Patent Publication Number: US-9430168-B2

Title: Recording medium storing a program for data relocation, data storage system and data relocating method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-063854, filed on Mar. 26, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to a recording medium, a system and a method. 
     BACKGROUND 
     Three types of disk drives, namely a solid state drive (SSD), a serial attached small computer system interface (SCSI) (SAS) disk, and a serial advanced technology attachment (SATA) disk, can be listed as examples of disk drives to be used in storage systems such as a disk array system. As illustrated in  FIG. 1 , in terms of reliability and performance, among the three types of disk drives, an SSD comes first, an SAS disk comes second, and an SATA disk comes third. 
     If a server employs a storage device that includes a hierarchized disk (see  FIG. 1 ) having the three types of disks that differ in characteristics as mentioned above, storage hierarchization control as described below is carried out. Specifically, with a configuration that employs a hierarchized disk, a data block to be relocated and a disk to which that data block is to be relocated are determined on the basis of the frequency of access at a disk side, and the data block is relocated accordingly, as illustrated in  FIG. 1 . 
     For example, if the frequency of access to a given data block, included in a file on the SSD is low, that data block is moved from the SSD to the SAS disk and is relocated on the SAS disk. Similarly, if the frequency of access to a given data block on the SAS disk is low, that data block is moved from the SAS disk to the SATA disk and is relocated on the SATA disk. Meanwhile, if the frequency of access to a given data block on the SATA disk increases, that data block is moved from the SATA disk to the SAS disk and is relocated on the SAS disk. Similarly, if the frequency of access to a given data block on the SAS disk increases, that data block is moved from the SAS disk to the SSD and is relocated on the SSD. 
     Japanese Laid-open Patent Publication No. 2006-4011 is an example of related art. 
     SUMMARY 
     According to an aspect of the embodiment, a non-transitory computer-readable recording medium has stored therein a program for causing a computer to execute a process. The process includes identifying a data block from among a plurality of data blocks in a first storage for relocation to a second storage, determining an access mode of the identified data block, the access mode including sequential access or random access, and relocating the identified data block to the second storage based on the determined access mode. 
     The object and advantages of the embodiment will be realized and attained by the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for describing a hierarchized disk and storage hierarchization control; 
         FIG. 2  is a block diagram illustrating a hardware configuration and a functional configuration of a storage system according to an embodiment; 
         FIG. 3  is a diagram for describing a function and an operation of the storage system illustrated in  FIG. 2 ; 
         FIG. 4  illustrates an exemplary file access mode table of the embodiment; 
         FIG. 5  is a flowchart for describing the operation of the storage system illustrated in  FIG. 2 ; 
         FIG. 6  is a flowchart for describing a procedure for determining an access mode of a target file in the embodiment; 
         FIG. 7  is a flowchart for describing a procedure for relocating a data block from an SSD to an SAS disk in the embodiment; 
         FIG. 8  illustrates a specific example on relocation of data blocks in the storage system illustrated in  FIG. 2 ; and 
         FIG. 9  illustrates a specific example on relocation of data blocks in the storage system illustrated in  FIG. 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     The inventor has found from a study that when some of the data blocks in a sequential access file on an SSD are relocated to an SAS disk, the arrangement of the data blocks forming the file becomes nonsequential. Thus, the sequential access performance of the data blocks on the SAS disk degrades as compared with the access performance of the data blocks on the SSD. As the frequency of relocation in the file increases, the data in the file is further fragmented, and the sequential access performance of the data blocks on the SAS disk further degrades accordingly. 
     In order to determine an access mode of a data block included in a file, that is, whether or not the data block is a sequential access data block, and to relocate the data block accordingly so as to suppress fragmentation of the data, information on an access (read/write) layer pertaining to the specifications of an application that accesses a hierarchized disk is desired. In other words, a server that employs a hierarchized disk does not have information as to with what access mode the hierarchized disk actually arranges the data. Thus, an existing hierarchized disk experiences such a disadvantage that the sequential access performance degrades and the data is fragmented through relocation. 
     According to an embodiment described hereinafter, data fragmentation caused by relocating a data block in a hierarchized disk can be suppressed. 
     Hereinafter, an embodiment will be described with reference to the drawings. 
     [1] Configuration of Storage System of Embodiment 
       FIG. 2  is a block diagram illustrating a hardware configuration and a functional configuration of a storage system  1  according to an embodiment. As illustrated in  FIG. 2 , the storage system  1  includes a storage device  10  and a server (information processing device)  20 . 
     The storage device  10 , which is accessed by the server  20  (application  211 ) and is also managed by the server  20  (management unit  213 ), includes a hierarchized disk  11 , a central processing unit (CPU)  12 , and a memory  13 . In  FIG. 2 , a logical disk  110  is illustrated in the storage device  10 . The logical disk  110  is identified by a logical unit number (LUN) and is a logical disk that is visible from the application  211 , which will be described later. 
