Abstract:
Some disk devices used for a disk array may shorten a total active time when they are turned on 24 hours a day in comparison to a case where the disk devices are turned on only when needed. The disk device&#39;s total active time shortens when the disk device is turned on 24 hours a day and is operated continuously 24 hours a day. As a result, the disk device&#39;s operation period shortens. Such disk device is not suited for 24-hour continuous operations. The disk array capacity increases year after year. There is a trend of increasing the number of disk devices to be mounted. The power consumption for disk arrays tends to increase. A computer accessing a disk array has a disk control instruction program which allows the disk array to turn on or off specific disk devices in it. The disk array has a disk power supply control instruction reception program and a disk power supply control circuit. The disk power supply control instruction reception program receives an instruction to turn on or off disk devices from the computer. The disk power supply control circuit turns on or off a disk device corresponding to an LU specified by the computer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an external storage device system. More specifically, the present invention relates to a technology for prolonging an operation period of a disk device (hereafter also referred to simply as a disk) and decreasing power consumption of a storage device system (hereafter referred to as a disk array). Here, the disk device&#39;s operation period signifies a period from the time to start using the disk device to the time when the disk device becomes unusable.  
         [0002]     A disk array is a type of storage device systems connected to a computer. The disk array is also referred to as a RAID (Redundant Arrays of Inexpensive Disks) and constitutes a storage device system comprising a plurality of disk devices arranged in an array and a control section to control them. The disk array concurrently operates disk devices to accelerate read requests (requests to read data) and write requests (requests to write data) and to provide data with redundancy. Disk arrays are categorized into five levels depending on types of redundant data to be added and disk array configurations. This is described in non-patent document 1 (Daved A. Patterson, Garth Gibson, and Randy H. Katz, “A Case for Redundant Arrays of Inexpensive Disks (RAID)”, Computer Science Division Department of Electrical Engineering and Computer Sciences, University of California Berkeley, 1988).  
       SUMMARY OF THE INVENTION  
       [0003]     Some disk devices used for a disk array may shorten a total active time when they are turned on 24 hours a day in comparison to a case where the disk devices are turned on only when needed. The total active time, signifies the accumulated time during which the disk device is turned on. Such problem caused by deterioration and wear of parts constituting the disk devices. The disk device&#39;s total active time shortens when the disk device is turned on 24 hours a day and is operated continuously 24 hours a day. As a result, the disk device&#39;s operation period shortens. Such disk device is not suited for 24-hour continuous operations. The disk array capacity increases year after year. There is a trend of increasing the number of disk devices to be mounted. The power consumption for disk arrays tends to increase.  
         [0004]     The present invention discloses a technology to prolong operation times of disk devices constituting a disk array. The present invention also discloses a technology to decrease the disk array&#39;s power consumption.  
         [0005]     A computer accessing a disk array has a disk control instruction program which allows the disk array to turn on or off specific disk devices in it. The disk array has a disk power supply control instruction reception program and a disk power supply control circuit. The disk power supply control instruction reception program receives an instruction to turn on or off disk devices from the computer. The disk power supply control circuit turns on or off a disk device corresponding to an LU specified by the computer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  shows an example of a system configuration according to a first embodiment;  
         [0007]      FIG. 2  shows an example of a computer configuration according to the first embodiment;  
         [0008]      FIG. 3  shows an example of a backup server configuration according to the first embodiment;  
         [0009]      FIG. 4  shows an example of a disk management table according to the first embodiment;  
         [0010]      FIG. 5  shows an example of an update position management table according to the first embodiment;  
         [0011]      FIG. 6  shows an example of obtaining or deleting a snapshot according to the first embodiment;  
         [0012]      FIG. 7  shows a process example of a mirror resynchronization program according to the first embodiment;  
         [0013]      FIG. 8  shows an example of a system configuration according to a second embodiment;  
         [0014]      FIG. 9  shows an example of a computer configuration according to the second embodiment;  
         [0015]      FIG. 10  shows an example of an address map according to the second embodiment; and  
         [0016]      FIG. 11  shows a process example of a disk power supply control instruction program according to the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Embodiments of the present invention will be described in further detail with reference to the accompanying drawings.  
