Abstract:
To optimize performance and power consumption of a storage system having many disk drives, the storage system contains a plurality of volumes. A first number of the volumes belong to a first volume set. The first number of the remaining volumes belong to a second volume set. The volumes that belong to the first volume set are allocated dispersedly to a second number of disk drives. The volumes that belong to the second volume set are allocated dispersedly to a third number of disk drives, the third number being larger than the second number. A computer selects one of the first volume set and the second volume set based on a predetermined condition to store data dispersedly in the volumes belonging to the selected volume set. The computer stops spinning of disks in the disk drives to which none of the volumes belonging to the selected volume set are allocated.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application JP2008-156523 filed on Jun. 16, 2008, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     This invention relates to management of power consumption and performance of a storage system. More particularly, a technology disclosed herein relates to a file system that provides data access in accordance with a policy regarding power consumption and performance. 
     Storage systems are becoming larger in scale as the amount of data handled by computer systems increases. Large-scale storage systems are used in data centers and the like, and their increased power consumption is posing a problem. 
     A technology called massive arrays of idle disks (MAID) has been proposed to deal with this problem (see U.S. Pat. No. 7,035,972). The MAID reduces the power consumption of a storage system by stopping the spinning of the disks in a disk drive that is not in use. 
     The MAID is applied to a storage system that is capable of power supply control (see JP 2005-157710 A). According to JP 2005-157710 A, in the case where a plurality of disk drives constitute redundant arrays of inexpensive disks (RAID), power supply is controlled separately for each management unit of a plurality of disk drives constituting the RAID. 
     When access to a disk drive that has stopped the spinning of its disks is requested, the access is executed after the disk drive is started up and the disks start spinning. The period of time required to start up the disk drive lowers the access performance. JP 2007-86843 A and JP 2007-164650 A disclose a technology of preventing the lowering of access performance from this cause by selecting a disk drive that is to store data based on the access frequency of the data. Specifically, the data storage location is selected such that data with a high access frequency is stored in a disk drive that is controlled to stop the spinning of its disks for a short period of time (or a disk drive that is controlled to never stop the spinning of its disks). 
     Another known technology of improving the access performance is to store data in a plurality of disk drives in a dispersed manner. Generally, the access performance (specifically, I/O throughput) improves more when data is dispersed among more disk drives. However, when data of one file, for example, is stored dispersedly among a plurality of disk drives, all those disk drives have to be accessed to read the file. Dispersing data in many disk drives therefore means that many disk drives need to be started up to read a single file. 
     JP 2007-286975 A discloses a technology of determining the placement of data in accordance with a policy regarding the above-mentioned access performance and start-up time. Specifically, in the case where improving the access performance is given priority, data is stored dispersedly in a relatively small number of disk drives, whereas data is stored dispersedly in a relatively large number of disk drives in the case where keeping the disk drive start-up time short is given priority. 
     SUMMARY 
     As mentioned above, dispersing data to be stored among more disk drives improves the access performance more. If the MAID is applied to those disk drives, however, dispersing data to be stored among more disk drives means more disk drives to be started up and accessed, thereby lowering the effect of reducing power consumption. Power consumption and the achieved performance thus have a trade-off relation. 
     Another fact to be considered is that the access performance required in accessing the same file varies depending on the characteristics of the job executed and other factors. For instance, executing numerous jobs in a batch environment requires a high I/O throughput whereas referring to a file in the normal dialogue environment does not require so high an I/O throughput. It has not been possible in the past to optimize the access performance and power consumption in cases as the one described above by switching to data placement that suits the characteristics of the job executed or the like. 
     According to a representative invention disclosed in this application, there is provided a computer system, comprising: a computer; and a storage system coupled to the computer through a network, wherein: the computer has an interface, which is coupled to the network, a processor, which is coupled to the interface, and a memory, which is coupled to the processor; the storage system has a plurality of disk drives and a controller, which controls the plurality of disk drives; the plurality of disk drives provide a data storage area that is partitioned into a plurality of logical volumes; the plurality of logical volumes include a first number of first logical volumes and as many second logical volumes as the first number; each of the plurality of logical volumes belongs to one of a plurality of volume sets; the plurality of volume sets include a first volume set to which the first number of the first logical volumes belong and a second volume set to which the first number of the second logical volumes belong; the first number of the first logical volumes are allocated dispersedly to a second number of the disk drives; the first number of second logical volumes are allocated dispersedly to a third number of the disk drives, the third number being larger than the second number; and the computer holds management information which associates each of the first logical volumes and each of the second logical volumes with each other, and is configured to: select one of the first volume set and the second volume set based on a result of judging whether or not a predetermined condition is met; store data of a file dispersedly in the first number of the logical volumes belonging to the selected volume set; and send an instruction to the storage system to stop spinning of disks in the disk drives to which none of the first number of the logical volumes belonging to the selected volume set are allocated. 
     According to an embodiment of this invention, the same file may be accessed at varying access performance and power consumption levels so that the access performance and power consumption are optimized for the characteristics of the job executed or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a computer system according to an embodiment of this invention. 
         FIG. 2  is a block diagram showing a configuration of a storage system according to the embodiment of this invention. 
         FIG. 3  is a block diagram showing structures of LU sets according to the embodiment of this invention. 
         FIG. 4  is an explanatory diagram of a configuration of a logically single file storage space according to the embodiment of this invention. 
         FIG. 5  is an explanatory diagram of LU set management information according to the embodiment of this invention. 
         FIG. 6  is an explanatory diagram of employed mode information according to the embodiment of this invention. 
         FIG. 7  is an explanatory diagram of power supply control information according to the embodiment of this invention. 
         FIG. 8  is an explanatory diagram of LU set switching condition information according to the embodiment of this invention. 
         FIG. 9  is an explanatory diagram of LU set selection hint information according to the embodiment of this invention. 
         FIG. 10  is a flow chart showing processing that is executed by an LU set selection module when a mount request is issued according to the embodiment of this invention. 
         FIG. 11  is a flow chart showing processing that is executed by the LU set selection module when LU sets are switched temporarily according to the embodiment of this invention. 
         FIG. 12  is a flow chart showing an LU set temporary switching permitted/prohibited judging processing, which is executed by the LU set selection module according to the embodiment of this invention. 
         FIG. 13  is a flow chart showing processing that is executed by an LU set synchronizing start-up module according to the embodiment of this invention. 
         FIG. 14  is a flow chart showing processing that is executed by a power supply control module according to the embodiment of this invention. 
         FIG. 15  is a flow chart showing a powering on processing which is executed by the power supply control module according to the embodiment of this invention. 
         FIG. 16  is a flow chart showing a powering off processing which is executed by the power supply control module according to the embodiment of this invention. 
         FIG. 17  is a flow chart showing processing that is executed by a switching condition registration module according to the embodiment of this invention. 
         FIG. 18  is a flow chart showing processing that is executed by a sub-data update information management module according to the embodiment of this invention. 
         FIG. 19  is a flow chart showing processing that is executed by an LU update information registration module according to the embodiment of this invention. 
         FIG. 20  is an explanatory diagram of sub-data update notification exclusion information according to the embodiment of this invention. 
         FIG. 21  is an explanatory diagram of sub-data update information according to the embodiment of this invention. 
         FIG. 22  is a flow chart showing processing that is executed by the LU set selection module to end the operation of a temporarily switched LU set according to the embodiment of this invention. 
         FIG. 23  is an explanatory diagram of configuration definition information according to the embodiment of this invention. 
         FIG. 24  is an explanatory diagram of file system mount location information according to the embodiment of this invention. 
         FIG. 25  is an explanatory diagram of selected LU set information according to the embodiment of this invention. 
         FIG. 26  is a flow chart showing processing that is executed by an LU activation module according to the embodiment of this invention. 
         FIG. 27  is a flow chart showing logical file creating processing, which is executed by a file system program according to the embodiment of this invention. 
         FIG. 28  is a flow chart showing logical file input/output processing, which is executed by the file system program according to the embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of this invention will be described below with reference to accompanying drawings. 
       FIG. 1  is a diagram showing a configuration of a computer system according to the embodiment of this invention. 
     The computer system of this embodiment has a host computer  101 , a service processor (SVP)  103 , and a power supply-controllable storage system  102  (hereinafter simply referred to as “storage system  102 ”). The host computer  101  is coupled to the SVP  103  through a local area network (LAN)  105 , and to the storage system  102  through a storage area network (SAN)  104 . The storage system  102  is coupled to the SVP  103  through a LAN  106 . 
     The host computer  101  is a computer that implements various applications using the storage system  102 . The host computer  101  of this embodiment has a CPU  161 , a memory  162 , an interface (I/F)  163 , and an I/F  164 , which are coupled to one another. 
     The CPU  161  is a processor that executes programs stored in the memory  162 . Processing that the respective programs execute in the following description is actually executed by the CPU  161 . 
     The memory  162  is a storage device that stores programs executed by the CPU  161 , data referred to by the CPU  161 , and others. When the memory  162  is a semiconductor memory, for example, a DRAM, those programs, data, and others may be stored in a hard disk drive (not shown), whereby all or some of them are copied to the memory  162  as the need arises. 
     The memory  162  of this embodiment stores at least an LU set management program  111 , a file system program or input/output library  112 , and a storage management program  113 . 
     The LU set management program  111  is a program for managing LU sets each of which is constituted of a plurality of logical volumes (logical units: LUs). The LU set management program  111  contains an LU set selection module  121 , a power supply control module  122 , an LU set synchronizing start-up module  123 , a switching condition registration module  124 , an LU update information registration module  125 , LU set management information  131 , employed mode information  132 , power supply control information  133 , LU set switching condition information  134 , and sub-data update notification exclusion information  135 . Those modules and information will be described later in detail. 
     The file system program or input/output library  112  may be a file system program provided as part of an operating system (OS) (not shown), or may be an input/output library used by a user application (not shown). Described below is a representative example in which the file system program or input/output library  112  is a file system program. The file system program or input/output library  112  in the following description will therefore be referred to simply as “file system program  112 ”. However, the following description applies to a case where the file system program or input/output library  112  is an input/output library. 
     The file system program  112  of this embodiment contains LU set selection hint information  141 , sub-data update information  142 , and a sub-data update information management module  143  details of which will be described later. 
     The storage management program  113  executes processing for managing the storage system  102 . The storage management program  113  may be provided as part of the OS. 
