold for deciding whether the sub-data in an SVOL-chunk corresponding to a PVOL-FC-chunk is stored in a pool chunk. This update frequency threshold is a value to be compared with the number of times sub-data in a PVOL-FC-chunk was updated during the above mentioned swap period (that is, the update frequency recorded in the volume copy management table  308 ) by the write command from an external device. 
     The disk ratio threshold is a threshold of the ratio of the storage capacity created by one or more SVOL-FC-chunks (hereafter SVOL-FC storage capacity) in an entire SVOL  206 . Specifically, if the disk ratio threshold is “0.3”, for example, the SVOL-FC storage capacity is 0.3 (that is 30%) in an entire SVOL  206 , which means that the storage capacity created by one or more SVOL-SATA-chunks (hereafter SVOL-SATA storage capacity) is the remaining 0.7 (that is 70%) in an entire SVOL  206 . 
     A plurality of setup value tables corresponding to the plurality of pair numbers (that is a plurality of volume pairs) may be provided. In this case, the CPU  210  of the storage control system  200  may perform management referring to the setup value tables corresponding to each pair number. This increases flexibility of management. 
       FIG. 6  shows a configuration example of a failure handling volume copy management table. 
     The failure handling volume copy management table  306  is a table for managing which FC-chunk, corresponding to each PVOL-FC chunk in a PVOL  204 , is switched to a PVOL-FC-chunk when a failure occurred to the PVOL  204 . The failure handling volume copy management table  306  is largely divided into a left column, middle column and right column, just like the volume copy management table  308 . The left column has the same configuration of the left column of the volume copy management table  308 , except that the update frequency is not registered in this case. The middle column has the same configuration as the middle column of the volume copy management table  308 . 
     In the right column, information on the failure handling shift destination chunk is registered. Here “failure handling shift destination chunk” is a chunk selected from the pool chunk group  208  as a shift destination of the sub-data in an SVOL-chunk. In the right column, the disk type, LU # and chunk # are registered for each failure handling shift destination chunk. As  FIG. 6  shows, the failure handling shift destination chunk is an FC-chunk in the present embodiment, and the SVOL-chunk corresponded to the failure handling shift destination chunk is an SVOL-SATA chunk. By this, when a failure occurs in a PVOL  204 , and if a copy of the sub-data in the PVOL-FC-chunk exists in the SVOL-SATA-chunk, the sub-data in the SVOL-SATA-chunk is shifted to the FC-chunk corresponded to the SVOL-SATA-chunk (that is, an open chunk on the FC disk device, which is a high reliability high performance disk device). A blank indicates that a failure handling shift destination chunk in an SVOL-chunk is not corresponded. 
     The above is the description on each table  309 - 306  and  911  which are stored in the control memory  220 . 
     Now the processing flow to be executed in the present embodiment will be described below with reference to the above mentioned tables  309 - 306  and  911 . 
       FIG. 7  shows a processing flow to be executed by the volume copy acquisition program  302  which is read by the CPU  210 . 
     The volume copy acquisition program  302  sets a pair of volume copy (step S 1 ). In this case, information is registered in the volume copy LU registration table  309  according to the flow shown in  FIG. 11 , for example. 
     For example, the volume copy acquisition program  302  displays the copy pair setup screen  912  on the display screen of the management terminal  600 . The LU # (internal LU number) of the PVOL-LU constituting the PVOL  204  and the LU # of a plurality of SVOL-LUs (one or more FC-LU and one or more SATA-LU) constituting the SVOL are input in the copy pair setup screen  912 . When PVOL-FC-LU (FC-LU constituting the PVOL  204 ), SVOL-FC-LU (FC-LU constituting the SVOL  206 ) and SVOL-SATA-LU (SATA-LU constituting the SVOL  206 ) are selected from the plurality of LUs not registered in the volume copy LU registration table  309  (in other words, LUs which are selectable in the LU management table  911 ), the respective LU #s are input in this copy pair setup screen  912 , and the volume copy acquisition program  302  writes each LU # which was input to the volume copy LU registration table  309 . The volume copy acquisition program  302  also acquires other LU information elements (e.g. disk type) corresponding to the LU #, which was input, from the LU management table  911 , and writes the acquired LU information elements to the volume copy LU registration table  309 . For example, if the internal LU # “ 1 ” is input as the PVOL-FC-LU, the internal LU # “ 2 ” is input as the SVOL-FC-LU, and the internal LU # “ 3 ” is input as the SVOL-SATA-LU, then in the LU management table  911 , the volume copy acquisition program  302  switches the selectability information corresponding to each internal LU # “ 1 ”-“ 3 ” from selectable to unselectable, and constructs the volume copy LU registration table  309  shown in  FIG. 3 . 
