Patent Publication Number: US-2009228674-A1

Title: Disk array apparatus for controlling start timing of disk drive

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent applications JP 2008-54987 filed on Mar. 5, 2008, the content of which are hereby incorporated by reference into this application. 
     BACKGROUND 
     This invention relates to a disk array device, and more particularly, to a disk array device where vibrations of a hard disk drive are suppressed. 
     As a conventional technology regarding a disk array device which includes hard disk drives (HDD&#39;s) mounted therein at a high density, for example, a disk array device described in JP 2004-178557 A has been known. According to the disk array device described in JP 2004-178557 A, a sufficient cooling effect can be obtained with a weak airflow by disposing a heat radiation member on one side of the HDD&#39;s. Thus, the HDD&#39;s can be mounted in the disk array device at a high density. 
     JP 2006-235964 A describes a disk array device where a plurality of HDD blades having HDD&#39;s arrayed in a depth direction are mounted in an enclosure. The disk array device described in JP 2006-235964 A can achieve both high-density mounting and maintenance performance of the HDD&#39;s. 
     However, the enclosure including the HDD&#39;s mounted therein at a high density is easily affected by heat and vibrations generated from another HDD. Once affected by the vibrations, positioning of an HDD header for writing or reading data takes time, causing a reduction in data transfer speed. The vibrations cause an error in header positioning, and correction of the error takes longer time for data writing or reading. 
     A main cause of the vibrations is rotation of a platter in the HDD. A basic frequency of generated vibrations is calculated from an inverse number of a rotational speed of the HDD. Components of the generated vibrations include, in addition to the basic frequency, a harmonic component whose frequency is an integral multiple of the basic frequency. As an HDD surface density increases, an influence of vibrations on accuracy of head positioning using servo control increases. 
     As countermeasures against the vibrations generated by the HDD, JP 2006-146616 A discloses a method which uses a leaf spring. Generally, there has been known a method of supporting an HDD by a rubber cushion washer. As a technology of suppressing vibrations based on arrangement of HDD&#39;s in an enclosure, JP 2007-517355 A discloses a technology involving arranging two HDD&#39;s simultaneously accessed such that vibrations by an actuator operation are cancelled. 
     SUMMARY 
     As a representative power-saving technology for a disk array device, a MAID technology of turning OFF the power of an unaccessed HDD has been known. According to a MAID function, when a host computer makes an I/O request to the power-OFF HDD, the power of the HDD is turned ON to complete spinning-up, and then data is input or output. To spin up the HDD, a platter has to be rotated by large torque. Thus, vibrations are larger than those during normal rotation. 
     The disk array device that includes the MAID function is expected to be applied to backing-up which requires only low performance or archive use, which requires HDD&#39;s to be mounted at a high density when used in place of a tape library device. However, when the HDD&#39;s are mounted at a high density, physical restrictions on countermeasures against vibrations are large because space is limited. 
     This invention has been made to solve the above problems of the conventional art, and an object of this invention is to realize a disk array device which suppresses vibrations not only during normal rotation but also during HDD spinning-up. 
     A representative aspect of this invention is as follows. That is, there is provided a disk array apparatus, comprising: a plurality of disk drives; and a controller for controlling the plurality of disk drives. The disk array apparatus controls power of the plurality of disk drives for each disk group including of the plurality of disk drives. The two disk drives included in the same disk group are arranged in sets on one base member. The controller controls to set start timings of the two disk drives constituting the set identical to each other. 
     According to an aspect of this invention, there can be realized the disk array device using a MAID function of controlling the power of the HDD&#39;s, which suppresses vibrations not only during normal rotation but also during HDD spinning-up. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
         FIG. 1  is a block diagram showing a configuration of a disk array device in accordance with an first embodiment of this invention; 
         FIG. 2  is a perspective view showing an additional chassis in accordance with the first embodiment of this invention; 
         FIG. 3  is an explanatory diagram showing an internal structure of the additional chassis in accordance with the first embodiment of this invention; 
         FIG. 4  is a perspective view showing an HDD blade in accordance with the first embodiment of this invention; 
         FIG. 5  is a right side elevation view showing the HDD blade in accordance with the first embodiment of this invention; 
         FIG. 6  is a left side elevation view showing the HDD blade in accordance with the first embodiment of this invention; 
         FIG. 7A  is a side view showing two HDD&#39;s attached to the HDD blade in accordance with the first embodiment of this invention; 
         FIG. 7B  is a sectional view showing the two HDD&#39;s attached to the HDD blade in accordance with the first embodiment of this invention; 
         FIG. 8  is an explanatory diagram showing a RAID configuration management table in accordance with the first embodiment of this invention; 
         FIG. 9  is an explanatory diagram showing a spare HDD management table in accordance with the first embodiment of this invention; 
         FIG. 10  is an explanatory diagram showing power supply control of the RAID group in accordance with the first embodiment of this invention; 
         FIG. 11A  to  FIG. 11D  are explanatory diagrams showing a data recovery process when a failure occurs in the HDD in accordance with the first embodiment of this invention; 
         FIG. 12  is a flowchart showing a data recovery process when a failure occurs in the HDD in accordance with the first embodiment of this invention; 
         FIG. 13  is a wave form chart showing residual vibrations generated by an HDD pair in accordance with a second embodiment of this invention; 
         FIG. 14  is an explanatory diagram showing power supply control of an HDD pair in accordance with the second embodiment of this invention; 
         FIG. 15  is a block diagram showing an offset circuit in accordance with the second embodiment of this invention; 
         FIG. 16  is a flowchart showing a process for correcting power supply timing in accordance with the second embodiment of this invention; and 
         FIG. 17  is an explanatory diagram showing power supply control of a RAID group in accordance with a third embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention will be described below referring to the drawings. 