     The hierarchized disk  11  includes multiple disks, each having distinct characteristics. As illustrated in  FIG. 2 , the hierarchized disk  11  of the embodiment includes two types of disks, namely an SSD  111  and an SAS disk  112 . Note, however, that the embodiment is not limited thereto, and the embodiment is applied similarly to a case where the hierarchized disk  11  includes, for example, three types of disks, namely an SSD, an SAS disk, and an SATA disk, as in the case described above with reference to  FIG. 1 . 
     Unlike a hard disk, an SSD does not include a disk, and thus no time is spent moving a reading device (head) over the disk (i.e., seek time) or waiting for the target data to rotate to a head position (i.e., search time). Thus, with an SSD, whether data blocks included in a sequential access file are arranged sequentially or fragmented and arranged randomly does not make a difference in terms of access performance. 
     On the other hand, with an SAS disk or an SATA disk, if data blocks included in a sequential access file are fragmented and arranged randomly, the seek time or the search time is extended as compared with the case where the data blocks are arranged sequentially, and thus the access performance degrades. 
     Therefore, in the embodiment, at least when a sequential access data block is relocated on an SAS disk or an SATA disk, whether or not the access mode of the data block to be relocated indicates sequential access is taken into consideration. 
     Thus, in the embodiment, when a data block is relocated from the SSD  111  to the SAS disk  112 , the access mode of the data block to be relocated is determined, and the data block is relocated accordingly on the basis of the determined access mode. If the hierarchized disk  11  includes the three types of disks described above, the data block is relocated on the basis of the access mode thereof, as in the embodiment, when a data block is relocated from the SSD to the SAS disk, from the SAS disk to the SATA disk, or from the SATA disk to the SAS disk. When a data block is relocated from the SAS disk to the SSD, the target data block may or may not be relocated on the basis of the access mode thereof. 
     The CPU (processing unit, computer)  12  loads a program stored in the memory  13  and executes the program to thus function as a disk control unit  121 , which will be described later. The disk control unit  121  controls the hierarchized disk  11  and also functions in the storage hierarchization control, which has been described with reference to  FIG. 1 . 
     The memory  13  is a random access memory (RAM) or the like and stores the program to be executed by the CPU  12  as well as various pieces of data and so on to be used by the CPU  12  when carrying out processing (control processing and so on by the disk control unit  121 ) in the storage device  10 . 
     The server  20  is coupled to the storage device  10  through Fibre Channel (FC)/iSCSI and accesses the hierarchized disk  11  of the storage device  10 . The server  20  is coupled to the disk control unit  121  (CPU  12 ) of the storage device  10  through a local area network (LAN) and manages the storage device  10 . The server  20  includes a CPU  21  and a memory  22 . 
     The CPU  21  loads the application (application program)  211  stored in the memory  22  and executes the application  211  to thus issue a read/write (R/W) command (input/output request) to the hierarchized disk  11  of the storage number  10 . As described above, the application  211  recognizes the hierarchized disk  11  as the logical disk  110  and issues the R/W command to the logical disk  110 . 
     In addition, the CPU  21  loads a program stored in the memory  22  and executes the program to thus realize a virtual driver (virtual device)  212  between the application  211  and the storage number  10 . The virtual driver  212  functions as an access mode identification unit, which will be described later. 
     Furthermore, the CPU  21  loads a program stored in the memory  22  and executes the program to thus create a file access mode table  221 , which will be described later, and to function as the management unit  213 , which will be described later, using the created table  221 . 
     The memory  22  is a RAM or the like and stores the programs to be executed by the CPU  21  and the table  221  as well as various pieces of data and so on to be used by the CPU  21  when carrying out processing (processing and so on by the application  211 , the virtual driver  212 , and the management unit  213 ) in the server  20 . 
     In the storage system  1  illustrated in  FIG. 2 , the single server  20  includes the application  211 , the virtual driver  212 , the management unit  213 , and the file access mode table  221 . Alternatively, the application  211 , the virtual driver  212 , the management unit  213 , and the file access mode table  221  may be distributed between two servers. For example, a business server that accesses the storage device  10  may include the application  211  and the virtual driver  212 , and a management server may be coupled to the business server and the storage device  10  through a LAN and may include the management unit  213  and the file access mode table  221 . 
     Hereinafter, specific functions of the disk control unit  121 , the virtual driver  212 , and the management unit  213  of the storage system  1  of the embodiment will be described with reference to  FIG. 3 . In addition, a specific example of the file access mode table  221  stored in the memory  22  of the server  20  will be described with reference to  FIG. 4 .  FIG. 3  schematically illustrates the function and the operation of the storage system  1 , and  FIG. 4  illustrates an example of the file access mode table  221 . 