       First Embodiment  
       [0018]     The first embodiment will now be described.  
         [0019]     (1) Description of the System Configuration  
         [0020]      FIG. 1  shows an example of the system configuration according to the first embodiment. In  FIG. 1 , a computer  100  is connected to a disk array  200  via a Fibre Channel interface (hereafter referred to as an FC I/F)  290  of the disk array  200 . A backup server  400  is connected to the disk array  200  via an FC I/F  300  of the disk array  200 . A tape apparatus  500  is connected to the backup server  400 . A management terminal  600  is connected to the disk array  200  via a management I/F  310  of the disk array  200 .  
         [0021]     The disk array  200  comprises a CPU  210 , memory  220 , a cache  240 , and a disk controller  250 . The CPU  210  controls the disk array  200 . The cache  240  stores user data of the disk array  200 . The disk controller  250  controls a plurality of disk devices.  
         [0022]     The disk controller  250  is connected to a plurality of FC disks  271  through  273  (hereafter referred to as an FC disk group) and a plurality of ATA disks  274  through  276  (hereafter referred to as an ATA disk group). The FC disk group and the ATA disk group each use a redundant configuration called RAID. The computer  100  accesses storage areas in the FC disks  271  through  273  as a SCSI logical unit (LU). Likewise, the computer  100  also accesses storage areas in the ATA disks  274  through  276  as a SCSI LU. An LU  261  represents the LU in the FC disks  271  through  273 . An LU  262  represents the LU in the ATA disks  274  through  276 .  FIG. 1  illustrates the LU  261  and the LU  262  each as a storage area covering a plurality of disks. Further, the LU may comprise a storage area on one disk.  
         [0023]     According to the embodiment, the snapshot management program  226  in the disk array  200  manages the LU  261  and the LU  262  duplicatively. When data is written to the LU  261  according to the embodiment, the snapshot management program  226  writes the same data to the LU  262  to duplicate the data. In this manner, storing the same data in the LU  261  and the LU  262  is referred to as mirroring data. The LU  261  contains original data and is referred to as a mirror origin LU. The LU  262  contains a copy of the original data and is referred to as a mirror destination LU. The LU  262  is used to obtain a snapshot. The snapshot is used to back up data on the tape apparatus.  
         [0024]     A disk power supply control circuit  280  is connected to the ATA disks  274  through  276 . The disk power supply control circuit  280  turns on or off each ATA disk independently of the other ATA disks. The disk power supply control circuit  280  is provided with registers corresponding to the ATA disks  274  through  276 . Turning on or off the register turns on or off the corresponding ATA disk.  
         [0025]     The following describes programs and management tables in the disk array  200 .  
         [0026]     The memory  220  of the disk array  200  contains a RAID control program  221 , a disk management program  222 , a disk power supply control instruction reception program  223 , a disk management table  224 , a disk power supply control program  225 , and a snapshot management program  226 . The RAID control program  221  controls the disk array  200 . The disk management program  222  manages the FC disks  271  through  273  and the ATA disks  274  through  276 . The disk management table  224  records operation parameters and operation states of the FC disks  271  through  273  and the ATA disks  274  through  276 . The disk power supply control instruction reception program  223  receives an instruction to turn on or off disk devices from the computer  100 . This instruction is hereafter referred to as a disk power supply control instruction. The disk power supply control program  225  turns on or off the ATA disks  274  through  276  based on instructions received from the computer  100 . The snapshot management program  226  controls snapshots. According to the embodiment, a snapshot instruction program  127  to be described is contained in the computer  100  and issues instructions to delete or obtain snapshots. In addition, the snapshot instruction program  127  issues ModeSelect commands concerning disk power supply control instructions for the ATA disks  274  through  276 . Accordingly, the snapshot management program  226  accepts not only instructions to delete or obtain snapshots, but also disk power supply control instructions for the ATA disks  274  through  276 .  