     First Embodiment 
       FIG. 1  illustrates a configuration of a disk array device  10  according to a first embodiment of this invention. 
     The disk array device  10  is coupled to a host computer  80  via a host interface  70 . The host computer  80  requests the disk array device  10  to input or output data. 
     The disk array device  10  includes a basic chassis  11 , an additional chassis  12 , a fan chassis (not shown), and a power supply chassis (not shown). 
     The basic chassis  11  includes controllers  20  and  30 , HDD&#39;s  15 , switches  13 , and logic circuits  14 . 
     The controller  20  includes a CPU  21 , a local memory  22 , a cache memory  25 , a channel control unit  23 , a disk control unit  24 , and a data transfer control unit  26 . 
     The CPU  21  is a processor for executing a program stored in the local memory  22  to control the controller  20 . The CPU  21  can control a process of inputting or outputting data to or from HDD&#39;s  15  installed in the basic and additional chassis  11  and  12 , for example, in response to data inputting or outputting requested from the host computer  80 . 
     The local memory  22  stores programs executed by the CPU  21  and various tables (e.g., RAID configuration management table and spare HDD management table described below). It should be noted that a program and data stored in the HDD  15  is copied to the local memory  22  when necessary. 
     The cache memory  25  is a storage area for temporarily storing write data entered from the host computer  80  to the disk array device  10 , and read data output from the disk array device  10  to the host computer  80 . The cache memory  25  may include, for example, a nonvolatile memory or a volatile memory backed up by a battery. It should be noted that in the case of the nonvolatile memory, the cache memory  25  can hold stored cache data even when power is cut off. 
     The channel control unit  23  is an interface coupled to the host computer  80  to receive data I/O request (e.g., block I/O request or file I/O request) from the host computer  80 . 
     The disk control unit  24  is an interface coupled to the HDD  15  to make a data I/O request to the HDD  15  by using a predetermined protocol. 
     The data transfer control unit  23  controls data transfer between the host computer  80  and an HDD  25  by the instruction from the CPU  21 . 
     It should be noted that the controllers  20  and  30  are duplicated to improve availability. 
     The controller  30  has the same configuration as that of the controller  20 , and includes a CPU  31 , a local memory  32 , a cache memory  35 , a channel control unit  33 , a disk control unit  34 , and a data transfer control unit  36 . It should be noted that the CPU  31 , the local memory  32 , the cache memory  35 , the channel control unit  33 , the disk control unit  34 , and the data transfer control unit  36  correspond to the CPU  21 , the local memory  22 , the cache memory  25 , the channel control unit  23 , the disk control unit  24 , and the data transfer control unit  26 , respectively. 
     The data transfer control units  26  and  36  of the controllers  20  and  30  are coupled. Data written in the cache memory  25  of the controller  20  is transferred to the controller  30  to be written in the cache memory  35 . Similarly, data written in the cache memory  35  of the controller  30  is transferred to the controller  20  to be written in the cache memory  25 . Accordingly, the cache data written in the cache memories  25  and  35  are duplicated. Hereinafter, there will be described the case where the controller  20  is operated. 
     The HDD  15  stores programs, user data, and the like, and includes, for example, a serial attached SCSI (SAS) interface. The HDD  15  includes a dual I/O ports to improve availability, and is coupled to the controllers  20  and  30  via the switch  13 . 
     The switch  13  is a device for transferring I/O data from the host computer  80  to the HDD  15 . The switch  13  can include, for example, an SAS expander. In an example shown in  FIG. 1 , the number of switches  13  is one corresponding to the logical circuit  14 . However, an optional number of switches  13  may be installed. 