     The memory  162  may further store a user application program (not shown) which provides an arbitrary function to the user. 
     The I/F  163  is an interface that is coupled to the LAN  105  to communicate with the SVP  103  over the LAN  105 . 
     The I/F  164  is an interface that is coupled to the SAN  104  to communicate with the storage system  102  over the SAN  104 . 
     The storage system  102  stores data written by the host computer  101 . The storage system  102  holds at least an LU-LU synchronization program  151 . The detailed configuration of the storage system  102  will be described later with reference to  FIG. 2 . 
     The SVP  103  is a computer for managing the storage system  102 . The SVP  103  may have a CPU (not shown), a memory (not shown), and interfaces (not shown), which are coupled to one another. The interfaces are coupled to the LAN  105  and the LAN  106 . 
     In this embodiment, the LANs  105  and  106  employ the Internet Protocol (IP) and the SAN  104  employs the Fibre Channel (FC) protocol. However, those networks can employ any other protocols than the ones described above. In the case where the networks employ other protocols than the above, the I/F  163  and the I/F  164  are replaced with interfaces that are suitable for the protocols employed by their respective coupled networks. 
       FIG. 2  is a block diagram showing a configuration of the storage system  102  according to the embodiment of this invention. 
     The storage system  102  of this embodiment has a controller part  201  and a plurality of hard disk drives (HDDs)  231  ( 231   a ,  231   b  . . . ),  232  ( 232   a ,  232   b  . . . ),  233  ( 233   a ,  233   b  . . . ),  234  ( 234   a ,  234   b  . . . ),  235  ( 235   a ,  235   b  . . . ) . . . . 
     The plurality of HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . constitute redundant arrays of inexpensive disks (RAID). A RAID group (denoted by  211   a ,  211   b ,  211   c  . . .  212   a ,  212   b  . . . ) is a management unit of the plurality of HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . constituting RAID. To give a specific example, when the RAID level  1  is implemented by two HDDs out of the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . , two mirrored HDDs constitute one RAID group. To give another example, when the RAID level 5 (3D (data)+1P (parity)) is implemented by four HDDs out of the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . , four HDDs in which the data and the parity generated from the data are stored constitute one RAID group. 
     In the example of  FIG. 2 , the HDDs  231   a ,  231   b  . . . constitute the RAID group  211   a . The HDDs  232   a  and  232   b  constitute the RAID group  211   b . The HDDs  233   a  and  233   b  constitute the RAID group  211   c . The HDDs  234   a ,  234   b  . . . constitute the RAID group  212   a . The HDDs  235   a ,  235   b  . . . constitute the RAID group  212   b.    
     Storage areas of HDDs within the respective RAID groups are provided as arbitrarily sized LUs to the host computer  101 . The host computer  101  recognizes each LU as a single storage device. In the example of  FIG. 2 , LUs  221   a  and  221   b  are set in the RAID group  211   a . An LU  222  is set in the RAID group  211   b . LUs  223   a ,  223   b , and  223   c  are set in the RAID group  211   c . LUs  224   a  and  224   b  are set in the RAID group  212   a . An LU  225  is set in the RAID group  212   b.    
     The controller part  201  controls data write to and data read from the plurality of HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . . The controller part  201  is also capable of controlling power on/power off of at least some of the plurality of HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . . Here, powering on an HDD means at least starting the spinning of the disks in the HDD, and powering off means at least stopping the spinning of the disks in the HDD. 
     The controller part  201  of this embodiment has a CPU  261 , a memory  262 , an I/F  263 , an I/F  264 , a disk controller  251 , and a power supply control part  252 , which are coupled to one another. 
     The configuration of the controller part  201  shown in  FIG. 2  is merely an example, and this embodiment can be carried out with a controller part that is configured differently from  FIG. 2 . For instance, the power supply control part  252  may be coupled under the disk controller  251  (i.e., on the HDD side). 
     The CPU  261  is a processor that executes programs stored in the memory  262 . 
     The memory  262  is a storage device that stores programs executed by the CPU  261 , data referred to by the CPU  261 , and others. The memory  262  of this embodiment stores at least the LU-LU synchronization program  151 . 
     The I/F  263  and the I/F  264  are coupled to the SAN  104  and the LAN  106  to communicate with the host computer  101  and the SVP  103 , respectively. 
     The disk controller  251  is coupled to the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . with the use of, for example, FC to control data write to the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . and data read from the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . . 
     The power supply control part  252  controls power on/power off of the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . . The power supply control part  252  of this embodiment is coupled to each power supply control unit constituted of a given number of HDDs out of the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . , and hence power on/power off can be controlled on a power supply control unit basis. 
     One power supply control unit can contain an arbitrary number of HDDs. For example, the plurality of HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . are housed in a plurality of chassis, and HDDs in one chassis may constitute one power supply control unit. In the example of  FIG. 2 , the RAID groups  211   a ,  211   b , and  211   c  constitute a power supply control unit  202 , and the RAID groups  212   a  and  212   b  constitute a power supply control unit  203 . The numbers of RAID groups constituting the respective power supply control units are not limited to the above example, and one RAID group may constitute one power supply control unit, for example. 
     While  FIG. 2  shows two power supply control units  202  and  203 , the storage system  102  can have as many power supply control units as desired. At least one of a plurality of power supply control units may be prohibited from turning its power off. For instance, powering off a power supply control unit containing an HDD that stores information necessary to manage the storage system  102  may be prohibited. 
     The storage system  102  contains nine LUs as shown in  FIG. 2 , but may have an arbitrary number of LUs. 
       FIG. 3  is a block diagram showing structures of LU sets according to the embodiment of this invention. 
     In this embodiment, a plurality of LUs constitute one LU set. Data is striped among the plurality of LUs contained in one LU set to improve the access performance (specifically, I/O throughput). In the example of  FIG. 3 , sixteen LUs constitute one LU set, and data contained in one file is stored dispersedly in sixteen LUs at maximum. 
     Further, two LU sets are associated with each other in this embodiment. Two associated LU sets store in principle the same data. However, as will be described later, data stored in two associated LU sets do not match after data stored in one of the LU sets is updated until data synchronization processing is executed. 
     The storage system  102  in the example of  FIG. 3  has four racks, a rack  1   301  to a rack  4   304 . Each rack holds five chassis. Specifically, the rack  1   301  holds a chassis  00   331   a , a chassis  10   331   e , a chassis  20   331   i , a chassis  30   331   m , and a chassis  40   331   q . This embodiment treats those chassis as power supply control units equivalent to the power supply control units  202  and  203  shown in  FIG. 2 . The number “00” affixed to the chassis  00   331   a  of  FIG. 3 , for example, is an identifier of this chassis. The same applies to the numbers affixed to the rest of the chassis. 
     Similarly, the rack  2   302  is provided with a chassis  01   331   b , a chassis  11   331   f , a chassis  21   331   j , a chassis  31   331   n , and a chassis  41   331   r . The rack  3   303  is provided with a chassis  02   331   c , a chassis  12   331   g , a chassis  22   331   k , a chassis  32   331   o , and a chassis  42   331   s . The rack  4   304  is provided with a chassis  03   331   d , a chassis  13   331   h , a chassis  23   331   l , a chassis  33   331   p , and a chassis  43   33   1   t.    
     The chassis  00   331   a  includes four LUs, in other words, an LU  000   311   a , an LU  001   311   e , an LU  002   311   i , and an LU  003   311   m . Each LU is identical to an LU  221   a  of  FIG. 2 . For example, the number “000” affixed to the LU  000   311   a  of  FIG. 3  is the identifier of that LU. Numbers affixed to other LUs are the same. 
     Similarly, the chassis  01   331   b  includes an LU  010   311   b , an LU  011   311   f , an LU  012   311   j , and an LU  013   311   n . The chassis  02   331   c  includes an LU  020   311   c , an LU  021   311   g , an LU  022   311   k , and an LU  023   311   o . The chassis  03   331   d  includes an LU  030   311   d , an LU  031   311   h , an LU  032   311   l , and an LU  033   311   p.    
     In the example of  FIG. 3 , the sixteen LUs, the LU  000   311   a  to the LU  033   311   p , constitute a first LU set  311 . 
     The chassis  10   331   e  includes at least an LU  100   321   a . Similarly, the chassis  20   331   i , the chassis  30   331   m , the chassis  40   331   q , the chassis  11   331   f , the chassis  21   331   j , the chassis  31   331   n , the chassis  41   331   r , the chassis  12   331   g , the chassis  22   331   k , the chassis  32   331   o , the chassis  42   331   s , the chassis  13   331   h , the chassis  23   331   l , the chassis  33   331   p , and the chassis  43   331   t  respectively include at least an LU  200   321   e , an LU  300   321   i , an LU  400   321   m , an LU  110   321   b , an LU  210   3211   f , an LU  310   321   j , an LU  410   321   n , an LU  120   321   c , an LU  220   321   g , an LU  320   321   k , an LU  420   321   o , an LU  130   321   d , an LU  230   321   h , an LU  330   321   l , and an LU  430   321   p.    
     In the example of  FIG. 3 , the sixteen LUs, the LU  100   321   a  to the LU  430   321   t , constitute a second LU set. In the following description, the second LU set is described as an LU set  321 . 
     The LU set  321  corresponds to the LU set  311 . Specifically, the LU  000   311   a , the LU  001   311   e , the LU  002   311   i , the LU  003   311   m , the LU  010   311   b , the LU  011   311   f , the LU  012   311   j , the LU  013   311   n , the LU  020   311   c , the LU  021   311   g , the LU  022   311   k , the LU  023   311   o , the LU  030   311   d , the LU  031   311   h , the LU  032   311   l , and the LU  033   311   p  respectively correspond to the LU  100   321   a , the LU  200   321   e , the LU  300   321   i , the LU  400   321   m , the LU  110   321   b , the LU  210   3211   f , the LU  310   321   j , the LU  410   321   n , the LU  120   321   c , the LU  220   321   g , the LU  320   321   k , the LU  420   321   o , the LU  130   321   d , the LU  230   321   h , the LU  330   321   l , and the LU  430   321   p.    
     As described above, the sixteen LUs belonging to the LU set  311  are allocated dispersedly to four chassis. On the other hand, the sixteen LUs belonging to the LU set  321  are allocated dispersedly to sixteen chassis. In the case where each chassis holds an equal number of HDDs, the sixteen LUs belonging to the LU set  321  are dispersed among more HDDs than the sixteen LUs belonging to the LU set  311  are. 