         [0027]     The snapshot management program  226  also contains a disk access sub-program  227  and an LU mirror sub-program  228 . The disk access sub-program  227  instructs the disk controller  250  to access disks in accordance with requests from the computer  100 . The LU mirror sub-program  228  applies an update to one LU and another predetermined LU and writes the same user data to two LUs. The LU mirror sub-program  228  duplicates data written to the LU  261  for the LU  262 .  
         [0028]     The snapshot management program  226  further contains a non-mirror update monitoring sub-program  230 , a non-mirror update position management sub-program  231 , and a mirror resynchronization sub-program  229 . When the data duplication is inactivated between the LU  261  and the LU  262  (non-mirror state), the non-mirror update monitoring sub-program  230  detects an update to the mirror origin LU. The non-mirror update position management sub-program  231  records the update position in an update position management table  232  to be described later. When the mirror resynchronization is performed to restart data duplication between the LU  261  and the LU  262 , the mirror resynchronization sub-program  229  copies the updated part of the mirror origin LU to the mirror destination LU.  
         [0029]     The update position management table  232  is used to manage the contents of data stored in the mirror origin LU and the mirror destination LU. When data in the mirror origin LU is updated in the non-mirror mode, the update position management table  232  records the update position in the mirror origin LU.  
         [0030]     For example, the update position management table  232  is a bit map as shown in  FIG. 5  and lists LBA set numbers in the mirror origin LU and update bit states corresponding to the LBA set numbers. Each LBA set comprises one, or the same number of LBAs (Logical Block Addresses) that are used as a unit to divide the entire area in the LU from the beginning. The LBA sets are assigned LBA set numbers, i.e., sequence numbers, from the beginning.  
         [0031]     The update bit indicates whether or not the LBA set corresponding to the update bit is updated in the non-mirror mode. The update bit is set to 1 when the LBA set is updated, or to 0 otherwise. The update bit is initially set to 0. For example, the update position management table  232  in  FIG. 5  shows that only the area corresponding to LBA set number 1 is updated in the non-mirror mode.  
         [0032]     Referring now to  FIG. 4 , an example of the disk management table  224  will now be described. The disk management table  224  comprises the following columns. The “disk No.” column shows identification numbers of disk devices in the disk array  200 . The “disk type” column shows the disk type, i.e., whether the disk device is an FC disk or an ATA disk. The “array configuration” column shows to which RAID group the disk device belongs. The “LU No.” column shows to which LU the disk device belongs. The “snapshot pair” column shows whether data stored in the disk is original or copy. The “snapshot status” column shows whether or not data to be stored in an FC disk is mirrored to an ATA disk. The “disk startup status” column shows whether the disk device is turned on or off. The “accumulated time” column shows the accumulated time in which the disk device is turned on. The “life setting” column shows the limit of the accumulated time in which the disk device is turned on.  
         [0033]     Returning now to  FIG. 1 , the management terminal  600  comprises an input section  610  and an output section  620 . The input section  610  accepts settings for the FC disks  271  through  273  and the ATA disks  274  through  276  from a user. The output section  620  displays information about the FC disks  271  through  273  and the ATA disks  274  through  276  to the user.  
         [0034]     The input section  610  inputs parameters to be assigned to the disk management table  224 . The output section  620  outputs information recorded in the disk management table  224 . The input section  610  represents a mouse, a keyboard, and the like. The output section  620  represents a display, an liquid crystal panel, a buzzer, and the like.  
         [0035]     Referring now to  FIG. 2 , an example of the computer  100  will now be described. The computer  100  comprises a CPU  110 , an FC I/F  140 , a communication I/F  160 , and memory  120 . Th CPU  110  controls the computer  100 . The I/F  140  connects the computer  100  to the disk array  200 . The communication I/F  160  provides communication between the computer  100  and the backup server  400 . The memory  120  stores programs.  