     The logic circuit  14  includes a power supply control register for an HDD pair described below, and a register for controlling lighting of an LED which notifies an access status of each HDD to the outside. Based on a command issued from the CPU  21  of the controller  20 , for example, each register can be set inbound. The logic circuit  14  may include, for example, an FPGA, a CPLD, or an ACIC. For example, one logical circuit  14  may be installed for each switch  13 . Alternatively, one logical circuit  14  may be installed for a plurality of switches  13 . 
     The switch  13  and the logic circuit  14  are duplexed to improve availability. The HDD  15  of the basic chassis  11  is mounted by a conventional method, and an influence of vibrations generated by rotation of the platter can be ignored. 
     As in the case of the basic chassis  11 , the additional chassis  12  includes HDD&#39;s  15 , switches  13 , and logic circuits  14 . Functions of the components (HDD  15 , switch  13 , and logic circuit  14 ) are similar to those of the components of the basic chassis  11 . 
     Each of the basic chassis  11  and the additional chassis  12  includes one or a plurality of RAID groups, and one or a plurality of spare disks. The RAID group is configured by, for example, grouping four HDD&#39;s  15  (3D+1P). The RAID group is configured by a disk array management terminal (not shown) coupled to the disk array device  10  based on information in the spare HDD management table described below. Among the HDD&#39;s  15  in the additional chassis  12 , an HDD not allocated as a RAID group is treated as a spare disk. 
     The fan chassis cools the HDD&#39;s  15  in the additional chassis  12 . The power supply chassis supplies power to the basic chassis  11  and the additional chassis  12 . 
     The basic chassis  11 , the additional chassis  12 , the fan chassis, and the power supply chassis can be mounted on, for example, a 19-inch rack. 
     In the example of  FIG. 1 , the disk array device  10  includes one basic chassis  11 . However, a plurality of basic chassis may be installed. The disk array device  10  includes one additional chassis  12 . However, a plurality of additional chassis may be installed. 
       FIG. 2  illustrates, in perspective, the additional chassis  12  according to the first embodiment of this invention. 
     As shown in  FIG. 2 , the additional chassis  12  can be mounted on a chassis (e.g., 19-inch rack) by using a panel  41 . 
     An HDD blade  40  is a base member (HDD blade) where a plurality of (e.g., 12) HDD&#39;s  15  are mounted in a depth direction. A plurality of (e.g., 10) HDD blades  40  are mounted on the additional chassis  12 . The number of HDD&#39;s mounted in the HDD blade  40  is preferably even. 
     As shown in  FIG. 2 , the additional chassis  12  includes many vent holes on its upper surface. Although not shown, the additional chassis  12  also includes many vent holes on its bottom surface. 
     In the 19-inch rack, by a fan chassis (not shown) installed near (e.g., directly above an HDD enclosure) the HDD enclosure (e.g., additional chassis  12 ), heat discharged from the HDD  15  passes through a gap between the HDD blades to be sucked up to the exterior of the HDD enclosure. 
       FIG. 3  illustrates an internal structure of the additional chassis  12  according to the first embodiment of this invention. 
     A substrate  42  is a back plane substrate on which the switch  13  and the logical circuit  14  are mounted. Connectors  43  and  44  of each HDD blade  40  and the back plane substrate  42  are coupled. The back plane substrates  42  are coupled to each other in the chassis (19-inch rack). 
       FIG. 4  illustrates, in perspective, the HDD blade  40  according to the first embodiment of this invention.  FIG. 5  illustrates a right side of the HDD blade  40  according to the first embodiment of this invention.  FIG. 6  illustrates a left side of the HDD blade  40  according to the first embodiment of this invention. 
     As shown in  FIG. 4 , a substrate  45  is a blade substrate. In the substrate  45 , a communication interface signal line for coupling the switch  13  and the HDD  15 , an HDD power control signal line drawn out from the logical circuit  14  to control power of the HDD, and an LED control signal line for lighting an LED are laid. 
     A material of the substrate  45  is preferably, for example, a glass epoxy resin. As shown in  FIG. 4 , 12 2.5-inch HDD&#39;s 15 in total, 6 in each side, may be installed in both sides of the substrate  45 . 
     As shown in  FIG. 5 , each HDD  15  is fixed to the substrate  45  by support plates  50  and  51 . A connector  54  is an HDD interface connector to couple the communication interface signal line and the HDD power supply line laid in the substrate  45  to the HDD  15 . 
     Although not shown, the substrate  45  includes a voltage regulator for generating power supply voltages for the HDD  15  and the LED  47 . A rail  46  is an inner rail to facilitate insertion/pulling-out of the HDD blade  40  into/from the additional chassis, and to prevent bending of the substrate  45 . A material of the inner rail  46  is preferably, for example, aluminum. 
     As shown in  FIG. 6 , the inner rail  46  is fixed to the substrate  45  (e.g., fixed by screws  48 ). 