     Now S 2  and later processing will be described with reference again to  FIG. 7 . 
     The volume copy acquisition program  302  receives input of the values of the setup value table  307  from the user via the management terminal  600 , for example. When various values (that is chunk size, swap period, update frequency threshold and disk ratio threshold) are input, the volume copy acquisition program  302  registers the values, that were input, in the setup value table  307  (S 2 ). 
     Then the volume copy acquisition program  302  registers the PVOL-FC-chunk in the volume copy management table  308  (S 3 ). A specific example of this processing will be described with reference to  FIG. 12  (A). For example, the volume copy acquisition program  302  calculates the number of chunks based on the LU capacity and the chunk size registered in the setup value table  307 , for the PVOL-FC-LU which is set in the volume copy LU registration table  309 . And the volume copy acquisition program  302  assigns the chunk # as a serial number to the calculated number of chunks respectively, and registers the assigned chunk # and the LU # of the PVOL-FC-LU thereof in the volume copy LU registration table  309 . The volume copy acquisition program  302  inputs “0” to the update frequency of each PVOL-FC-chunk as an initial value. When data is updated by a write command from an external device, such as the host device  100 , the volume copy acquisition program  302  adds “1” to the update frequency corresponding to the PVOL-FC-chunk for which data was updated. 
     Then the volume copy acquisition program  302  registers the SVOL-FC-chunk in the volume copy management table  308  (S 4 ). A specific example of this processing will be described with reference to  FIG. 12  (B). For example, the volume copy acquisition program  302  calculates the number of chunks of the PVOL-FC-LU and assigns a chunk # to each chunk in a same method as the case of registering the PVOL-FC-chunk, and registers the assigned chunk # and the LU # of the SVOL-FC-LU thereof in the volume copy LU registration table  309 . 
     If an SVOL-chunk was set for all the PVOL-FC-chunks in S 4  (Y in S 5 ), the volume copy acquisition program  302  moves to S 11  without executing the later mentioned operations in S 6 -S 10 . 
     When the SVOL-chunk is not set for at least one PVOL-FC-chunk, and the SVOL-FC-chunk remains without being corresponded with the PVOL-FC-chunk in S 4  (N in S 5  and Y in S 6 ), the volume copy acquisition program  302  executes the operation in S 4  for the remaining PVOL-FC-chunk. 
     When the SVOL-chunk is not set for at least one PVOL-FC-chunk and SVOL-FC-chunks do not remain (N in S 5  and N in S 6 ) in S 4 , the volume copy acquisition program  302  calculates the number of chunks for SVOL-SATA-LU and assigns a chunk # to each chunk, just like the case of SVOL-FC-LU, and registers the allocated chunk # and LU # in the volume copy LU registration table  309  (S 7 ).  FIG. 12  (C) shows an example of this result. 
     When the SVOL-chunk is set for all the PVOL-FC-chunks in S 7  (Y in S 8 ), the volume copy acquisition program  302  moves to S 11  without executing the later mentioned operations in S 9 -S 10 . 
     When the SVOL-chunk is not set for at least one PVOL-chunk and the SVOL-SATA-chunk remains without being corresponded to the PVOL-FC-chunk in S 7  (N in S 8  and Y in S 9 ), the volume copy acquisition program  302  returns to the beginning of S 7 . 