         [0036]     The memory  120  stores a database program  126 , a snapshot instruction program  127 , and a disk power supply control instruction program  128 . The snapshot instruction program  127  allows the disk array  200  to obtain snapshots. The disk power supply control instruction program  128  instructs to turn on or off disk devices in the disk array  200 . The CPU  110  to control the computer  100  executes these programs. The snapshot instruction program  127  generates and issues SCSI ModeSelect commands. The ModeSelect commands include an instruction to turn on or off the ATA disks  274  through  276  from the disk power supply control instruction program  128  to the disk array  200 . The ModeSelect commands also include an instruction to delete or obtain snapshots from the snapshot instruction program  127  to the disk array  200 . The database program  126  accesses the LU  261 , i.e., the mirror origin LU while the database is referenced or updated. The database program  126  also controls data update. The database program  126  stops accessing the disk array  200  at a check point that indicates a meaningful separation as database information. When data in the LU  261  becomes meaningful as database information, the database program  126  changes the disk array to a backup mode to back up that data. The backup mode ensures the consistency of data in the LU  261 .  
         [0037]     Referring now to  FIG. 3 , an example of the backup server  400  will now be described. The backup server  400  comprises a CPU  410 , an FC I/F  440 , a communication I/F  460 , a SCSI I/F  450 , and memory  420 . The CPU  410  controls the backup server  400 . The FC I/F  440  connects the backup server  400  with the disk array  200 . The communication interface  460  provides communication between the backup server  400  and the computer  100 . The SCSI I/F  450  connects the backup server  400  with the tape apparatus. The memory  420  stores programs. The memory  420  stores a backup program  426 , an I/O control program  427 , and a tape control program  428 . The backup program  426  backs up data in the disk array  200  onto the tape apparatus  500 . The I/O control program  427  transfers data between the disk array  200  and the tape apparatus  500 . The tape control program  428  control the tape apparatus  500 . The CPU  410  to control the backup server  400  executes these programs.  
         [0038]     There has been described the system configuration of the embodiment.  
         [0039]     (2) Obtaining and Deleting Snapshots  
         [0040]     With reference to the flowchart in  FIG. 6 , the following describes an example of processes performed by the snapshot instruction program  127  and the snapshot management program  226  when snapshots are obtained or deleted. The embodiment assumes that the disk array  200  obtains a snapshot of the LU  261  in synchronization with the timing when the backup server  400  backs up data. It also assumes to turn on the ATA disks  274  through  276  for the LU  262  only during a period in which a snapshot is obtained and data is backed up from mirror destination LU.  
         [0041]     Since the LU  262  stores data of the previously obtained snapshot, the snapshot instruction program  127  running on the computer  100  needs to nullify that snapshot and obtain a new snapshot. Hereafter, this is referred to as deletion of the snapshot. For this purpose, the snapshot instruction program  127  issues a ModeSelect command to the disk array  200  to delete the snapshot (step  2001 ).  
         [0042]     The snapshot management program  226  in the disk array  200  receives the ModeSelect command (step  3001 ). Based on the disk power supply control instruction included in the ModeSelect command, the snapshot management program  226  turns on the ATA disks  274  through  276  (step  3002 ). The disk power supply control instruction turns on the disk devices corresponding to the LU  262 . According to the instruction, the disk array  200  locates ATA disks constituting the LU  262  from the disk management table  224  in  FIG. 4  and turns on the ATA disks  274  through  276  in this embodiment. The LU mirror sub-program  228  is activated to restart duplication of the LU  261  and the LU  262  (step  3003 ). In this embodiment, the activation means starting or initiating a process or a program. The inactivation means terminating or stopping a process or a program. At step  3003 , the LU  262  reflects the update to the LU  261 .  