     The LED  47  is a 3-color LED, and displays an access status (e.g., ready status, active status, or error status) to each HDD  15  mounted on the HDD blade  40  by three colors (e.g., green, blue, and red). 
     The connector  43  is coupled to the back plane substrate  42  in the additional chassis  12 , and fixed to the substrate  45  (e.g., by soldering). A lever  49  fixes the HDD blade  40  to the additional chassis  12 . The HDD blade  40  is inserted into the additional chassis  12  to be coupled to the back plane substrate  42 , and operates the lever  49  (e.g., rotates the lever  49 ), thereby fixing the HDD blade  40  to the additional chassis  12 . 
     Next, a method of fixing the HDD  15  to the substrate  45  will be described. 
       FIG. 7A  is a side view of two HDD&#39;s  15 A and  15 B attached to the HDD blade  40  of this invention.  FIG. 7B  is a sectional view of the two HDD&#39;s  15 A and  15 B attached to the HDD blade  40  of this invention. 
     The HDD blade  40  is attached to the substrate  45  by the support plates  50  and  51 . 
     Specifically, as shown in  FIGS. 7A and 7B , there are four screw holes on a side face of the HDD  15 , and the support plates  50  and  51  are attached by screws  52 . 
     The two HDD&#39;s  15 A and  15 B to which the support plates  50  and  51  are attached are stuck to each other from both sides of the substrate  45  to be fastened together by screws  53 . In this case, the HDD&#39;s  15 A and  15 B are coupled to HDD interface connectors  54 A and  54 B fixed to the substrate  45 , respectively. The HDD interface connectors  54 A and  54 B are mounted in positions of both sides of the blade substrate  45  where Y-axis coordinates are equal to each other. 
     A platter  55  is a recording medium in the HDD  15 . A rotational center of the platter  55  is normally present on a straight line bisecting a short side of the HDD  15 . Accordingly, through the fixing method of the HDD  15 , a rotational axis of the platter  55  of the HDD  15 A matches that of the platter  55  of the HDD  15 B. 
     Rotational speeds of the platters  55  (platter rotational speeds) of the HDD&#39;s  15  are equal to each other in the HDD pair (e.g., HDD  15 A and HDD  15 B). For example, a rotational speed is 10000 rpm. The equality in this case means that numerical values of catalog specifications are equal to each other. The numbers of platters of the HDD&#39;s  15  are preferably equal in the HDD pair (e.g., HDD  15 A and HDD  15 B). As a rotational direction of the platter  55  is constant, rotational directions are opposite between the platters  55  of the HDD&#39;s  15 A and  15 B. As rotational speeds of the platters are normally equal, rotation moments of the platters  55  of the HDD&#39;s  15 A and  15 B indicating vector quantity are opposite in direction but equal in length. Thus, when HDD&#39;s  15  are attached as an HDD pair to the HDD blade  40 , rotation moments generated from the HDD&#39;s  15  cancel each other. 
     As the rotation moment has a correlation with vibrations, vibrations can be reduced by this embodiment. Specifically, when a rotation moment is generated, a force is applied on the HDD  15  to generate vibrations. Accordingly, by canceling the rotation moment, vibrations generated from the HDD  15  can be suppressed. 
     A disk management method in the disk array device  10  will be described below. 
       FIG. 8  illustrates a RAID configuration management table according to the first embodiment of this invention. 
     The RAID configuration management table of  FIG. 8  is for managing correspondence between logical and physical HDD&#39;s constituting a RAID group, and stored in the local memory  22  of the controller  20 . 
     The RAID configuration management table includes a RAID Gr. number, a logical HDD number, a chassis number, a blade number, a physical HDD number, and an HDD pair number. 
     The RAID Gr. number identifies a RAID group formed of HDD&#39;s  15 . The RAID group is configured by dividing or combining a plurality of HDD&#39;s  15 . 
     The logical HDD number identifies an HDD in the RAID group. 
     The chassis number identifies a chassis which constitutes the disk array device  10 . For example, as shown in  FIG. 8 , a chassis number of the basic chassis  11  may be “0”, and a chassis number of the additional chassis  12  may be larger than “1”. 
     The blade number is allocated to the HDD blade  40  mounted in the additional chassis  12 . For the blade number, for example, values of “0” to “9” are allocated. As the basic chassis  11  includes no HDD blade, a value of the blade number is F. 
     The physical HDD number identifies a physical HDD  15 . As shown in  FIG. 8 , for the physical HDD number, for example, values of “0” to “11” are used. In the example of  FIG. 8 , “0” and “1”, “2” and “3”, “4” and “5”, “6” and “7”, “8” and “9”, and “10” and “11”, are HDD pairs where rotation moments cancel each other in the HDD blade  40 . 