     For example, to save data of some file to the LU set  311 , the data is stored dispersedly in the sixteen LUs contained in the LU set  311 . When processing of synchronizing the LU set  311  and the LU set  321  with each other is executed subsequently, the data stored in the sixteen LUs of the LU set  311  is copied to the sixteen LUs of the LU set  321  on a one-on-one basis. As a result, data of the same file is stored in the two LU sets. 
     Storing data of one file dispersedly in a plurality of LUs improves the performance in accessing the file. This is because a plurality of LUs (more strictly, a plurality of HDDs to which the plurality of LUs are allocated) are accessed concurrently. However, in the case where the plurality of LUs physically share one HDD, access to the HDD is executed sequentially, and hence the access performance is not improved. 
     In the example of  FIG. 3 , the LU  000   311   a , the LU  001   311   e , the LU  002   311   i , and the LU  003   311   m  out of the sixteen LUs constituting the LU set  311  are allocated to the same chassis  00   331   a . In other words, there is a possibility that those four LUs share an HDD. In particular, when the chassis  00   331   a  contains only one RAID group, those four LUs share an HDD without exception. The improvement in access performance cannot be expected from dispersing data among those four LUs. The same applies to LUs contained in the chassis  01   331   b , the chassis  02   331   c , and the chassis  03   331   d.    
     On the other hand, the sixteen LUs constituting the LU set  321  are all allocated to different chassis. Accordingly, data of a file stored in the LU set  321  is accessed at an improved access performance level by concurrent access to the sixteen LUs. 
     However, access to data stored in the LU set  321  requires powering on HDDs in sixteen chassis whereas access to data stored in the LU set  311  requires powering on HDDs in four chassis. This means that accessing data in the LU set  321  consumes more power than when data in the LU set  311  is accessed. 
     In the case where the LU set  311  and the LU set  321  store the same data as described above, one of the LU set  311  and the LU set  321  is chosen to be accessed based on the required access performance and power consumption. When reducing power consumption is given priority over high access performance, the LU set  311  is chosen to be accessed whereas the LU set  321  is chosen when high access performance is given priority over reducing power consumption. 
     One of LUs may store data necessary to manage the storage system  102 . This data may be made available for access at any time by prohibiting powering off a chassis that stores the data. In this case, LUs that are used when the reduction of power consumption is given priority may be stored in the chassis that is prohibited from being powered off. This prevents an increase in number of chassis that are powered on. In the example of  FIG. 3 , the chassis  00   331   a  to the chassis  03   331   d  may be prohibited from being powered off. 
     Next, a method of enabling the host computer  101  to access chosen one of two LU sets will be described with reference to  FIG. 4  and  FIGS. 23 to 26 . 
       FIG. 4  is an explanatory diagram of a configuration of a logically single file storage space according to the embodiment of this invention. 
     Of the components of the computer system shown in  FIG. 1 , ones that are not necessary for the description of the logically single file storage space are omitted from  FIG. 4 . 
     The file system program  112  contains, in addition to the modules and information shown in  FIG. 1 , an LU activation module  413 , configuration definition information  411 , and file system mount location information  412 . The LU set management program  111  contains, in addition to the modules and information shown in  FIG. 1 , selected LU set information  421 . Those additional module and information will be described later in detail. 
     A disk  401  stores information equivalent to the configuration definition information  411 , the file system mount location information  412 , and the LU set management information  131 . 
     The disk  401  can be implemented by any physical storage device or logical storage area. For example, the disk  401  may be any one of the HDDs  231 ,  232 ,  233 ,  234 ,  235  . . . contained in the storage system  102 , may be any one of the LUs  221  ( 221   a ,  221   b ),  222 ,  223  ( 223   a ,  223   b ,  223   c ),  224  ( 224   a ,  224   b ),  225  . . . , or may be a storage device that is coupled to the SAN  104  independently of the storage system  102 . However, the LU set management program  111  needs to be prohibited from controlling the power on/power off of the disk  401 . In other words, the LU set management program  111  is not allowed to power off the disk  401 . 
     Sub-data  1   431   a  to sub-data  16   431   p  shown in  FIG. 4  are each a dispersed file storage space. Those sixteen pieces of sub-data are respectively associated with one of the storage areas of sixteen LUs that are contained in the LU set  311 , namely, the LU  000   311   a  to the LU  033   311   p , and the storage areas of sixteen LUs that are contained in the LU set  321 , namely, the LU  100   321   a  to the LU  430   321   p.    
       FIG. 4  shows, for the sake of convenience, a state in which the sub-data  1   431   a  is associated with the LU  000   311   a  and with the LU  100   321   a  both. Actually, the sub-data  1   431   a  is under no circumstances associated with the LU  000   311   a  and the LU  100   321   a  both at the same time, but is associated with chosen one of the two LUs. 
     When the sub-data  1   431   a  is associated with the LU  000   311   a , the sub-data  2   431   b  to the sub-data  16   431   p  are associated with the LU  001   311   e  to the LU  033   311   p , respectively. When the sub-data  1   431   a  is associated with the LU  100   321   a , the sub-data  2   431   b  to the sub-data  16   431   p  are associated with the LU  201   321   e  to the LU  430   321   p , respectively. Those sixteen pieces of sub-data are together treated as a logically single file storage space. 
     “1” to “16” affixed to the sub-data  1   431   a  to the sub-data  16   431   p  of  FIG. 4  are information for identifying the respective pieces of sub-data. This information is referred to as sub-data labels in  FIGS. 24 and 25 . 
     Master data  432  shown in  FIG. 4  is stored in a disk  402 . The master data  432  holds information with which partial files stored in sixteen pieces of sub-data are treated as one logical file when sixteen pieces of sub-data are treated as a logically single file storage space. The master data  432  holds, for example, the name of a logical file and information for managing a plurality of logical files in a tree structure. 
     As is the disk  401 , the disk  402  can also be implemented by any physical storage device or logical storage area. The disk  402  is also similar to the disk  401  in that the LU set management program  111  is not allowed to power off the disk  402 . 
     While  FIG. 4  shows the disk  401  and the disk  402 , the disks  401  and  402  may be implemented by a single disk by, for example, adding the master data  432  to the disk  401  and omitting the disk  402 . 
       FIG. 23  is an explanatory diagram of the configuration definition information  411  according to the embodiment of this invention. 
     The configuration definition information  411  contains a directory name  2301 . The directory name  2301  is a list of directory names for identifying directories to which LUs associated with the respective pieces of sub-data are mounted (namely, mount points). The information held in the configuration definition information  411  does not need to be directory names themselves, and can be any kind of information as long as directory names can be generated from the information. 
     The configuration definition information  411  may further contain information necessary for striping (e.g., information indicating the striping size). 
       FIG. 24  is an explanatory diagram of the file system mount location information  412  according to the embodiment of this invention. 
     The file system mount location information  412  contains a directory name  2401  and sub-data label information  2402 . 
     The sub-data label information  2402  is information for identifying each piece of sub-data. 
     The directory name  2401  is a directory name for identifying a directory to which an LU associated with a piece of sub-data is to be mounted. 
     In the example of  FIG. 24 , “1” is registered as the sub-data label information  2402  in association with a value “/sub/io01” of the directory name  2401 . This indicates that any one of the LU  000   311   a  and the LU  100   321   a  which are associated with the sub-data  1   431   a  identified by a sub-data label “1” is mounted to a mount point “/sub/io01”. Which of the LU  000   311   a  and the LU  100   321   a  is to be mounted is determined based on the selected LU set information  421 , which will be described next. 
       FIG. 25  is an explanatory diagram of the selected LU set information  421  according to the embodiment of this invention. 
     The selected LU set information  421  contains sub-data label information  2501  and a selected LU ID  2502 . 
     The sub-data label information  2501  is information for identifying each piece of sub-data, as is the sub-data label information  2402 . 
     The selected LU ID  2502  is information for identifying an LU that is associated with sub-data identified by the sub-data label information  2501 . The selected LU ID  2502  can be any kind of information as long as an LU can be identified uniquely. For example, the selected LU ID  2502  may be a device file name, a universally unique identifier (UUID), or a label name. 
       FIG. 25  shows an example in which the LU set  321  is chosen to be accessed. In this example, “100” is registered as the selected LUID  2502  in association with the value “1” of the sub-data label information  2501 . This indicates that, out of the LU  000   311   a  and the LU  100   321   a , the LU  100   321   a  is chosen as an LU that is associated with the sub-data  1   431   a.    
     The LU set selection is executed by the LU set selection module  121  as will be described later with reference to  FIGS. 10 ,  11 , and  22 . 
       FIG. 26  is a flow chart showing processing that is executed by the LU activation module  413  according to the embodiment of this invention. 
     First, the LU activation module  413  refers to the configuration definition information  411  to obtain the directory name  2301  (i.e., information indicating the mount point) of a directory used as sub-data (Step  2601 ). 
     Next, the LU activation module  413  refers to the file system mount location information  412  to obtain information that indicates the mount location of each directory (i.e., the sub-data label information  2402 ) (Step  2602 ). 
     Then, the LU activation module  413  receives the selected LU set information  421  from the LU set management program  111  (Step  2603 ). 
     From the information received in Step  2603 , the LU activation module  413  obtains the selected LU ID  2502  that is associated with each piece of the sub-data label information  2501 , and executes mounting (Step  2604 ). Specifically, the LU activation module  413  looks up the sub-data label information  2501  registered in association with the selected LU ID  2502 , searches for the sub-data label information  2402  that matches the sub-data label information  2501 , obtains the directory name  2401  that is associated with the found sub-data label information  2402 , and mounts an LU that is identified by the selected LU ID  2502  to a directory that is identified by the obtained directory name  2401 . 
     An LU is activated through the above processing. 
     For example, in the case where the selected LU ID  2502  contained in the information received in Step  2603  identifies an LU within the LU set  321  as shown in  FIG. 25 , each LU contained in the LU set  321  is mounted to a directory identified by the directory name  2301  through the processing of  FIG. 26 . Mounted to directories, LUs contained in the LU set  321  are thus activated. 
     As described above, the LU set  311  and its associated LU set  321  store data of the same file. When the file system program  112  receives a request to access the file after each LU contained in the LU set  321  is activated in the manner described above, the file system program  112  executes access to the LUs contained in the selected LU set  321 . 