         [0043]     The snapshot management program  226  inactivates the non-mirror update monitoring sub-program  230  and the non-mirror update position management sub-program  231  to stop recording updates to the LU  261  (step  3004 ). Thereafter, the update bit in the update position management table  232  is not changed by the non-mirror update position management sub-program  231 .  
         [0044]     The snapshot management program  226  then activates the mirror resynchronization sub-program  229 . The mirror resynchronization sub-program  229  references the update position management table  232 . When the LU  261  contains data that differs from data stored in the LU  262 , the mirror resynchronization sub-program  229  copies that data from the LU  261  to the LU  262  to resume the mirror state between the LU  261  and LU  262  (step  3005 ). This is also referred to as resynchronization.  
         [0045]     Upon completion of the resynchronization, the snapshot management program  226  inactivates the mirror resynchronization sub-program  229  (step  3006 ). The snapshot management program  226  sends the termination status of the ModeSelect command to the snapshot instruction program  127  on the computer  100  (step  3007 ). The snapshot instruction program  127  receives the termination status of the ModeSelect command to terminate the operation (step  2002 ).  
         [0046]     The snapshot instruction program  127  on the computer  100  then issues a ModeSelect command to obtain a snapshot on the disk array  200  (step  2003 ).  
         [0047]     The snapshot management program  226  on the disk array  200  receives th ModeSelect (step  3008 ). The snapshot management program  226  activates the non-mirror update monitoring sub-program  230  and the non-mirror update position management sub-program  231  to allow the update position management table  232  to record the position information about the data update applied to the LU  261  (step  3009 ). That is to say, when the LU  261  is updated, the update position management table  232  indicates the update bit set to 1 for the LBA set including the updated LBA to record the update.  
         [0048]     The snapshot management program  226  then inactivates the LU mirror sub-program  228  to stop duplication of the LU  261  and the LU  262  (step  3010 ). This prevents the LU  262  as the mirror destination LU from reflecting the update to the LU  261  as the mirror origin LU.  
         [0049]     The snapshot management program  226  then sends the termination status of the ModeSelect command to the snapshot instruction program  127  on the computer  100  (step  3011 ).  
         [0050]     The snapshot instruction program  127  on the computer  100  receives the termination status of the ModeSelect command from the snapshot management program  226  (step  2004 ). The snapshot instruction program  127  supplies the backup server  400  with an instruction to obtain a backup via the communication I/F  160 . The backup server  400  receives the termination report (step  2005 ). The snapshot instruction program  127  then issues a ModeSelect command to turn off the ATA disks  274  through  276  to the snapshot management program  226  (step  2006 ). The procedure to turn off the ATA disks  274  through  276  is the same as that at step  3002 .  
         [0051]     When receiving the ModeSelect command, the snapshot management program  226  in the disk array  200  turns off the ATA disks  274  through  276  according to the disk power supply control instruction contained in the ModeSelect command (step  3012 ).  
         [0052]     There have been described the operations of the snapshot instruction program  127  and the snapshot management program  226  when snapshots are obtained or deleted.  
         [0053]     (3) Write Operation  
         [0054]     The following describes a write operation of the RAID control program  221  in the disk array  200 . When the computer  100  writes data to the LU  261  to update the contents stored in the LU  261 , the snapshot management program  226  receives a WRITE command and data issued to the LU  261 .  
         [0055]     Let us assume that the LU mirror sub-program  231  is active and that the non-mirror update monitoring sub-program  230  and the non-mirror update position management sub-program  231  are inactive. When receiving write data from the computer  100 , the snapshot management program  226  writes the write data to both the LU  261  as the mirror origin LU and the LU  262  as the mirror destination LU.  
         [0056]     Let us assume that the LU mirror sub-program  231  is inactive and the non-mirror update monitoring sub-program  230  and the non-mirror update position management sub-program are active. In this case, the snapshot management program  226  writes data to the LU  261  as the mirror origin LU. While the update position management table  232  records update positions, the snapshot management program  226  allows this table to set the update bit to 1 for the LBA set including the updated LBA.  