     The HDD pair number identifies an HDD pair disposed in the additional chassis  12 . HDD&#39;s  15  that store identical HDD pair numbers constitute an HDD pair. 
     For example, the HDD pair number may indicate an HDD  15  used without constituting any HDD pair in the case of “0”, and HDD&#39;s  15  constituting an HDD pair in cases other than “0”. In this case, by retrieving an HDD pair number, for example, an HDD  15  not constituting any HDD pair as in the case of the HDD  15  disposed in the basic chassis  11  can be distinguished from HDD&#39;s  15  constituting an HDD pair as in the case of HDD&#39;s  15  disposed in the additional chassis  12 . 
     The controller  20  can configure a RAID group by registering correspondence between a logical HDD and a physical HDD (physical HDD  15 ) in the RAID configuration management table. Identical HDD pair numbers indicate HDD&#39;s  15  stuck together to be arranged in the HDD blade  40 . 
       FIG. 9  illustrates a spare HDD management table according to the first embodiment of this invention. 
     The spare HDD management table shown in  FIG. 9  is for managing correspondence between a spare HDD and a physical HDD, and stored in the local memory  22  of the controller  20 . 
     The spare HDD management table includes a spare HDD number, a chassis number, a blade number, a physical HDD number, an HDD pair number, and a status. 
     The spare HDD number identifies a spare HDD. The spare HDD number is, for example, a logical number starting from “0”. The spare HDD is an HDD  15  used for recovering data when a failure occurs in an HDD  15  constituting a RAID group. Specifically, in the data recovery process, contents of data stored in the failed HDD  15  are copied in the spared HDD. 
     Contents of the chassis number, the blade number, the physical HDD number, and the HDD pair number are similar to those of the chassis number, the blade number, the physical HDD number, and the HDD pair number included in the RAID configuration management table, and thus description thereof will be omitted. 
     The status is a flag indicating whether the spare HDD is being used. When the spare HDD is used in the data recovery process, the controller  20  changes the status flag from “UNUSED” to “BEING USED”. When data stored in the HDD  15  of the RAID group is deleted to cancel the RAID group, or when the failed HDD  15  is replaced to configure a new RAID group, the status flag is changed from “BEING USED” to “UNUSED”. 
     The disk array device  10  has a MAID function of turning OFF power of a RAID group to which no access is made from the host computer  80 . In other words, if power of a RAID group including a logical unit (LU) to which the host computer  80  has made a data I/O request is OFF, the power supply of the RAID group is changed to ON. On the other hand, if the host computer  80  has made no data I/O request to the LU constituting the RAID group for a predetermined time, the power supply of the RAID group is changed to OFF. 
     Referring to  FIG. 10 , a method of controlling power supply of the HDD  15  constituting the RAID group will be described. 
       FIG. 10  illustrates power supply control of the RAID group according to the first embodiment of this invention. 
     As shown in  FIG. 10 , the RAID group includes four HDD&#39;s  15 . For example, a RAID group # 1  includes HDD pairs # 1  and # 7 , and a RAID group # 2  includes HDD pairs # 2  and # 8 . 
     A value is set in a power supply control register  60  included in the logical circuit  14  to control power supply of the RAID group. 
     Each bit set in the power supply control register  60  corresponds to the pair of HDD&#39;s stuck together to be arranged in the HDD blade  40 . The power supply control register  60  may be set based on commands entered from the CPU&#39;s  21  and  31 . For example, the register may be set inbound. 
     An HDD pair constituting the RAID group can be specified by referring to the RAID configuration management table. Accordingly, by setting a bit of the power supply control register  60  corresponding to the HDD pair to “1”, power supply of the HDD  15  constituting the RAID group can be simultaneously changed to ON. By setting a bit of the power supply control register  60  corresponding to the HDD pair to “0”, the power supply of the HDD  15  of the RAID group can be simultaneously changed to OFF. To control power supply by a RAID group unit, equal values are set in bits corresponding to an HDD pair of the same RAID group. 
     When “1” is set in the power supply control register  60 , a voltage regulator  61  coupled to the HDD pair is turned ON. The voltage regulator  61  converts a voltage of 12 V supplied from the power supply chassis into 5 V which is an HDD driving voltage. 
     The voltage regulator  61  is preferably arranged near the HDD  15  in the HDD blade  40 . The voltage regulator  61  may be arranged in the back plane substrate  42 . 
     In an example of  FIG. 10 , each HDD  15  includes one voltage regulator  61 . However, when the voltage regulator  61  has an extra output current capacity, each HDD pair may include one voltage regulator  61 . In place of the voltage regulator  61 , a gate circuit to which a driving voltage of the HDD  15  is entered may be used. 