     Specifically, to process a request to write a file, the file system program  112  stores data of the single file dispersedly in sixteen LUs contained in the selected LU set  321 . To process a request to read a file, the file system program  112  reads data of the single file stored dispersedly in sixteen LUs contained in the selected LU set  321 . The write and the read are accomplished through processing shown in  FIGS. 27 and 28 . 
     In the case where the LU set  311  is chosen to be accessed, values of the selected LU IDs  2502  that are associated with values “1” to “16” of the sub-data label information  2501  of  FIG. 25  are the identifiers of the LUs contained in the LU set  311 , namely, “000” to “033”. In this case, the processing of  FIG. 26  activates the LUs within the LU set  311  and then access to those LUs is executed. 
       FIG. 27  is a flow chart showing logical file creating processing, which is executed by the file system program  112  according to the embodiment of this invention. 
     The processing of  FIG. 27  is executed by a logical file creation module (not shown) within the file system program  112 . 
     First, the logical file creation module determines based on the configuration definition information  411  which sub-data is to be used (Step  2701 ). 
     Next, the logical file creation module creates a master file (not shown) in the master data  432  (Step  2702 ). A master file is determined uniquely from the name of a logical file. The master file contains information for uniquely determining a sub-file (not shown) created in each piece of sub-data to be used in order to hold partial contents of the logical file. 
     Next, the logical file creation module creates sub-files for holding partial contents of the logical file in the sub-data to be used (Step  2703 ). 
       FIG. 28  is a flow chart showing logical file input/output processing, which is executed by the file system program  112  according to the embodiment of this invention. 
     The processing of  FIG. 28  is executed by a logical file input/output module (not shown) within the file system program  112 . 
     First, the logical file input/output module obtains information of a sub-file determined from the contents of a master file in the master data  432  (Step  2801 ). 
     Next, the logical file input/output module executes read or write with respect to the sub-file based on the obtained sub-file information (Step  2802 ). 
       FIG. 5  is an explanatory diagram of the LU set management information  131  according to the embodiment of this invention. 
     The LU set management information  131  contains a sub-data ID  501 , a performance-first LU  502 , a power-saving-first LU  503 , and a synchronization completion flag  504 . 
     The sub-data ID  501  is information for identifying each piece of sub-data similarly to the sub-data label information  2402  and  2501 . In the example of  FIG. 5 , “1” to “16” with which the sixteen pieces of sub-data described with reference to  FIG. 4  are identified are registered as the sub-data ID  501 . 
     The performance-first LU  502  indicates the identifier of an LU that is selected in a performance-first mode. The performance-first mode is a mode set when the improvement of the access performance is given priority over the reduction of power consumption. In the following description, the performance-first mode is also referred to as “mode 1”. “100” to “430” are registered as the performance-first LU  502  in the example of  FIG. 5 , with which the LUs contained in the LU set  321  are identified. 
     The power-saving-first LU  503  indicates the identifier of an LU that is selected in a power-saving-first mode. The power-saving-first mode is a mode set when the reduction of power consumption is given priority over the improvement of the access performance. In the following description, the power-saving-first mode is also referred to as “mode 2”. “000” to “033” are registered as the power-saving-first LU  503  in the example of  FIG. 5 , with which the LUs contained in the LU set  311  are identified. 
     The synchronization completion flag  504  is a flag indicating whether or not processing of synchronizing two LUs that are associated with one piece of sub-data has been completed. When the processing of synchronizing two LUs that are associated with one piece of sub-data is completed, a value “Yes” is registered as the synchronization completion flag  504  in association with those LUs. Those LUs at this point are storing the same data. In the case where data in only one of the two LUs is updated subsequently, the value of the synchronization completion flag  504  associated with the LUs is updated to “No”. Data in the updated LU and data in the other LU differ from each other, at least partially, at this point. 
       FIG. 6  is an explanatory diagram of the employed mode information  132  according to the embodiment of this invention. 
     The employed mode information  132  contains information for managing modes set in this embodiment. Specifically, the employed mode information  132  contains a default mode  601 , a switch-from-default-mode permission/prohibition  602 , a last mode  603 , a current mode  604 , a switch-from-current-mode permission/prohibition  605 , and a temporary mode  606 . 
     The default mode  601  is information indicating a mode that is chosen when no explicit instruction is given about which mode is to be employed (i.e., default mode). 
     The switch-from-default-mode permission/prohibition  602  is a default value of information that indicates whether or not the user is permitted to switch the currently set mode. As will be described later with reference to  FIG. 10 , in the case where an option to a mount request does not specify whether mode switching is permitted or prohibited, a value registered as the switch-from-default-mode permission/prohibition  602  determines whether to allow mode switching. In this case, the same value that is registered as the switch-from-default-mode permission/prohibition  602  is registered as the switch-from-current-mode permission/prohibition  605 , which will be described later. 
     The last mode  603  is information indicating a mode that has been set the last time. Specifically, when an LU is unmounted, a value indicating a mode that has been set immediately before the unmounting (in other words, a value that has been registered as the current mode  604  immediately before the unmounting) is registered as the last mode  603 . In some cases, synchronization processing described later is executed when an LU unmounted in this manner is mounted again. The last mode  603  is referred to in order to determine which LU is the copy source and which LU is the copy destination in the synchronization processing. 
     The current mode  604  is information indicating a mode that is currently set. 
     The switch-from-current-mode permission/prohibition  605  is information indicating whether or not the user is permitted to switch the current mode. When a value of the switch-from-current-mode permission/prohibition  605  is “permitted”, the user is allowed to switch the current mode. 
     The temporary mode  606  is information indicating a mode that replaces the current mode when a switch from the current mode is permitted. For instance, when the power-saving-first mode is set at present and the user is allowed to switch the current mode, “performance-first mode” is registered as the temporary mode  606 . 
       FIG. 7  is an explanatory diagram of the power supply control information  133  according to the embodiment of this invention. 
     The power supply control information  133  contains an LU ID  701 , an in-use flag  702 , an RG ID  703 , a power supply control unit ID  704 , and a power on/off state  705 . 
     The LU ID  701  is information for identifying each LU. 
     The in-use flag  702  is information indicating whether or not an LU is in use, in other words, whether or not an LU is mounted. In the case of an LU that is mounted, “Yes” is registered as the in-use flag  702  in association with this LU. 
     The RG ID  703  is information for identifying each RAID group (RG) which contains LUs. 
     The power supply control unit ID  704  is information for identifying each power supply control unit which contains LUs. 
     The power on/off state  705  is information indicating the power on/off state of each power supply control unit which contains LUs. A value “ON” registered as the power on/off state  705  in association with a power supply control unit indicates that the power of HDDs contained in the power supply control unit is on. 
     In the example of  FIG. 7 , in an entry where a value of the LU ID  701  is “000”, “Yes”, “00”, “00”, and “ON” are registered as the in-use flag  702 , the RG ID  703 , the power supply control unit ID  704 , and the power on/off state  705 , respectively. This indicates that the LU  000   311   a , which is identified by “000”, is currently mounted and is contained in a RAID group identified by “00”, that this RAID group is contained in a power supply control unit identified by “00” (chassis  00   331   a ), and that the power of HDDs contained in the chassis  00   331   a  is on. 
     One power supply control unit may contain a plurality of LUs as shown in  FIG. 3 . In the case where none of LUs contained in one power supply control unit are in use, this power supply control unit can be powered off. When at least one of the plurality of LUs contained in the power supply control unit is in use, on the other hand, the power supply control unit needs to be powered on despite the rest of the LUs not being in use. Therefore, while the value of the power on/off state  705  is always “ON” when the value of the in-use flag  702  is “Yes”, when the value of the in-use flag  702  is “No”, the value of the power on/off state  705  is “ON” in some cases and “OFF” in other cases. 
       FIG. 8  is an explanatory diagram of the LU set switching condition information  134  according to the embodiment of this invention. 
     The LU set switching condition information  134  contains information for managing conditions based on which whether to switch modes is judged (conditions that provide a basis for judging whether to switch LUs to be mounted). The judging is executed in Step  1001  of  FIG. 10  and Step  1203  of  FIG. 12  which will be described later. 
     The LU set switching condition information  134  contains an evaluation order  801 , an evaluation item  802 , a performance-first condition  803 , and a power-saving-first condition  804 . 
     The evaluation order  801  indicates an order in which judging conditions are evaluated. 
     The evaluation item  802  indicates an item evaluated as a judging condition. 
     The performance-first condition  803  indicates a condition based on which whether to switch to the performance-first mode is judged. When a condition registered as the performance-first condition  803  is met, a switch to the performance-first mode (in other words, a switch to an LU set that is associated with the performance-first mode) is executed. 
     The power-saving-first condition  804  indicates a condition based on which whether to switch to the power-saving-first mode is judged. When a condition registered as the power-saving-first condition  804  is met, a switch to the power-saving-first mode (in other words, a switch to an LU set that is associated with the power-saving-first mode) is executed. 
     In the example of  FIG. 8 , in an entry where a value of the evaluation order  801  is “1” (entry  811 ), “specified upon mounting”, “perf”, and “psave” are registered as the evaluation item  802 , the performance-first condition  803 , and the power-saving-first condition  804 , respectively. In this case, a mode designation contained in a mount request is judged first. Specifically, when “perf” is designated upon mounting (for example, when an option to a mount command or a flag argument of the mount function contains “perf”), the performance-first mode is set, which means that the LUs contained in the LU set  321  are to be mounted. When “psave” is designated upon mounting, on the other hand, the power-saving-first mode is set by mounting LUs that are contained in the LU set  311  as in Steps  1001  and  1003  of  FIG. 10 . 
     In the case where none of “perf” and “psave” are designated upon mounting, a mode registered as the default mode  601  in the employed mode information  132  are set. 
     After LUs associated with any one of the modes are mounted in the manner described above, whether to switch modes is judged at various time points based on values that are held in entries where the value of the evaluation order  801  is “2” or larger as in Step  1203  of  FIG. 12 . 
     Specifically, whether to switch modes is judged first based on values that are held in an entry where the value of the evaluation order  801  is “2”. In the case where none of “O_PERF” and “O_PSAVE” are designated as a result, the judging is executed based on values that are held in an entry where the value of the evaluation order  801  is “3”. The subsequent entries are processed in the same manner and, when no mode is designated by processing one entry, the judging is executed based on a mode designation of the next entry where the value of the evaluation order  801  is larger by one. 