         [0057]     Finally, the status is sent to the computer  100 .  
         [0000]     (4) Mirror Resynchronization  
         [0058]     The following describes an example of the mirror resynchronization to copy data from the LU  261  to the LU  262  at step  3003  in  FIG. 6 .  FIG. 7  exemplifies an operation of the mirror resynchronization sub-program  229  during mirror resynchronization. The mirror resynchronization sub-program  229  checks whether or not the update position management table  232  contains the update bit set to 1 to record the update (step  1001 ). When there is no update bit set to 1 to record the update, the mirror resynchronization is complete. The mirror resynchronization sub-program  229  terminates the process (step  1002 ).  
         [0059]     When there is an update record, the mirror resynchronization sub-program  229  references the update position management table  232 . Based on the LBA set number corresponding to the update bit set to 1, the mirror resynchronization sub-program  229  calculates a read position on the LU  261  and a write position on the LU  262  (step  1003 ).  
         [0060]     The mirror resynchronization sub-program  229  prevents data from being updated to the corresponding read/write position (step  1004 ). Based on the calculated read/write position, the mirror resynchronization sub-program  229  copies the data stored at the read position in the LU  261  as the mirror origin LU to the write position in th LU  262  as the mirror destination LU (step  1005 ). It should be noted that preventing the update is equivalent to inhibiting writing. The update is prevented to ensure the consistency of data between the LU  261  and the LU  262  after the mirror resynchronization.  
         [0061]     The mirror resynchronization sub-program  229  then releases the prevention against updating data at the corresponding read/write position (step  1006 ). The mirror resynchronization sub-program  229  sets 0 to the update bit in the update position management table  232  correspondingly to the mirrored position due to the copy at step  1005  (step  1007 ), and then returns to step  1001 .  
         [0062]     There has been described the mirror resynchronization.  
         [0000]     (5) Disk Operation Time Management  
         [0063]     Using the input section  610  of the management terminal  600 , a user can set the life of each of the FC disks  271  through  273  and the ATA disks  274  through  276  in the disk management table  224 . The life signifies a product cycle estimated from the disk design specifications of the disk device, or a warranty period, or an accumulated operation time over which the disk is highly possibly subject to errors. It is assumed to find an accumulated time by totaling the time during which the disk device is turned on. When the accumulated time exceeds the life, there is a high possibility of causing an error.  
         [0064]     The disk management program  222  records the disk device&#39;s accumulated time in the disk management table  224 . The disk management program  222  counts the time during which the FC disks  271  through  273  and the ATA disks  274  through  276  are turned on. The disk management program  222  records the counted time as the accumulated time in the disk management table  224 . The disk management program  222  compares the life of each of the FC disks  271  through  273  and the ATA disks  274  through  276  with the accumulated time. When the accumulated time exceeds the life, the disk management program  222  displays an alarm on the output section  620  of the management terminal  600 .  
         [0065]     According to the first embodiment as mentioned above, the snapshot management program obtains a snapshot of the LU  262  in synchronization with the timing when the backup server obtains a backup. It is possible to turn on the ATA disk device storing the snapshot data only when the snapshot management program obtains a snapshot and backs it up on the tape. In other words, the disk device can be turned off while no backup is created. This can extend the operation period of disk devices in the disk array  200  and decrease the power consumption of the disk array  200 .  
         [0066]     The disk array  200  receives the ModeSelect command issued from the computer  100 . At this time, the disk controller  250  controls turning on or off the disk device. While the disk device is turned off, the computer  200  does not access the disk device. This can prevent a time-out condition of access requests from the computer  200 .  
       Second Embodiment  
       [0067]     The second embodiment will now be described.  