     When the SVOL-chunk is not set for at least one PVOL-FC-chunk and selectable SVOL-SATA-chunk does not remain at S 7  (N in S 8  and N in S 9 ), the volume copy acquisition program  302  outputs a warning, to add an LU to SVOL, on the management terminal  600 , for example (S 10 ), because this means that the number of chunks of SVOL are insufficient. 
     If data is stored in PVOL after Y in S 5  and Y in S 8 , the volume copy acquisition program  302  judges the correspondence of the chunk of PVOL and the chunk of SVOL referring to the volume copy management table  308 , and stores the sub-data in the PVOL-FC-chunk and the SVOL-chunk corresponded thereto (S 11 ). Specifically, the volume copy acquisition program  302  duplicates the sub-data registered in the cache memory  230 , stores one sub-data on the cache memory  230  in the PVOL-FC-chunk, and stores the other sub-data on the cache memory  230  in the SVOL-chunk corresponded to that PVOL-FC-chunk. 
     After S 11 , the volume copy acquisition program  302  sets the access attribute of each LU constituting the SVOL to R/W enable (enabling both read and write) in the volume copy LU registration table  309  at an arbitrary timing (S 12 ). 
     In this processing, data may be written to the PVOL by random write access, or data may be written by sequential write access. In the case of random write access, for example, the volume copy acquisition program  302  receives a corresponding write command for each PVOL-FC-chunk, and stores the sub-data to the PVOL-FC-chunk and the SVOL-chunk corresponded thereto each time one write command is processed. In the case of sequential write access, for example, the volume copy acquisition program  302  receives write commands corresponding to a plurality of PVOL-FC-chunks (e.g. all the PVOL-FC-chunks), and when one write command is processed, sub-data is written to the plurality of PVOL-FC-chunks and the plurality of SVOL-chunks corresponding thereto sequentially from a smaller chunk #. 
       FIG. 8  shows the processing flow to be executed by the update frequency threshold swap program  303  which is read by the CPU  210 . 
     When the time of the swap period registered in the setup value table  307  comes (S 21 -A), or when the user inputs a data swap instruction via a predetermined terminal (e.g. management terminal  600  or host device  100 ) (S 21 -B), the basic control program  301  starts up the update frequency threshold swap program  303 . 
     When it is detected that the volume pair selected from one or more volume pairs (hereafter target volume pair (s)) are in non-mirror status, the update frequency threshold swap program  303  sets the access attribute of each LU constituting the SVOL of the target volume pair to update disable (e.g. R only) in the volume copy LU registration table  309 . When the target volume pair is in mirror status, a warning is output (S 22 ). Whether the target volume pair is non-mirror status or mirror status can be judged by referring to the pair management table  914  (e.g. provided in the control memory  220 ) in which the status information corresponding to each pair number (information to indicate mirror status or non-mirror status) is registered. Mirror status is a status where data is duplicated. In other words, in this status if data is updated in the PVOL, the same updated data is copied to the SVOL (in other words, the SVOL is synchronized with the PVOL). Non-mirror status is a status where duplication is not being done, in other words, in this status even if data is updated in the PVOL, the updated data is not written to the SVOL (in other words, the SVOL is not synchronized with the PVOL). 
     The update frequency threshold swap program  303  compares the update frequency of the PVOL-chunk registered in the first row of the volume copy management table  308  and the update frequency threshold registered in the setup value table  307  (S 23 ). 
     When it is judged that the update frequency of the PVOL-FC-chunk is the update frequency threshold or more (Y in S 23 ) and the SVOL-chunk of the chunk pair partner of the PVOL-FC-chunk is an FC-chunk based on the judgment according to the volume copy management table  308  (Y in S 24 ) in S 23 , the update frequency threshold swap program  303  advances to the later mentioned S 28 . 