     In the example of  FIG. 10 , by setting values in the power supply control register  60 , the power supply of the HDD&#39;s  15  (HDD&#39;s  15  of the HDD pairs # 1  and # 7 ) of the RAID group  1  is turned ON, while the power supply of the HDD&#39;s  15  (HDD&#39;s  15  of the HDD pairs # 2  and # 8 ) of the RAID group  2  is turned OFF. 
     In the example of  FIG. 10 , by setting a value in the power supply control register  60 , the power of the RAID group is controlled. However, the power of the RAID group may be controlled based on a command entered from the controller  20 . 
     When a failure occurs in one of the HDD&#39;s  15  of the additional chassis  12  during use of the disk array device  10 , data is recovered in the HDD pair. Referring to  FIGS. 11 and 12 , the data recovery process when the HDD  15  of the RAID group fails will be described. 
       FIG. 11A  to  FIG. 11D  illustrate a data recovery process when a failure occurs in the HDD  15  according to the first embodiment of this invention.  FIG. 12  is a flowchart illustrating a data recovery process when a failure occurs in the HDD  15  according to the first embodiment of this invention. 
     The data recovery process shown in  FIG. 12  is carried out by executing a program stored in the memory  22  via the CPU  21  of the controller  20 . The data recovery process may be carried out by executing a program stored in the memory  32  via the CPU  31  of the controller  30 . 
     Referring to  FIG. 11A , a process when a failure occurs in the HDD  15 A of the RAID group  1  will be described. The RAID group  1  employs a RAID level 5 (RAID 5). A substrate  63  is similar to the substrate  45 . HDD&#39;s  15 A and  15 B are arranged so that rotation moments generated by rotation of platters thereof can cancel each other. 
     First, the controller  20  refers to the spare HDD management table to retrieve an HDD pair of spare HDD&#39;s (spare HDD pair) whose status flag in the spare HDD management table is “UNUSED” (S 101 ). 
     The controller  20  judges whether a spare HDD pair retrieved in Step S 101  is present (S 102 ). The controller  20  preferentially selects a spare HDD pair not present in the same HDD blade  40  of the failed HDD  15 . 
     If no spare HDD pair is present, data cannot be recovered in the additional chassis  12 . Accordingly, data is recovered in the basic chassis  11 . On the other hand, if a spare HDD pair is present, the process proceeds to Step S 103 . 
     As shown in  FIG. 11B , the controller  20  executes collection copying in one of the HDD&#39;s  15  of the spare HDD pair retrieved in Step S 101  by using, in the RAID group including the failed HDD  15 A, a normally operating HDD  15  (normal HDD  15 ) (S 103 ). 
     As shown in  FIG. 11C , the controller  20  copies data of the normal HDD  15 B arranged by sandwiching a blade substrate (substrate  63 ) with the failed HDD  15 A in the other HDD  15  of the spare HDD pair retrieved in Step S 101  (S 104 ). 
     As shown in  FIG. 11D , the controller  20  updates values of the blade number, the physical HDD number, and the HDD pair number of the RAID configuration management table, and the status flag of the spare HDD management table (S 105 ). Specifically, the controller  20  changes the blade number from blade  0  to blade  2 , the physical HDD number to a physical HDD number of a copy destination spare HDD, the HDD pair number to an HDD pair number of the copy destination spare HDD, and the status flag from “UNUSED” to “BEING USED”. 
     To replace the failed HDD  15 A, data stored in the HDD  15  present in the same HDD blade  40  of the failed HDD  15 A has to be copied in each spare HDD to be replaced by an HDD blade unit. In this case, as in the case of the process shown in  FIG. 12 , a spare HDD pair is retrieved, and the data is copied in the retrieved spare HDD pair. After completion of the data copying, the HDD blade  40  is replaced. In the example of  FIG. 11D , a spare HDD pair of HDD&#39;s  1101  and  1102  of the RAID group  2  which are present in the same HDD blade  40  as the failed HDD  15 A is retrieved. Then, data of the HDD&#39;s  1101  and  1102  are copied in retrieved HDD&#39;s  1103  and  1104 . After completion of data copying, the HDD blade  40  of the blade  0  is replaced. 
     According to the first embodiment of this invention, the power supply is controlled for each RAID group. However, for example, power supply is controlled for each group (disk group) which includes a plurality of HDD pairs as units. 
     According to the first embodiment of this invention, the HDD&#39;s constituting the RAID group are arranged as a pair in the HDD blade so that rotation moments can cancel each other, and the power supply is controlled so that start timings of the HDD&#39;s constituting the RAID group can be identical to each other. Thus, rotation moments between the HDD&#39;s of the pair can be cancelled during spinning-up, enabling suppressing of vibrations. 
     By suppressing vibrations of the HDD, a collision of the head with the platter can be prevented. Moreover, response to access to the HDD can be improved. 