         [0068]     (1) Description of the System Configuration  
         [0069]      FIG. 8  shows an example of the system configuration according to the second embodiment. The following describes only differences from the first embodiment. Unlike the example in  FIG. 1 , the system in  FIG. 8  uses entirely ATA disks  281  through  289  connected to the disk controller  250 . The computer  100  accesses a storage area in the ATA disks  281  through  283  as a SCSI LU. Likewise, the computer  100  accesses a storage area in the ATA disks  284  through  286  as another SCSI LU. The computer  100  accesses a storage area in the ATA disks  287  through  289  as yet another SCSI LU. The LU in the ATA disks  281  through  283  is defined as an LU  263 . The LU in the ATA disks  284  through  286  is defined as an LU  264 . The LU in the ATA disks  287  through  289  is defined as an LU  265 .  FIG. 8  shows that each of the LU  263 , the LU  264 , and the LU  265  constitutes a storage area extending to a plurality of disks. Each LU may comprise a storage area in one disk. According to an instruction from the disk power supply control program  225 , the disk power supply control circuit  280  turns on or off the ATA disks  281  through  289  independently of the other ATA disks.  
         [0070]     The memory  220  stores an LU protection program  234  that protects the LUs against reading or writing instead of the snapshot management program  226 .  
         [0071]     Unlike the example in  FIG. 4 , the disk management table  224  according to the second embodiment does not record the snapshot pair or the snapshot status.  
         [0072]      FIG. 9  shows an example of the computer  100  according to the second embodiment. Unlike the example in  FIG. 2 , the computer  100  contains an E-mail application program  130 , an LU protection instruction program  131 , and a disk power supply control instruction information storage area  132  instead of the database program  126  and the snapshot instruction program  127 . The LU protection instruction program  131  instructs protection against reading or writing to the LUs. The E-mail application stores electronic mail archives in the memory  120 , sequentially writes data, and stores data at consecutive addresses. This embodiment will be described using the E-mail application as an example. The E-mail application specifies a sequence of LUs to be written. According to the embodiment, the E-mail application accesses the LU  263 , LU  264 , and LU  265  in this order at scheduled times to write data. The embodiment is not limited to the E-mail application.  
         [0073]     The disk power supply control instruction information storage area  132  stores an address map as shown in  FIG. 10 . The address map lists LU numbers, LBA numbers, and disk startup status of disk devices constituting LUs. The disk startup status provides information about power-on/off states of the disk devices.  
         [0074]     The computer further contains a management I/F  150  that is connected to the management terminal  600 .  
         [0075]     There has been described the system configuration according to the second embodiment.  
         [0076]     (2) Operation of the Disk Power Supply Control Instruction Program  
         [0077]     The E-mail application  130  issues a request to write data to the disk array  200  for sequential access. Based on this request, the disk power supply control instruction program  128  in the computer  100  references the address map in  FIG. 10 . The disk power supply control instruction program  128  issues an instruction to turn on disk devices belonging to the LU corresponding to the address for writing. The disk power supply control instruction program  128  issues an instruction to turn off disk devices belonging to the LU corresponding to the address for which the writing is complete. When reading data, a user uses the input section  610  of the management terminal  600  to issue an instruction to turn on disk devices belonging to the LU that stores data to be read.  