     When the update frequency of the PVOL-FC-chunk is the update frequency threshold or more (Y in S 23 ) and the SVOL-chunk corresponding to the PVOL-FC-chunk is not an FC-chunk based on the judgment according to the volume copy management table  308  (N in S 24 ) in S 23 , the update frequency threshold swap program  303  selects a pool FC-chunk from the plurality of pool chunks, and writes the chunk # and LU # of the selected pool FC-chunk to the right column (column of the swap partner chunk) of the volume copy management table  308  with corresponding to the SVOL-chunk which is not the above FC-chunk (that is, SVOL-SATA-chunk) (S 25 ). By this, for the SVOL-SATA-chunk corresponded to the PVOL-FC-chunk of which the update frequency of the sub-data is the update frequency threshold or more, the FC-chunk existing on a high reliability high performance disk device is corresponded as the data swap partner. The chunk # corresponded to the SVOL-SATA-chunk is selected from a plurality of pool chunks, therefore it is a chunk # not registered on the volume copy management table  308 . 
     When the update frequency of the PVOL-FC-chunk is less than the update frequency threshold (N in S 23 ) and the SVOL-chunk corresponding to the PVOL-FC-chunk is an SATA-chunk based on the judgment according to the volume copy management table  308  (Y in S 26 ) in S 23 , the update frequency threshold swap program  303  advances to the later mentioned S 28 . 
     When the update frequency of the PVOL-FC-chunk is less than the update frequency threshold (N in S 23 ) and the SVOL-chunk corresponding to the PVOL-FC-chunk is not an SATA-chunk (N in S 26 ) based on the judgment according to the volume copy management table  308  in S 23 , the update frequency threshold swap program  303  selects the SATA-chunk from a plurality of pool chunks, and writes the chunk # and LU # of the selected SATA-chunk to the right column (column of the swap partner chunk) of the volume copy management table  308  with corresponding to the SVOL-chunk which is not the above SATA-chunk (that is the SVOL-FC-chunk) (S 27 ). By this, for the SVOL-FC-chunk corresponded to the PVOL-FC-chunk of which the update frequency of the sub-data is less than the update frequency threshold, the SATA-chunk existing on a low reliability low performance but inexpensive disk device is corresponded as the data swap partner. 
     In the case of Y in S 24 , Y in S 26  and a chunk existing in the swap destination (that is, a selectable pool chunk does not exist) in S 25  or S 27  (N in S 28 ), the pools of chunks at the swap destination are insufficient, so the update frequency threshold swap program  303  outputs a warning, to have the SVOL add an LU or change the threshold, to the management terminal  600  or host device  100 , for example (S 29 ). The later mentioned processings in S 31 -S 36  may be executed without confirming all the chunks. In this case, processing in S 22  or later may be executed after these processings. 
     When the swap destination chunks are sufficient in S 28  (Y in S 28 ), the update frequency threshold swap program  303  judges whether the comparison processing of the update frequency and the update frequency threshold has been completed for all the PVOL-FC-chunks (S 30 ). 
     If there is a PVOL-FC-chunk for which comparison processing has not been executed in S 30  (N in S 30 ), the update frequency threshold swap program  303  returns to S 23 , and executes the processings in S 23 -S 28  for the next PVOL-FC-chunk. 
     If it is judged that the comparison processing has been completed for all the PVOL-FC-chunks in S 30  (Y in S 30 ), the update frequency threshold swap program  303  judges whether data is being read from the SVOL by backup so as to execute processing to swap data in the corresponding copy destination chunk to the swap destination chunk having the chunk # registered in the volume copy management table  308  (S 31 ). 
     When data is being read from the SVOL in S 31  (Y in S 31 ), the update frequency threshold swap program  303  outputs a warning to stop the reading operation, such as backup, or to stop the swap program (S 32 ). 
     When data is not being read from the SVOL in S 31  (N in S 31 ), the update frequency threshold swap program  303  sets the access attribute of each LU constituting the SVOL to read disable (e.g. R/W disabled) (S 33 ). 
     After S 33 , the update frequency threshold swap program  303  shifts the sub-data in the SVOL-chunk having a chunk # registered in the middle column (SVOL column) to the swap destination chunk corresponded to the SVOL-chunk based on the volume copy management table  308  (S 34 ). When this completes, the update frequency threshold swap program  303  overwrites the content of the swap destination chunk (disk type, LU # and chunk #) on the content of the SVOL-chunk corresponding to the swap destination chunk in the volume copy management table  308 , and deletes the content of the swap destination chunk from the right column (swap partner chunk column) (S 35 ). In this case, the update frequency threshold swap program  303  may register the deleted content of the swap destination chunk (e.g. chunk #) to the LU management table  911 , for example, as a content of a pool chunk. 