     In the example of  FIG. 8 , in an entry where the value of the evaluation order  801  is “2” (entry  812 ), “specified upon opening”, “O_PERF”, and “O_PSAVE” are registered as the evaluation item  802 , the performance-first condition  803 , and the power-saving-first condition  804 , respectively. In this case, whether to switch LU sets is judged based on a designation contained in a file open request. 
     Specifically, when “O_PERF” is designated upon file opening (for example, when an option to an open command or a flag argument of the open function contains “O_PERF”), a condition registered as the performance-first condition  803  is met. In the case where the mode set at that point is the power-saving-first mode, a switch from the power-saving-first mode to the performance-first mode is executed by unmounting LUs that are contained in the LU set  311  and mounting LUs that are contained in the LU set  321 . 
     When “O_PSAVE” is designated upon file opening, a condition registered as the power-saving-first condition  804  is met. In the case where the mode set at that point is the performance-first mode, a switch from the performance-first mode to the power-saving-first mode is executed. 
     With the entry  811  and the entry  812 , a mode can be set (or switched) explicitly through a mount command or the mount function, or through an open command or the open function. For example, when executing access that requires high performance, the user may designate the performance-first mode through a mount command or the mount function, or through an open command or the open function. 
     In an entry where the value of the evaluation order  801  is “3” (entry  813 ), “file attribute”, “PERF”, and “PSAVE” are registered as the evaluation item  802 , the performance-first condition  803 , and the power-saving-first condition  804 , respectively. In this case, whether to switch LU sets is judged based on the attributes of a file. 
     Specifically, when any one of “PERF” and “PSAVE” is contained in meta data that indicates the attributes of a file specified by an open command or the open function, or a file designated as the access target, for example, the attributes of the file are judged by referring to the meta data. When the meta data indicating the attributes of the file contains “PERF”, a condition registered as the performance-first condition  803  is met and the performance-first mode is set. When the meta data indicating the attributes of the file contains “PSAVE”, a condition registered as the power-saving-first condition  804  is met and a switch to the power-saving-first mode is executed. 
     For instance, the system administrator can add “PERF” to meta data of a file that needs to be accessed at high speed. In this manner, a mode can be set separately for each file to be accessed. 
     In an entry where the value of the evaluation order  801  is “4” (entry  814 ), “environmental variable”, “ENV_PERF”, and “ENV_PSAVE” are registered as the evaluation item  802 , the performance-first condition  803 , and the power-saving-first condition  804 , respectively. In this case, whether to switch LU sets is judged based on an environmental variable. 
     Specifically, when an environmental variable “ENV_PERF” is defined, or when a given value (e.g., 1) is specified for the environmental variable “ENV_PERF”, a condition registered as the performance-first condition  803  is met and the performance-first mode is set. When an environmental variable “ENV_PSAVE” is defined, or when a given value (e.g., 1) is specified for the environmental variable “ENV_PSAVE”, a condition registered as the power-saving-first condition  804  is met and a switch to the power-saving-first mode is executed. 
     In an entry where the value of the evaluation order  801  is “5” (entry  815 ), “parent process name” and “job scheduler name” are registered as the evaluation item  802  and the performance-first condition  803 , respectively. In this case, whether or not the name of a parent process that has started up the currently executed job matches a name registered as the performance-first condition  803  is judged. When the two match, a condition registered as the performance-first condition  803  is met and a switch to the performance-first mode is executed. When the two do not match, a condition registered as the performance-first condition  803  is not met and mode switching is not executed. 
     In an entry where the value of the evaluation order  801  is “6” (entry  816 ), “executed job name” and “job 1 ” are registered as the evaluation item  802  and the performance-first condition  803 , respectively. In this case, whether or not the name of the currently executed job matches “job 1 ” registered as the performance-first condition  803  is judged. When the two match, a condition registered as the performance-first condition  803  is met and a switch to the performance-first mode is executed. 
     With the entry  815  and the entry  816 , a mode can be set based on the type of a job to be executed. For example, when a large-size job is registered in a job scheduler to be executed in a given time zone (e.g., at night), the name of this job scheduler may be registered as the performance-first condition  803  in the entry  815 , or the name of the job itself may be registered as the performance-first condition  803  in the entry  816 . 
     In evaluation according to the entries  815  and  816 , an executed job name  907  and a parent process name  909  are in some cases not designated in the LU set selection hint information  141  which is received from the user and will be described later. To deal with that event, this embodiment provides an option of evaluation by process ID which is described next. 
     In evaluation by process ID, a parent name “job scheduler name” and an executed job name “job 1 ” which are registered as the performance-first mode  803  are used to obtain the process ID of each process from process execution state information, which is managed by the OS. By judging whether or not the obtained process ID matches an executed job process ID  908  or a parent process ID  910  in the LU set selection hint information  141  described later, evaluation equivalent to the one according to the entries  815  and  816  can be made. 
     In an entry where the value of the evaluation order  801  is “7” (entry  817 ), “group ID” and “hpc” are registered as the evaluation item  802  and the performance-first condition  803 , respectively. In this case, whether or not the name of a group to which a user who has executed the current job belongs is “hpc” is judged. When the name of the group is “hpc”, a condition registered as the performance-first condition  803  is met and a switch to the performance-first mode is executed. 
     In an entry where the value of the evaluation order  801  is “8” (entry  818 ), “user ID” and “foo, bar” are registered as the evaluation item  802  and the performance-first condition  803 , respectively. In this case, whether or not the name of a user who has executed the current job belongs is “foo” or “bar” is judged. When the name of the user is any one of “foo” or “bar”, a condition registered as the performance-first condition  803  is met and a switch to the performance-first mode is executed. 
     With the entry  817  and the entry  818 , a mode can be set based on the name of a user who accesses a file. For example, a user who executes access that requires high performance all the time may register his/her own user name or the name of a group to which he/she belongs as the performance-first condition  803 . 
     The system administrator or users can register arbitrary values in the LU set switching condition information  134 . For example, the entries  811  to  814  are for conditions that are set by the system administrator and common to all users, whereas the entries  815  to  818  can be set by users at their discretion. The LU set switching condition information  134  may further contain information set by the respective users in the entries  815  to  818 . 
       FIG. 9  is an explanatory diagram of the LU set selection hint information  141  according to the embodiment of this invention. 
     The LU set selection hint information  141  is created by the file system program  112  from a file operation request (e.g., file open request) received from a user, or the like, to be sent to the LU set management program  111 . 
     Specifically, the LU set selection hint information  141  contains a used file path name  901 , a specified-upon-file-use option  902 , a used file attribute  903 , an environmental variable  904 , a job executing user ID  905 , a job executing group ID  906 , the executed job name  907 , the executed job process ID  908 , the parent process name  909 , and the parent process ID  910 . 
     The LU set management program  111  compares the received LU set selection hint information  141  against the LU set switching condition information  134  to judge whether or not a condition for executing mode switching (i.e., switching of LU sets) is met as in Step  1203  of  FIG. 12 . 
     To give a specific example, when the specified-upon-file-use option  902  contains “O_PERF” or “O_PSAVE”, this value is compared against values registered in an entry of the LU set switching condition information  134  where the evaluation item  802  is “specified upon opening” (entry  812 ) in order to judge whether or not a mode switching condition is met. In the case where the specified-upon-file-use option  902  contains “O_PERF”, for example, it is judged that a condition registered as the performance-first condition  803  in the entry  812  is met. Then, the performance-first mode is selected as will be described later. 
     Similarly, the used file attribute  903 , the environmental variable  904 , the job executing user ID  905 , the job executing group ID  906 , the executed job name  907 , and the parent process name  909  correspond to the evaluation items “file attribute”, “environmental variable”, “user ID”, “group ID”, “executed job name”, and “parent process name”, respectively, in the LU set switching condition information  134 . 
       FIG. 10  is a flow chart showing processing that is executed by the LU set selection module  121  when a mount request is issued according to the embodiment of this invention. 
     Receiving a mount request, the LU set selection module  121  judges whether or not the performance-first mode or the power-saving-first mode is designated in an option to the mount request (Step  1001 ). The judging is executed by comparing the option to the mount request against values registered in an entry of the LU set switching condition information  134  where the evaluation item  802  is “specified upon mounting”. 
     When it is judged in Step  1001  that none of the two modes are designated, the LU set selection module  121  sets a mode that is designated as the default mode  601  in the employed mode information  132  (Step  1002 ). For example, in the case where the power-saving-first mode is designated as the default mode  601  as shown in  FIG. 6 , the LU set selection module  121  sets the power-saving-first mode (by registering a value that is registered as the default mode  601  as the current mode  604  in the employed mode information  132 ). 
     On the other hand, when it is judged in Step  1001  that any one of the two modes is designated, the LU set selection module  121  registers the designated mode as the current mode  604  in the employed mode information  132  (Step  1003 ). 
     Next, the LU set selection module  121  judges if the option to the mount request specifies whether mode switching is permitted or prohibited (Step  1004 ). 
     When it is judged in Step  1004  that whether mode switching is permitted or prohibited is not specified, the LU set selection module  121  registers a value that is registered as the switch-from-default-mode permission/prohibition  602  in the employed mode information  132  (“permitted” in the example of  FIG. 6 ) as the switch-from-current-mode permission/prohibition  605  (Step  1005 ). 
     On the other hand, when it is judged in Step  1004  that whether mode switching is permitted or prohibited is specified, the LU set selection module  121  registers the specified value (any one of “permitted” and “prohibited”) as the switch-from-current-mode permission/prohibition  605  (Step  1006 ). 
     Next, the LU set selection module  121  designates, as a control object, an LU set that is associated with a mode registered as the current mode  604  in the employed mode information  132 , and instructs the power supply control module  122  to power on the LU set (Step  1007 ). 
     Specifically, the LU set selection module  121  identifies from the LU set management information  131  LUs that are associated with a mode registered as the current mode  604 . For example, in the case where the power-saving-first mode is registered as the current mode  604  as shown in  FIG. 6 , the LU  000   311   a  to the LU  033   311   p  (i.e., LU set  311 ) which are registered as the power-saving-first LU  503  of  FIG. 5  are identified. Then, the LU set selection module  121  identifies from the power supply control information  133  a power supply control unit that contains the identified LUs. For example, in the case where the LU  000   311   a  to the LU  033   311   p  are identified as in the above example, the power supply control unit  00   331   a  to the power supply control unit  03   331   d  which contain the identified LUs are identified. The LU set selection module  121  instructs the power supply control module  122  to power on the identified power supply control unit  00   331   a  to the identified power supply control unit  03   331   d.    