         [0078]      FIG. 11  is a flowchart showing an operation example of the disk power supply control instruction program  128  in the E-mail application. Let us assume that the E-mail application  130  makes preparations for writing to the LU  263 . Based on an request from the E-mail application  130  to write to the disk array  200 , the disk power supply control instruction program  128  instructs the disk array  200  to turn on the disks constituting the LU  263  (step  5001 ). The E-mail application  130  starts writing to the LU  263 . The disk power supply control instruction program  128  then determines whether or not the remaining time is shorter than or equal to the specified time until the scheduled time of terminating the access to the LU  263  (step  5002 ). When the remaining time is longer than the specified time until the scheduled time of terminating the access, the disk power supply control instruction program  128  repeats the determination at step  5002 . When the remaining time becomes shorter than the specified time, the disk power supply control instruction program  128  instructs the disk array  200  to turn on the LU  264  (step  5003 ). The disk power supply control instruction program  128  then determines whether or not the E-mail application  130  starts accessing the LU  264  (step  5004 ). When the E-mail application  130  does not start accessing the LU  264 , the disk power supply control instruction program  128  repeats the determination at step  5004 . When the E-mail application  130  starts accessing the LU  264 , the disk power supply control instruction program  128  instructs the disk array  200  to turn off the LU  263  that terminated the access from the E-mail application  130  (step  5005 ). The disk power supply control instruction program  128  then determines whether or not the remaining time is shorter than or equal to the specified time until the scheduled time of terminating the access to the LU  264  (step  5006 ). When the remaining time is longer than the specified time until the scheduled time of terminating the access, the disk power supply control instruction program  128  repeats the determination at step  5006 . When the remaining time becomes shorter than th specified time, the disk power supply control instruction program  128  instructs the disk array  200  to turn on the LU  265  (step  5007 ). The disk power supply control instruction program  128  then determines whether or not the E-mail application  130  starts accessing the LU  265  (step  5008 ). When the E-mail application  130  does not start accessing the LU  265 , the disk power supply control instruction program  128  repeats the determination at step  5008 . When the E-mail application  130  starts accessing the LU  265 , the disk power supply control instruction program  128  instructs the disk array  200  to turn off the LU  264  that terminated the access from the E-mail application  130  (step  5009 ). Finally, the disk power supply control instruction program  128  determines whether or not the access to the LU  265  terminates (step  5010 ). When the access does not terminate, the disk power supply control instruction program  128  repeats the determination at step  5010 . When the access from the E-mail application  130  terminates, the disk power supply control instruction program  128  instructs the disk array  200  to turn off the LU  265  (step  5011 ). There has been described the operation of the disk power supply control instruction program  128 . Like the first embodiment, the disk array  200  receives a disk power supply control instruction from the computer  100 . The disk array  200  references the disk management table  224  to locate the disk device corresponding to the LU to be turned on or off and turns on or off the disk device.  
         [0079]     It may be preferable to instruct the disk array  200  to turn on or off disk devices corresponding to the LU based on a user input. For example, the user may use the input section  610  of the management terminal  600  to issue an instruction to the LU protection instruction program  131  so as to protect an LU against reading or writing. In this case, the disk power supply control instruction program  128  instructs the disk power supply control program  225  of the disk array  200  to turn off disk devices belonging to the LU that is instructed to be protected. The user may use the input section  610  of the management terminal  600  to issue an instruction to the LU protection instruction program  131  so as to release the protection of an LU against reading or writing. In this case, the disk power supply control instruction program  128  instructs the disk power supply control program  225  of the disk array  200  to turn on disk devices belonging to the LU that is instructed to be unprotected.  
         [0080]     The embodiment has been described using the E-mail application as an example. Consequently, the flowchart in  FIG. 11  shows the operation of specifying the sequence of LUs for writing data and sequentially turning on or off the disk devices belonging to the LU at specified times. However, the embodiment is not limited to the E-mail application and is capable of turning on or off disk devices constituting the LU to be accessed for reading or writing.  
         [0081]     The embodiment can operate only disk devices reading or writing data and stop the other disk devices not reading or writing data, making it possible to extend the disk array&#39;s operation period and decrease its power consumption.  
         [0082]     The disk controller  250  controls turning on or off the disk devices in synchronization with the timing when the disk array  200  receives an instruction from the computer  100  to turn on or off the disk devices. While the disk device is turned off, the computer  200  does not access the disk device. This can prevent a time-out condition of access requests from the computer  200 .  
         [0083]     It is possible to extend the operation period and the total active time of the disk devices constituting the disk array and decrease the disk array&#39;s power consumption.