     After S 35 , the update frequency threshold swap program  303  sets the access attribute of each LU constituting the SVOL to read enable (e.g. R only) (S 36 ). After S 36 , if processing after S 31  had been executed without performing the above comparison processing for all the PVOL-FC-chunks, the update frequency threshold swap program  303  returns to S 3 , as shown by the dotted line. 
     After S 36 , the update frequency threshold swap program  303  resets the update frequency of each v on the volume copy management table  308  to the initial value (S 37 ). And the update frequency threshold swap program  303  sets the access attribute of each LU constituting the SVOL to updatable (e.g. R/W enabled) (S 38 ). 
     The above is the processing flow to be executed by the update frequency threshold swap program  303 . 
       FIG. 9  shows the processing flow to be executed by the disk usage ratio swap program  304  which is read by the CPU  210 . 
     When the swap period registered in the setup value table  307  comes (S 41 -A), or when the user inputs a data swap instruction via a predetermined terminal (e.g. management terminal  600  or host device  100 ) (S 41 -B), the basic control program  301  starts up the disk usage ratio swap program  304 . 
     The disk usage ratio swap program  304  sets the access attribute of each LU constituting the SVOL of the target volume pair to update disable (e.g. R only) in the volume copy LU registration table  309  with the same method as S 22  in  FIG. 8  (S 42 ). 
     Then the disk usage ratio swap program  304  sorts a plurality of rows on the volume copy management table  308  in the descending order of data update frequency (S 43 ). Hereafter the number of rows in the volume copy management table  308  is assumed to be n and the row number after the above sorting is i, and the volume copy management table  308  after the above sorting is P (i). The disk usage ratio swap program  304  executes the following S 44  and the later processings in the sequence of lower row number i after the sorting (in other words, starting from the higher data update frequency). 
     The disk usage ratio swap program  304  selects one row number i from the plurality of row numbers after the above sorting, and compares the value i/n when the selected row number i is divided by the number of rows n and the disk ratio threshold T registered in the setup value table  307  (S 44 ). 
     When i/n is T or more as a result of S 44 , the disk usage ratio swap program  304  judges whether the SVOL-chunk corresponding to the PVOL-FC-chunk with the above selected row number i is an FC-chunk or not (S 45 ). In S 45 , if a positive judgment result is acquired (Y in S 45 ), the disk usage ratio swap program  304  executes the later mentioned processing in S 51 . If a negative judgment result is acquired in S 45  (N in S 45 ), the disk usage ratio swap program  304  selects an FC-chunk from a plurality of pool chunks, and sets the selected FC-chunk in P(i) as a swap partner chunk of the above mentioned corresponded SVOL-chunk (S 46 ). At this time, if a selectable FC-chunk does not exist in the plurality of pool chunks (N in S 49 ), the disk usage ratio swap program  304  outputs a warning, to increase the selectable pool FC-chunks, to the user (S 50 ), and if not executes S 51  (Y in S 49 ). 
     When i/n is less than T as a result of S 44 , the disk usage ratio swap program  304  judges whether the SVOL-chunk corresponding to the PVOL-FC-chunk with the above mentioned selected row number i is an SATA-chunk or not (S 47 ). In S 45 , if a positive judgment result is acquired (Y in S 47 ), the disk usage ratio swap program  304  executes the later mentioned processing in S 51 . If a negative judgment result is acquired in S 47  (N in S 47 ), the disk usage ratio swap program  304  selects an SATA-chunk from a plurality of pool chunks, and sets the selected SATA-chunk in P (i) as a swap partner chunk of the above mentioned corresponded SVOL-chunk (S 48 ). At this time, if a selectable SATA-chunk does not exist in the plurality of pool chunks (N in S 49 ), the disk usage ratio swap program  304  outputs a warning, to increase the selectable pool SATA-chunks, to the user (S 50 ), and if not executes S 51  (Y in S 49 ). 