     An influence of vibrations of the HDD is known to have a correlation with the number of error correction times. Thus, suppression of vibrations can be checked by a self-monitoring analysis and reporting technology (SMART) function which is a self-diagnosing function of the HDD. 
     In a mode where HDD&#39;s are mounted at a high density in a disk array device of limited physical space, vibrations of the HDD can be suppressed. 
     Second Embodiment 
     A second embodiment of this invention will be described below. 
     In the case of the first embodiment, depending on tolerance of the HDD  15  attached to the HDD blade  40  and eccentricity of the platter in the HDD  15 , cancellation of rotation moments between the HDD&#39;s of the pair may be insufficient. 
     Thus, according to the second embodiment, regarding residual vibrations generated by two HDD&#39;s  15 , when vibrations shifted from each other in phase by a predetermined time and opposite to each other in displacement in a direction (hereinafter, direction of the Z axis) vertical to the HDD blade  40  are superimposed as shown in  FIG. 13 , the residual vibrations are suppressed by using an acceleration sensor. The direction vertical to the HDD blade  40  is, for example, a direction vertical to a surface of the HDD blade  40  where vent holes are present. 
       FIG. 14  illustrates power supply control of an HDD pair according to the second embodiment of this invention. 
     The additional chassis  12  includes an acceleration sensor  62 , a preamplifier  63 , and an A/D converter  64  for each HDD pair. 
     The acceleration sensor  62  is arranged near an HDD pair disposed in a substrate  45  to measure acceleration of an HDD  15  of a z-axis direction. 
     The preamplifier  63  amplifies an analog signal output from the acceleration sensor  62 . 
     The A/D converter  64  converts the analog signal amplified by the preamplifier  63  into a digital signal. 
     The preamplifier  63  and the A/D converter  64  are preferably arranged near the acceleration sensor  62  disposed in the substrate  45 . They may be arranged in the back plane substrate  42 , or incorporated in the logic circuit  14 . 
     The logic circuit  14  includes a power supply control register  60 , an HDD pair select register  65 , a sensor I/F control circuit  66 , a built-in memory  67 , and an offset circuit  71 . 
     As in the case of the power supply control register shown in  FIG. 10 , the power supply control register  60  controls power supply of an HDD  15  constituting a RAID group by setting a value in the register. 
     The HDD pair select register  65  selects an HDD pair upon setting of a value in the register. For example, the HDD pair select register  65  includes a bit corresponding to each HDD pair, and an HDD pair corresponding to a bit set to “1” is selected. 
     The sensor I/F control circuit  66  fetches digital signals converted by the A/D converter  64  at a predetermined cycle. The fetched digital signals are written in the built-in memory  67 . Specifically, the sensor I/F control circuit  66  obtains, according to contents of the HDD pair select register  65  set by the controller  20 , acceleration data from the A/D converter  64  corresponding to the HDD pair at a sampling frequency of 10 kHz, and stores the obtained acceleration data in the built-in memory  67 . The sensor I/F control circuit  66  may include, for example, a microcontroller or a state machine. 
     The offset circuit  71  controls timing of switching power supply according to correction of power supply timing described below referring to  FIG. 16 . 
     The controller  20  corrects, after the HDD&#39;s  15  of the additional chassis  12  constitute a RAID group, timing of controlling power supply of one of the HDD&#39;s of a pair (e.g., turning power ON). 
       FIG. 15  is a block diagram showing the offset circuit  71  according to the second embodiment of this invention. 
     The offset circuit  71 B includes a counter  68 , an offset  69 , and a comparator  70 . The counter  68  counts the number of clocks based on a clock cycle. In the offset  69 , a correction value of power supply timing is set. 
     The comparator  70  compares a value of the counter  68  with a value of the offset  69 . When a value of the counter  68  reaches a value of the offset  69 , the comparator  70  outputs a signal for controlling power to the voltage regulator  61 . 
       FIG. 16  is a flowchart showing a process for correcting power supply timing according to the second embodiment of this invention. 
     The process shown in  FIG. 16  is carried out by executing a program stored in a memory  22  by the CPU  21  of the controller  20 . 
     First, the controller  20  turns OFF power supply of all the HDD pairs of the additional chassis  12  (S 201 ). 
     The controller  20  selects an HDD pair constituting a RAID group, and sets a value in the HDD pair select register  65  of the logic circuit  14 . The controller  20  obtains acceleration data corresponding to the selected HDD pair (selected HDD pair) (S 202 ). 
     The controller  20  sets a value in the power supply control register  60  corresponding to the selected HDD pair to turn ON power supply of the selected HDD pair (S 203 ). 
     The controller  20  judges whether two HDD&#39;s  15  constituting the selected HDD pair are idle (S 204 ). 