     Processing that the power supply control module  122  executes upon instructed to power on power supply control units will be described later with reference to  FIG. 14 . 
     Next, the LU set selection module  121  judges whether or not the value registered as the current mode  604  differs from the value registered as the last mode  603  in the employed mode information  132  (Step  1008 ). 
     When the value registered as the current mode  604  differs from the value registered as the last mode  603 , it means that the LU set to be mounted this time (LU set identified in Step  1007 ) is not the same as the LU set mounted last time before this mounting is executed. In this case, there is a possibility that the latest data has not been stored yet in the LU set to be mounted this time. 
     For example, when the mode designated in the last mounting is the performance-first mode, the mounted LUs are the ones contained in the LU set  321 . When those LUs are unmounted after data stored in the LUs is updated, the latest data stored in the LU set  321  is not reflected on the LU set  311  at that point (in this case, a value indicating the performance-first mode is registered as the last mode  603  in the employed mode information  132 ). In the case where the power-saving-first mode is designated in this mounting, the latest data needs to be reflected on the LU set  311  which is associated with the power-saving-first mode. 
     For that reason, when the value registered as the current mode  604  differs from the value registered as the last mode  603 , the LU set selection module  121  designates as a control object an LU set that is associated with a mode registered as the last mode  603  in the employed mode information  132 , and instructs the power supply control module  122  to power on the LU set (Step  1009 ). The control object LUs are identified in the same manner as in Step  1007 . 
     Next, the LU set selection module  121  designates, as the copy source, an LU set that is associated with a mode registered as the last mode  603  and, as the copy destination, an LU set that is associated with a mode registered as the current mode  604  to execute the LU set synchronizing start-up module  123  (Step  1010 ). Processing that is executed by the LU set synchronizing start-up module  123  in this step will be described later with reference to  FIG. 13 . 
     Then, the LU set selection module  121  designates as a control object an LU set that is associated with a mode registered as the last mode  603  in the employed mode information  132 , and instructs the power supply control module  122  to power off the LU set (Step  1011 ). Processing that the power supply control module  122  executes upon receiving this instruction will be described later with reference to  FIGS. 14 to 16 . 
     Next, the LU set selection module  121  sends a notification informing that the preparation of the LU set for use has been completed (Step  1012 ). As the notification, the LU set selection module  121  may send the identifiers of the LUs belonging to the selected LU set (LUs identified in Step  1007 ). 
     On the other hand, when it is judged in Step  1008  that the value registered as the current mode  604  is the same as the value registered as the last mode  603 , it means that the LUs identified in Step  1007  are storing the latest data. Then, Steps  1009  to  1011  are skipped and Step  1012  is executed. 
     As a result of executing Step  1012 , the identifiers of the LUs belonging to the selected LU set are registered as the selected LU ID  2502  in the selected LU set information  421 . For example, when the LU set  321  is chosen, the identifiers “100” to “430” of the LU  100   321   a  to the LU  430   321   p  which belong to the LU set  321  are registered as shown in  FIG. 25 . 
     The processing executed by the LU set selection module  121  when a mount request is issued is thus completed. 
       FIG. 11  is a flow chart showing processing that is executed by the LU set selection module  121  when LU sets are switched temporarily according to the embodiment of this invention. 
     First, the LU set selection module  121  receives the LU set selection hint information  141  from the file system program  112  (Step  1101 ). 
     Next, the LU set selection module  121  judges whether to execute temporary switching of LU sets (temporary mode switching) (Step  1102 ). Specifically, the LU set selection module  121  executes LU set temporary switching permitted/prohibited judging processing, which will be described later with reference to  FIG. 12 . 
     When it is judged in Step  1102  that temporary switching of LU sets is to be executed, the LU set selection module  121  sets an LU set that is associated with a mode registered as the current mode  604  in the employed mode information  132  (in examples of  FIG. 6  and  FIG. 5 , LU set  311 ) to a quiescent state (Step  1103 ). Setting an LU set to a quiescent state means stopping of I/O to and from the LU set. In the case where the OS is holding data that is going to be written in an LU belonging to an LU set, the data is written in the LU before I/O to and from the LU set is stopped. 
     Next, the LU set selection module  121  designates, as a control object, an LU set that is associated with a mode registered as the temporary mode  606  in the employed mode information  132  (in examples of  FIG. 6  and  FIG. 5 , LU set  321  which is associated with performance-first mode), and instructs the power supply control module  122  to power on the LU set (Step  1104 ). 
     Next, the LU set selection module  121  designates, as the copy source, an LU set that is associated with a mode registered as the current mode  604  and, as the copy destination, an LU set that is associated with a mode registered as the temporary mode  606  to execute the LU set synchronizing start-up module  123  (Step  1105 ). 
     Next, the LU set selection module  121  designates, as a control object, an LU set that is associated with a mode registered as the current mode  604  in the employed mode information  132 , and instructs the power supply control module  122  to power off the LU set (Step  1106 ). 
     Next, the LU set selection module  121  sends a notification informing that the preparation of the LU set for use has been completed (Step  1107 ). As the notification, the LU set selection module  121  may send the identifiers of the LUs belonging to the selected LU set (LUs designated in Step  1104 ). 
     When it is judged in Step  1102  that temporary switching of LU sets is not to be executed, on the other hand, the LU set selection module  121  executes Step  1107 , skipping Steps  1103  to  1106 . 
     As a result of executing Step  1107 , the identifiers of the LUs belonging to the selected LU set are registered as the selected LU ID  2502  in the selected LU set information  421 , as in Step  1012  of  FIG. 10 . 
     The processing executed by the LU set selection module  121  when an open request is issued is thus completed. 
       FIG. 12  is a flow chart showing the LU set temporary switching permitted/prohibited judging processing, which is executed by the LU set selection module  121  according to the embodiment of this invention. 
     The LU set temporary switching permitted/prohibited judging processing shown in  FIG. 12  is executed in Step  1102  of  FIG. 11 . 
     First, the LU set selection module  121  judges whether or not the value of the switch-from-current-mode permission/prohibition  605  in the employed mode information  132  is “permitted”. 
     When the value of the switch-from-current-mode permission/prohibition  605  is not “permitted”, it means that temporary mode switching (temporary switching of LU sets) is not allowed. Then the LU set selection module  121  clears the value of the temporary mode  606  in the employed mode information  132  (Step  1208 ), returns a value indicating that temporary mode switching is not executed (for example, “prohibited”) (Step  1209 ), and ends the LU set temporary switching permitted/prohibited judging processing. In the case where Step  1209  is executed, it is judged that temporary switching of LU sets is not executed in Step  1102  of  FIG. 11 . 
     When the value of the switch-from-current-mode permission/prohibition  605  is “permitted”, it means that temporary mode switching is allowed. Then the LU set selection module  121  refers to the LU set management information  131  to judge whether or not a plurality of LU sets of different modes are registered in association with each other (Step  1202 ). 
     In the example of  FIG. 5 , the LU set  321  of performance-first mode and the LU set  311  of power-saving-first mode are registered in association with each other. In this case, it is judged in Step  1202  that a plurality of LU sets of different modes are registered in association with each other. 
     When it is judged in Step  1202  that a plurality of LU sets of different modes are not registered in association with each other, the LU set selection module  121  cannot execute temporary mode switching. Then the LU set selection module  121  executes Steps  1208  and  1209  and ends the LU set temporary switching permitted/prohibited judging processing. 
     When it is judged in Step  1202  that a plurality of LU sets of different modes are registered in association with each other, the LU set selection module  121  evaluates the LU set switching condition information  134  based on the LU set selection hint information  141 , to thereby judge whether or not there is an LU set in the mode that satisfies a switching condition (i.e., whether or not a condition registered as the performance-first condition  803  or the power-saving-first condition  804  is met with respect to any evaluation item  802 ) (Step  1203 ). This is judged in the manner described above with reference to  FIGS. 8 and 9 . 
     When it is judged in Step  1203  that there is no LU set in the mode that satisfies a switching condition, the LU set selection module  121  does not need to execute temporary mode switching. Then the LU set selection module  121  executes Steps  1208  and  1209  and ends the LU set temporary switching permitted/prohibited judging processing. 
     When it is judged in Step  1203  that there is an LU set in the mode that satisfies a switching condition, the LU set selection module  121  registers a value indicating the mode that satisfies a switching condition as the temporary mode  606  in the employed mode information  132  (Step  1204 ). 
     Next, the LU set selection module  121  judges whether or not the value registered as the temporary mode  606  in the employed mode information  132  differs from the value registered as the current mode  604  (Step  1205 ). 
     When it is judged in Step  1205  that the value registered as the temporary mode  606  is the same as the value registered as the current mode  604 , there is no need to switch modes. Then the LU set selection module  121  executes Steps  1208  and  1209  and ends the LU set temporary switching permitted/prohibited judging processing. 
     When it is judged in Step  1205  that the value registered as the temporary mode  606  differs from the value registered as the current mode  604 , on the other hand, the LU set selection module  121  judges whether or not an LU set that is associated with a mode registered as the current mode  604  is being used (Step  1206 ). Specifically, the LU set selection module  121  refers to, for example, information indicating whether an LU can be unmounted which is managed by the OS to judge whether or not the unmounting is possible. When an LU can be unmounted, it is judged that the LU is not in use whereas it is judged that the LU is in use when the unmounting is not possible. Whether an LU can be unmounted may be judged from, for example, information obtained through a fuser command. 
     When it is judged in Step  1206  that an LU set that is associated with a mode registered as the current mode  604  is in use, a power supply control unit that contains LUs belonging to this LU set cannot be powered off. In other words, modes cannot be switched in this case. Then the LU set selection module  121  executes Steps  1208  and  1209  and ends the LU set temporary switching permitted/prohibited judging processing. 
     When it is judged in Step  1206  that an LU set that is associated with a mode registered as the current mode  604  is not in use, the LU set selection module  121  returns a value indicating that temporary mode switching is executed (for example, “permitted”) (Step  1207 ), and ends the LU set temporary switching permitted/prohibited judging processing. In the case where Step  1207  is executed, it is judged that temporary switching of LU sets is executed in Step  1102  of  FIG. 11 . 