     The disk usage ratio swap program  304  executes the above mentioned processings in S 44 -S 48  for all the row numbers i (N in S 51 ), and when this processing is completed for all the row numbers i (Y in S 51 ), the same processing as S 31 -S 38  in  FIG. 8  are executed. 
     In the above processing flow, if the SVOL-chunk corresponding to the PVOL-FC-chunk is not a chunk with an appropriate attribute level according to the update frequency of the sub-data in the PVOL-FC-chunk, the sub-data in the SVOL-chunk is shifted to another chunk with an appropriate attribute level, and the ratio of the SVOL-FC storage capacity in SVOL  206  (in other words, the ratio of the SVOL-SATA storage capacity) is adjusted to the disk ratio threshold T. 
       FIG. 10  shows the processing flow to be executed by the failure swap program  305  which is read by the CPU  210 . In the description below, to make the description simple and clear, the PVOL when failure occurs is called the “original PVOL”, the SVOL when failure occurs is called the “original SVOL”, and the PVOL and SVOL created by the failure swap program are called the “new PVOL” and the “new SVOL” respectively. Also in the description below, it is assumed that a failure occurred to the original PVOL, while the write data received from the host device  100  is being written to the original PVOL. 
     When a failure occurs to the original PVOL (S 61 ), the basic control program  301  detects this, and starts up the failure swap program  305 . 
     The failure swap program  305  sets the access attribute of each LU constituting the original PVOL where a failure occurred to read disable (e.g. R/W disable) in the volume copy LU registration table  309  (S 62 ). The failure swap program  305  saves the write data from the host device  100  to the original PVOL in the cache memory  230  or in another LU (S 63 ). 
     Then the failure swap program  305  selects an FC-chunk out of a plurality of pool chunks, and writes the chunk # and LU # of the selected FC-chunk in the right column of the failure handling management table  306  (failure handling shift destination chunk column) with corresponding to the original SVOL-SATA-chunk (S 64 ). At this time, if there are sufficient allocatable FC-chunks (Y in S 65 ), the failure swap program  305  executes the later mentioned S 67 , and if no allocatable FC-chunk exists in the plurality of pool chunks, the failure swap program  305  outputs a warning to notify that FC-LU is insufficient (N in S 65  and S 66 ). 
     In S 67 , the failure swap program  305  shifts the sub-data in the original SVOL-SATA-chunk, having a chunk # registered in the middle column (SVOL column), to the swap destination chunk (FC-chunk) corresponded to that original SVOL-SATA-chunk, based on the failure handling volume copy management table  306  (S 67 ). 
     Then the failure swap program  305  overwrites the content (LU # and chunk #) of the swap destination chunk on the content of the original PVOL-FC-chunk corresponding to that swap destination chunk in the failure handling volume copy management table  306 , and deletes the content of the swap destination chunk. For the original PVOL-FC-chunk where no swap destination chunk exists, the failure swap program  305  overwrites the content (LU# and chunk #) of the original SVOL-FC-chunk on the content of the original PVOL-FC-chunk, and deletes the content of that original SVOL-FC-chunk (S 68 ). If failure occurs to the original PVOL by this processing, the original SVOL-chunk is switched to the new PVOL-chunk if the original SVOL-chunk corresponding to the original PVOL-FC-chunk is an FC-chunk. And if the original SVOL-chunk corresponding to the original PVOL-chunk is an SATA-chunk, then an FC-chunk selected from the plurality of pool chunks is corresponded to that SATA-chunk, and the selected FC-chunk is switched to the new PVOL-chunk. As a result, each of the plurality of original PVOL-chunks registered in the left column (original data column) of the failure handling volume copy management table  306  is switched to the original SVOL-FC-chunk or the above mentioned selected FC-chunk, and a new PVOL comprised of the original SVOL-FC-chunk and the above mentioned selected FC-chunk are generated.  FIG. 13  (A) shows the status of the failure handling volume copy management table  306  before update, and  FIG. 13  (B) shows the status of the table  306  after update. 