     If the two HDD&#39;s  15  are idle, the process proceeds to Step S 205 . On the other hand, if neither of the two HDD&#39;s  15  are idle, the process stands by until the two HDD&#39;s  15  become idle. 
     The controller  20  clears the HDD pair select register  65  to finish the acquisition of acceleration data (S 205 ). 
     Then, the controller  20  obtains acceleration data from the built-in memory  67  to integrate the obtained acceleration data by time (S 206 ). Data obtained by integrating the obtained acceleration data indicates a speed of vibrations. 
     The controller  20  further integrates the data integrated in Step S 206  by time (S 207 ). The data integrated in Step S 207  indicates displacement of vibrations (amount of vibrations). A platter rotational speed of the HDD is 10.000 rpm, and a cycle of one rotation of the platter is 6 milliseconds. Accordingly, for example, when measuring time is 10 milliseconds, the platter makes one rotation or more, and thus vibrations caused by a platter rotational frequency can be obtained.  FIG. 13  illustrates an example of a vibration waveform. 
     Then, based on the displacement data obtained in Step S 207 , the controller  20  observes relative time when a vibration takes a maximum value (e.g., from time when obtaining acceleration data is started to time when a vibration takes a maximum value) and relative time when a vibration takes a minimum value (e.g., from time when obtaining acceleration data is started to time when a vibration takes a minimum value), and obtains time of a difference (timing correction value) between the retrieved relative time when a vibration takes the maximum value and the retrieved relative time when a vibration takes the minimum value (S 208 ). 
     The controller  20  divides the timing correction value obtained in Step S 208  by a clock cycle, and sets a divided value in the offset register  69  of the offset circuit  71  (S 209 ). An initial value of the offset register  69  is 0. In this case, the timing is not corrected, and thus power supplies of the HDD pairs are simultaneously turned ON. 
     The controller  20  turns OFF power supply of a selected HDD pair (S 210 ). 
     The controller  20  judges whether timings of all the HDD pairs have been corrected (S 211 ). Specifically, the controller  20  obtains acceleration data of all the HDD pairs, and judges whether a timing correction value has been set in the offset register  69 . 
     If the timings of all the HDD pairs have been corrected, the process is finished. On the other hand, if the timing of at least one HDD pair has not been corrected, the process proceeds to Step S 212 . Upon end of the process, according to a request from a client (e.g., host computer  80 ), a status which enables control of power supplies of the HDD pair is set (ready status). 
     The controller  20  changes values of the HDD pair select register  65  in order (e.g., sets a value in next bit) (S 212 ). Then, the process returns to Step S 202  to similarly set, for a next HDD pair (e.g., another HDD pair of the same RAID group), a timing correction value for turning power supply ON in the offset register  69 . 
     In the example of  FIG. 14 , the process of  FIG. 16  is executed, and the power supply control register  60  is set to first turn ON power supply of one HDD  15 A of the HDD pair constituting the RAID group  1 . At this time, power supply of the other HDD  15 B of the HDD pair is kept OFF. Subsequently, a value of the counter  68  is incremented in synchronization with a clock of the logic circuit  14 . When the value of the counter  68  reaches the value of the offset register  69 , the power supply of the other HDD  15 B of the HDD pair is turned ON in response to an output from the comparator  70 . 
     As described above, as shown in  FIG. 13 , the maximum and minimum components of displacement of vibrations of opposite displacing directions which are shifted in phase by predetermined time and superimposed together are canceled each other. Thus, vibrations applied on the other HDD can be further reduced. 
     Third Embodiment 
     According to the first embodiment of this invention, the number of HDD&#39;s  15  constituting the RAID group is even. According to a third embodiment, however, by using an HDD  15  of another chassis (e.g., basic chassis  11 ) for which no influence of vibrations has to be taken into consideration, this embodiment can be applied to a case where the number of HDD&#39;s  15  constituting a RAID group is odd. 
       FIG. 17  illustrates power supply control of a RAID group according to the third embodiment of this invention. 
     As shown in  FIG. 17 , for example, a RAID group can consist of the even number of HDD&#39;s  15  including HDD pairs of an additional chassis  12  and the odd number of HDD&#39;s  15  of a basic chassis  11 . 
     In an example shown in  FIG. 17 , a RAID group consists of four HDD&#39;s  15  of the additional chassis  12  and one HDD  15  of the basic chassis  11 . 
     Each bit of a power supply control register  60 B of the basic chassis  11  is allocated for each HDD. A method of controlling power supply of the HDD pair of the additional chassis  12  is similar to the method of the first embodiment of this invention. 
     Thus, according to the third embodiment of this invention, even when the number of HDD&#39;s  15  constituting a RAID group is odd, HDD vibrations can be suppressed. 
     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.