       FIG. 13  is a flow chart showing processing that is executed by the LU set synchronizing start-up module  123  according to the embodiment of this invention. 
     The processing shown in  FIG. 13  is executed in Step  1010  of  FIG. 10  and Step  1105  of  FIG. 11 . 
     First, the LU set synchronizing start-up module  123  refers to the synchronization completion flag  504  of the LU set management information  131  to judge whether or not every LU pair consisting of an LU that is designated as the copy source and a corresponding LU that is designated as the copy destination has been synchronized (Step  1301 ). 
     When it is judged in Step  1301  that at least one LU pair has not been synchronized, it means that the latest data stored in one of the LUs that form this LU pair is not reflected on the other LU. Then the LU set synchronizing start-up module  123  sends an instruction about the unsynchronized LU pair to the LU-LU synchronization program  151  to synchronize the copy destination LU with the copy source LU (in other words, to copy data stored in the copy source LU to the copy destination LU) (Step  1302 ). After the copying is completed, the LU set synchronizing start-up module  123  updates the value of the synchronization completion flag  504  to “Yes” in an entry for the paired LUs between which the copying has been executed, and ends the processing. 
     When it is judged in Step  1301  that every LU pair has been synchronized, the LU set synchronizing start-up module  123  ends the processing without executing Step  1302 . 
     Through the processing of  FIG. 13 , whether or not the LU set selected this time is the same as the previously selected LU set is judged. When the two are not the same, data in the previously selected LU set that is updated after the selection is made is copied to the LU set selected this time. 
     As will be described later with reference to  FIGS. 18 and 19 , in the case where the only data that differs between two associated LUs out of all data stored in the two associated LUs is one designated as data that does not need to be synchronized (e.g., meta data), the value of the synchronization completion flag  504  is “Yes”. The copying for synchronization (Step  1302 ) is not executed in this case. 
       FIG. 14  is a flow chart showing processing that is executed by the power supply control module  122  according to the embodiment of this invention. 
     The processing shown in  FIG. 14  is executed by the power supply control module  122  upon reception of the instructions sent in Steps  1007 ,  1009 , and  1011  of  FIG. 10  and Steps  1104  and  1106  of  FIG. 11 . 
     First, the power supply control module  122  judges whether or not the received instruction is to power on (Step  1401 ). 
     When the received instruction is to power on, the power supply control module  122  executes powering on processing (Step  1402 ). The powering on processing will be described later with reference to  FIG. 15 . 
     When the received instruction is not a power-on instruction, it means that powering off is instructed. Then the power supply control module  122  executes powering off processing (Step  1403 ). The powering off processing will be described later with reference to  FIG. 16 . 
     Finishing Step  1402  or Step  1403 , the power supply control module  122  ends the processing. 
       FIG. 15  is a flow chart showing the powering on processing which is executed by the power supply control module  122  according to the embodiment of this invention. 
     The powering on processing is executed in Step  1402  of  FIG. 14 . 
     First, the power supply control module  122  refers to the power supply control information  133  to judge whether or not the power on/off state  705  is “ON” for every power supply control unit to which the LUs in an LU set designated as the control object belong (Step  1501 ). 
     When it is judged in Step  1501  that the power on/off state  705  is “OFF” for at least one of power supply control units to which the LUs in an LU set designated as the control object belong, the power supply control module  122  sends an instruction to the storage system  102  or the SVP  103  to power on any power supply control unit whose power on/off state  705  is “OFF” among power supply control units to which the LUs in an LU set designated as the control object belong (Step  1502 ). 
     However, power supply control units identified in Step  1502  may include one that is prohibited from being powered off. For instance, a power supply control unit that stores information necessary to manage the storage system  102  is set such that the power supply control unit is on all the time and is prohibited from being powered off. As for such power supply control units that are prohibited from being powered off, there is no need to send a power-on instruction in Step  1502 . 
     In the case where the instruction in Step  1502  is sent to the SVP  103 , the SVP  103  sends an instruction to the storage system  102  to power on a power supply control unit specified in the received instruction. The same applies to Step  1603  described later. 
     Next, the power supply control module  122  updates the value of the power on/off state  705  to “ON” in an entry for a power supply control unit that has been powered on as a result of the instruction given in Step  1502  (Step  1503 ). 
     Next, the power supply control module  122  updates the value of the in-use flag  702  to “Yes” for the LUs contained in the LU set that is designated as the control object (Step  1504 ). 
     When it is judged in Step  1501  that the power on/off state  705  is “ON” for every power supply control unit to which the LUs in the LU set designated as the control object belong, it means that the power of those power supply control units is already on. Then the power supply control module  122  executes Step  1504 , skipping Steps  1502  and  1503 . 
     After finishing Step  1504 , the power supply control module  122  ends the powering on processing. 
       FIG. 16  is a flow chart showing the powering off processing which is executed by the power supply control module  122  according to the embodiment of this invention. 
     The powering off processing is executed in Step  1403  of  FIG. 14 . 
     First, the power supply control module  122  refers to the power supply control information  133  to update the value of the in-use flag  702  to “No” for the LUs contained in the LU set that is designated as the control object (Step  1601 ). 
     Next, the power supply control module  122  refers to the value of the power supply control unit ID  704  in each entry for an LU, where the value of the in-use flag  702  has been updated to “No” in Step  1601 . Then the power supply control module  122  judges whether or not the value of the in-use flag  702  is “No” for every LU that is associated with the referred-to value of the power supply control unit ID  704  (Step  1602 ). 
     For example, in the case where the value of the in-use flag  702  is updated in Step  1601  from “Yes” to “No” in an entry for the LU  000   311   a , the power supply control module  122  judges in Step  1602  whether or not the value of the in-use flag  702  is “No” for other LUs stored in a power supply control unit that stores the LU  000   311   a  (in the examples of  FIG. 7  and  FIG. 3 , the other LUs are the LU  001   311   e , the LU  002   311   i , and the LU  003   311   m , which are stored in the chassis  00   331   a ). 
     When it is judged in Step  1602  that the value of the in-use flag  702  is “No” for every LU that fits the criterion (in other words, a power supply control unit that stores an LU whose value of the in-use flag  702  has been updated to “No” in Step  1601  contains only LUs that are not in use), the power supply control unit containing those LUs can be powered off. Then the power supply control module  122  sends an instruction to the storage system  102  or the SVP  103  to power off a power supply control unit that contains LUs for which the value of the in-use flag  702  has been updated to “No” in Step  1601  (Step  1603 ). The spinning of the disks is thus stopped in an HDD to which an LU selected to be accessed is not allocated. 
     However, in the case where this power supply control unit is prohibited from being powered off, the power supply control module  122  does not send the instruction in Step  1603 . 
     Next, the power supply control module  122  updates the value of the power on/off state  705  to “OFF” for a power supply control unit that has been powered off in Step  1603  (Step  1604 ). 
     After finishing Step  1604 , the power supply control module  122  ends the powering off processing. 
     On the other hand, when it is judged in Step  1602  that the value of the in-use flag  702  is “Yes” for at least one of LUs that fit the criterion, it means that a power supply control unit that stores an LU whose value of the in-use flag  702  has been updated to “No” in Step  1601  contains at least one LU that is in use. In this case, the power supply control unit containing an LU whose value of the in-use flag  702  has been updated to “No” in Step  1601  cannot be powered off. Then the power supply control module  122  ends the powering off processing without executing Steps  1603  and  1604 . 
       FIG. 17  is a flow chart showing processing that is executed by the switching condition registration module  124  according to the embodiment of this invention. 
     The switching condition registration module  124  executes the processing shown in  FIG. 17  in order to register in the LU set switching condition information  134 . The processing of  FIG. 17  may be executed at an arbitrary time point. For example, the processing of  FIG. 17  may be executed before the computer system shown in  FIG. 1  is put into operation. The processing of  FIG. 17  may also be executed at an arbitrary time point in order to change information registered in the LU set switching condition information  134 . 
     First, the switching condition registration module  124  receives an evaluation item, information to be registered as an evaluation item, and mode identification information that is associated with the information to be registered as an evaluation item (or identification information of LU set that is associated with mode) (Step  1701 ). Those pieces of information may be entered to the host computer  101  by the system administrator or a user, for example. 
     Next, the switching condition registration module  124  registers the information received in Step  1701  in the LU set switching condition information  134  (Step  1702 ). Specifically, the switching condition registration module  124  registers the received evaluation item as the evaluation item  802 . The switching condition registration module  124  registers the information to be registered as an evaluation item as the performance-first condition  803  or the power-saving-first condition  804 . Which of the performance-first condition  803  and the power-saving-first condition  804  is chosen is determined from the received mode identification information. 
     For example, when the switching condition registration module  124  receives in Step  1701  “specified upon mounting” as an evaluation item, “perf” as information to be registered as an evaluation item associated with the performance-first mode, and “psave” as information to be registered as an evaluation item associated with the power-saving-first mode, the switching condition registration module  124  registers “specified upon mounting”, “perf”, and “psave” as the evaluation item  802 , the performance-first condition  803 , and the power-saving-first condition  804 , respectively, in the LU set switching condition information  134  shown in  FIG. 8 . 
     Next, referring to  FIGS. 18 to 21 , processing of managing update of sub-data (e.g., sub-data  1   431   a  to sub-data  16   431   p ) and information that is used in the processing are described. 
     As has been described with reference to  FIG. 5 , when data stored in one of two LUs that are associated with one piece of sub-data is updated, the synchronization completion flag  504  is updated to “No” in an entry for this sub-data. Synchronization processing shown in  FIG. 13  is thereafter executed for the LU pair whose synchronization completion flag  504  is “No”. 
     In practice, however, not all of data stored in LUs that form an LU pair as the one described above needs to be synchronized. It is generally unnecessary to synchronize information that indicates file management attributes (information indicating attributes of file, meta data of file system of entire storage space, and the like). To give a specific example, when an LU is mounted, management information indicating that the mounting has been executed is written in the LU, and management information as this does not need to be copied to other LUs. 
     This embodiment includes processing of  FIGS. 18 and 19  so that synchronization is executed only when data that needs to be synchronized is updated. Executing unnecessary synchronization can thus be avoided. 