     By this update processing in S 68 , the plurality of FC-chunks out of the original SVOL-chunks are all switched to new PVOL-FC-chunks, so new SVOL-chunks for this amount of chunks are required. So the failure swap program  305  selects the required number of FC-chunks from the plurality of pool chunks, and registers the selected FC-chunks in the middle column of the failure handling volume copy management table  306  and the volume copy management table  308  as new SVOL-chunks. And the failure swap program  305  copies the data in the FC-chunks, which were original SVOL-chunks, to the new SVOL-chunks (S 69 ). 
     Then the failure swap program  305  writes the write data saved in S 63  to the new PVOL. The failure swap program  305  provides the information on the new PVOL (e.g. external LU number and storage capacity) to the host device  100  when a predetermined inquiry command (e.g. inquiry command based on SCSI protocol) from the host device  100  (S 69 ). By this, the host device  100  can recognize the new PVOL. 
     The above is the processing flow executed by the failure swap program  305 . In the processing in S 68 , the failure swap program  305  may update the content of the volume copy management table  308  in the same way. The content of the original SVOL-chunk switched to the new PVOL-chunk may be deleted from the failure handling volume copy management table  306  and volume copy management table  308 . 
     According to the above mentioned embodiment, each of the plurality of LUs existing on the storage control system  200  is divided into a plurality of chunks. The PVOL is comprised of only FC-chunks, but an SVOL is comprised of both FC-chunks and SATA-chunks. And to each of the plurality of SVOL-chunks, either an FC-chunk or SATA-chunk selected from the plurality of pool chunks is dynamically corresponded. The type of corresponded chunk is switched depending on the status of data write to the PVOL. Specifically, to the SVOL-SATA-chunk corresponding to the PVOL-FC-chunk with a high data update frequency, for example, an FC-chunk existing on a high reliability high performance FC disk device is corresponded, and to an SVOL-FC-chunk corresponding to a PVOL-FC-chunk with a low data update frequency, a SATA-chunk existing on a low reliability low performance but inexpensive SATA disk device is corresponded. By this, the drop in speed of copy processing by a low reliability low performance disk device and an increase in cost can both be addressed. 
     According to the above mentioned embodiment, the storage capacity ratio of an SVOL-FC in an SVOL (in other words the SVOL-SATA storage capacity ratio) is automatically adjusted to be a preset disk ratio threshold. Therefore the FC storage capacity ratio in an SVOL becomes the ratio desired by the user, even if the user does not periodically perform complicated settings. 
     According to the above mentioned embodiment, the storage capacity ratio of an SVOL-FC is adjusted in the sequence of SVOL-chunks corresponding to the PVOL-FC-chunks with a higher update frequency. By this, the storage capacity ratio of an FC is efficiently adjusted. 
     According to the above mentioned embodiment, when a failure occurs to the original PVOL, even if the chunk corresponded to the original PVOL-chunk is an SATA-chunk, the data in the SATA-chunk is shifted to the FC-chunk selected from the plurality of pool chunks, and the FC-chunk is switched to the new PVOL-chunk. By this, the new PVOL-chunk constituting the new PVOL can be an FC-chunk regardless the type of chunk corresponded to the original PVOL-chunk. 
     An embodiment of the present invention was described above, but this is just an example in order to describe the present invention, and it is not intended to limit the scope of the present invention to only this embodiment. The present invention can be implemented by various other embodiments. For example, the above embodiment can be applied to a storage device with an attribute level other than reliability or performance. The above embodiment can be applied even when a plurality of LUs are distributed in two or more devices (e.g. a PVOL exists in a storage control system  200  and an SVOL exists in another storage control system). Also in the above embodiment, there are two levels of disk devices, one has high reliability and high performance, and the other has low reliability and low performance, but more levels of disk devices may be used. Also in the present embodiment, a plurality of thresholds may be used for at least one of the update frequency threshold and the disk ratio threshold, for a more refined adjustment. Two types of disk ratio thresholds may be provided for the FC storage capacity ratio and the SATA storage capacity ratio. The data update frequency is a number of times of data updates in a predetermined period, but may simply be an update frequency, regardless the period.