       FIG. 20  is an explanatory diagram of the sub-data update notification exclusion information  135  according to the embodiment of this invention. 
     The sub-data update notification exclusion information  135  is used to manage data that does not need to be synchronized among data stored in the above LU pair. Specifically, the sub-data update notification exclusion information  135  contains a classification  2001  and an excluded item  2002 . 
     The classification  2001  indicates a classification of data that does not need to be synchronized. In the example of  FIG. 20 , meta data, data (directory), and data (file) are registered as the classification  2001 . 
     The excluded item  2002  is information for identifying data that does not need to be synchronized. 
     For example, in an entry of  FIG. 20  where the value of the classification  2001  is “meta data” as shown in an entry  2011 , “upon-mounting update information” and “atime (access time)” are registered as the excluded item  2002 . This indicates that, out of meta data (i.e., data for managing sub-data) stored in an LU, upon-mounting update information and information indicating an access time do not need to be synchronized. The upon-mounting update information here is, for example, information indicating that the LU has been mounted. 
     In an entry of  FIG. 20  where the value of the classification  2001  is “data (directory)” as shown in an entry  2012 , “/tmp” is registered as the excluded item  2002 . This indicates that, out of data stored in an LU, data of a file under a directory “/tmp” does not need to be synchronized. 
     In an entry of  FIG. 20  where the value of the classification  2001  is “data (file)” as shown in an entry  2013 , “/large/log” and “/large/messages” are registered as the excluded item  2002 . This indicates that, out of data stored in an LU, data of files “/large/log” and “/large/messages” does not need to be synchronized. 
       FIG. 21  is an explanatory diagram of the sub-data update information  142  according to the embodiment of this invention. 
     The sub-data update information  142  is information for managing update of data that is contained in each piece of sub-data (i.e., data stored in LUs that are associated with each piece of sub-data). Specifically, the sub-data update information  142  contains sub-data  2101 , an updated flag  2102 , and a notified flag  2103 . 
     The sub-data  2101  indicates the identifier of each piece of sub-data. For example, the sub-data labels “ 1 ” to “ 16 ” of the sub-data  1   431   a  to the sub-data  16   431   p  are registered as the sub-data  2101 . 
     The updated flag  2102  is information that indicates whether or not data contained in each piece of sub-data has been updated. A value “Yes” registered as the updated flag  2102  indicates that the data has been updated. 
     The notified flag  2103  is information that indicates whether or not the LU set management program  111  has been notified of an update of data contained in each piece of sub-data after the update. A value “Yes” registered as the notified flag  2103  indicates that the update has been notified. 
     The sub-data update information  142  may be created when, for example, sub-data is newly associated with LUs that belong to one of two associated LU sets. At that point, the two LU sets are synchronized and the initial values of the updated flag  2102  and the notified flag  2103  are therefore “No”. 
       FIG. 18  is a flow chart showing processing that is executed by the sub-data update information management module  143  according to the embodiment of this invention. 
     The processing shown in  FIG. 18  is executed by the sub-data update information management module  143  when the file system program  112  executes write processing. 
     First, the sub-data update information management module  143  refers to the sub-data update information  142  to judge whether or not the value of the updated flag  2102  is “Yes” in an entry for sub-data in which data is to be written through the write processing (Step  1801 ). 
     When it is judged in Step  1801  that the value of the updated flag  2102  is “No”, the sub-data update information management module  143  judges whether or not the updated data corresponds to the excluded item  2002  in the sub-data update notification exclusion information  135  (Step  1802 ). 
     When it is judged in Step  1802  that the updated data does not correspond to the excluded item  2002 , it means that the updated data is data that needs to be synchronized. Then the sub-data update information management module  143  updates the value of the updated flag  2102  in Step  1803  to “Yes” in the entry for this sub-data. 
     When it is judged in Step  1802  that the updated data corresponds to the excluded item  2002 , it means that the updated data is data that does not need to be synchronized. Then the sub-data update information management module  143  proceeds to Step  1804 , skipping Step  1803 . 
     When it is judged in Step  1801  that the value of the updated flag  2102  is “Yes”, it automatically means that the updated data is data that needs to be synchronized. Then the sub-data update information management module  143  proceeds to Step  1804 , skipping Steps  1802  and  1803 . 
     When it is judged in Step  1804  that the value of the notified flag  2103  is “No”, the sub-data update information management module  143  notifies the LU set management program  111  of the fact that data in the sub-data has been updated (Step  1805 ). The notification contains information for identifying the sub-data that contains the updated data (hereinafter, referred to as “updated sub-data”). The sub-data update information management module  143  also updates the value of the notified flag  2103  to “Yes” in the entry for the updated sub-data. 
     The processing of the sub-data update information management module  143  is thus completed. 
     The sub-data update information management module  143  ends the processing without executing Step  1805  when it is judged in Step  1804  that the value of the notified flag  2103  is “Yes”, which means that the LU set management program  111  has already been notified of the existence of data that needs to be synchronized. 
       FIG. 18  shows that Steps  1801  to  1803  and Step  1804  are executed in succession. Alternatively, Steps  1804  and  1805  may be executed independently of Steps  1801  to  1803  at a different time point. In this case, a plurality of sub-data pieces that need to be synchronized can be notified to the LU set management program  111  at one time. 
       FIG. 19  is a flow chart showing processing that is executed by the LU update information registration module  125  according to the embodiment of this invention. 
     The LU update information registration module  125  executes the processing shown in  FIG. 19  when receiving a notification that is sent in Step  1804  of  FIG. 18 . 
     First, the LU update information registration module  125  receives the identifier of updated sub-data (Step  1901 ). This identifier is contained in the notification sent in Step  1804  of  FIG. 18 . 
     Next, the LU update information registration module  125  judges whether or not an LU that stores the updated sub-data (LU associated with updated sub-data) is being used for Read Only (Step  1902 ). 
     When it is judged in Step  1902  that the LU that stores the updated sub-data is not being used for Read Only, it means that writing new data in this LU and updating data that is stored in this LU are allowed. Then the LU update information registration module  125  updates the synchronization completion flag  504  to “No” in an entry of the LU set management information  131  for the LU that stores the updated sub-data (Step  1903 ), and ends the processing. 
     When it is judged in Step  1902  that the LU that stores the updated sub-data is being used for Read Only, it means that writing new data in this LU and updating data that is stored in this LU are both prohibited. Then the LU update information registration module  125  ends the processing without executing Step  1903 . 
     In the processing of  FIGS. 18 and 19 , the value of the synchronization completion flag  504  is updated to “No” for two LUs associated with each other when data stored in one of the two LUs is updated after processing of synchronizing the two (Step  1302  of  FIG. 13 ) is executed. However, the value of the synchronization completion flag  504  remains “Yes” in the case where data that does not need to be synchronized (e.g., meta data) alone is updated. 
     In other words, among data held in two associated LUs whose value of the synchronization completion flag  504  is “No”, at least some of data that needs to be synchronized differs between the two whereas, for two associated LUs whose value of the synchronization completion flag  504  is “Yes”, at least data that needs to be synchronized out of all data held in the two LUs does not differ between the two. 
       FIG. 22  is a flow chart showing processing that is executed by the LU set selection module  121  to end the operation of a temporarily switched LU set according to the embodiment of this invention. 
     Specifically,  FIG. 22  shows processing executed in order to stop the operation of the switched LU set after a temporary switch of LU sets is made by the processing of  FIG. 11  and start running the original LU set (LU set that is associated with mode registered as current mode  604 ) again. 
     First, the LU set selection module  121  judges whether or not an LU set that is associated with a mode registered as the temporary mode  606  in the employed mode information  132  (in examples of  FIG. 6  and  FIG. 5 , LU set  321  which is associated with performance-first mode) is in use (Step  2201 ). Specifically, when the value of the in-use flag  702  is “No” for all LUs that belong to the LU set, it is judged that the LU set is not in use. When the value of the in-use flag  702  is “Yes” for at least one of LUs that belong to the LU set, on the other hand, it is judged that the LU set is in use. 
     When it is judged in Step  2201  that the LU set is not in use, the LU set selection module  121  makes the LU set quiescent (Step  2202 ). 
     Next, the LU set selection module  121  designates, as a control object, an LU set that is associated with a mode registered as the current mode  604  in the employed mode information  132  (in examples of  FIG. 6  and  FIG. 5 , LU set  311  which is associated with power-saving-first mode), and instructs the power supply control part  252  to power on the LU set (Step  2203 ). 
     Next, the LU set selection module  121  designates, as the copy source, the LU set that is associated with the mode registered as the temporary mode  606  and, as the copy destination, the LU set that is associated with the mode registered as the current mode  604  to execute the LU set synchronizing start-up module  123  (Step  2204 ). 
     Next, the LU set selection module  121  designates, as a control object, the LU set that is associated with the mode registered as the temporary mode  606  in the employed mode information  132 , and instructs the power supply control part  252  to power off the LU set (Step  2205 ). 
     Then, the LU set selection module  121  clears the value registered as the temporary mode  606  in the employed mode information  132  (Step  2206 ). 
     Next, the LU set selection module  121  sends a notification informing that the preparation of the LU set for use has been completed (Step  2207 ). As the notification, the LU set selection module  121  may send the identifiers of the LUs belonging to the LU set that is associated with the mode registered as the current mode  604 . 
     As a result of executing Step  2207 , the identifiers of the LUs belonging to the selected LU set are registered as the selected LU ID  2502  in the selected LU set information  421 , as in Step  1012  of  FIG. 10 . 
     The LU set selection module  121  thus finishes the processing. 
     The LU set selection module  121  ends the processing without executing Steps  2202  to  2207  when it is judged in Step  2201  that the LU set is in use, which means that the LU set cannot be powered off. 
     As has been described, according to the embodiment of this invention, two LU sets that store the same data are used. LUs in one of those LU sets store data dispersedly among a relatively small number of power supply control units whereas LUs in the other LU set store data dispersedly among a relatively large number of power supply control units. Of reducing the power consumption of the storage system and improving the access performance of the storage system, which one is to be given priority is judged from conditions set at the system administrator&#39;s, or a user&#39;s, discretion. When the reduction in power consumption is given priority, the former LU set is used and, when the improvement in access performance is given priority, the latter LU set is used. In this way, power consumption and access performance can be optimized to suit the file type, the job type, or the like. 
     While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.