     Upon receiving an R/W command (input/output request) to the storage device  10  from the application  211 , the virtual driver (virtual device)  212  functions as the access mode identification unit that identifies access mode information on a file-by-file basis. The access mode information includes information for identifying the application  211  that uses the storage device  10  (i.e., application name or the like), information for identifying the target file of the R/W command (i.e., file name or the like), and an access mode of the target file. The virtual driver  212  provides the management unit  213  with the obtained and identified access mode information (see arrow A 1  in  FIG. 3 ). 
     The virtual driver  212  identifies the access mode of the target file, that is, whether the target file is a sequential access file or a random access file, through the following process. The virtual device  212  determines whether or not the R/W command seeks an R/W position (input/output position) in the target file through an index. If the R/W command does not seek the R/W position through an index, that is, when the R/W command reads the target file from the beginning thereof, the virtual device  212  determines that the target file is a sequential access file. Meanwhile, if the R/W command seeks the R/W position through an index, the virtual device  212  determines that the target file is a random access file. 
     The virtual driver  212  identifies the access mode of each target file that is not registered in the file access mode table  221 , among the files to be used by the application  211 . In addition, the virtual driver  212  continues with the identification of the access mode for a specific period of time after the application  211  starts accessing the storage device  10 . 
     The disk control unit  121  obtains block information, which contains storage locations (physical locations) of data blocks included in each file stored in the hierarchized disk  11 , and access information of each data block (see arrow A 2  in  FIG. 3 ) and provides the management unit  213  of the server  20  with the obtained block information and access information (see arrow A 3  in  FIG. 3 ). The block information includes information for identifying the application  211  (i.e., application name or the like), information for identifying a target file to be used by the application  211  (i.e., file name or the like), and information on the physical locations of the data blocks included in the target file (i.e., a physical storage where the logical disk  110 , which is recognized by the application  211  of the server  20 , is located). Meanwhile, the access information includes the frequency of access to each data block by the application  211 . 
     The management unit  213  registers the access mode information provided by the virtual device  212  and the block information and the access information provided by the disk control unit  121  into the file access mode table  221  in the memory  22  (see arrow A 4  in  FIG. 3 ). 
     As illustrated in  FIG. 4 , the application name of the application  211 , the file name of the target file to be used by the application  211 , and the access mode (sequential access or random access) of the target file identified by the virtual driver  212  are registered into the file access mode table  221  on the basis of the access mode information provided by the virtual device  212 . In addition, as illustrated in  FIG. 4 , the block information and the access information provided by the disk control unit  121  are associated with the access mode information through the application name and the file name, which are then registered into the file access mode table  221 . In other words, the storage location of each data block included in the target file and the frequency of access to each data block are registered. In  FIG. 4 , the storage location of and the frequency of access to a single data block are illustrated. In reality, however, the storage location of and the frequency of access to each data block are registered. In  FIG. 4 , the unit for the frequency of access is input/output per second (IOPS) indicating the reading/writing frequency per second. 
     The management unit  213  refers to the file access mode table  221  (see arrow A 5  in  FIG. 3 ) and determines a data block to be relocated. Specifically, the management unit  213  determines a data block that is located in the SSD  111  and has a low frequency of access to be the data block to be relocated to the SAS disk  112 . Meanwhile, the management unit  213  determines a data block that is located in the SAS disk  112  and has a high frequency of access to be the data block to be relocated to the SSD  111 . In addition, the management unit  213  refers to the file access mode table  221  (see arrow A 5  in  FIG. 3 ) and determines the access mode (sequential access or random access) of the data block to be relocated. In other words, the management unit  213  determines the access mode of the data block on the basis of the access mode information on each file that is used by the application  211  and the block information on each file provided by the disk control unit  121 . 
     In addition, the management unit  213  determines the content of relocation in the hierarchized disk  11  on the basis of the block information (storage location) of the data block to be relocated, which has been determined to be relocated on the basis of the frequency of access thereto, and the access mode, which has been determined by the management unit  213 , of the data block to be relocated, as will be described later. The management unit  213  instructs the disk control unit  121  of the storage device  10  to relocate the data block in accordance with the determined relocation content (see arrow A 6  in  FIG. 3 ). 
     The disk control unit  121  of the storage device  10  relocates the data block in the hierarchized disk  11  as specified by the management unit  213 . 
     In the storage system  1  of the embodiment, when a data block is relocated from the SSD  111  to the SAS disk  112 , the content of that relocation is determined by the management unit  213  in a manner that will be described below with reference to items (A1) to (A4), and the disk control unit  121  relocates the target data block. A specific method for determining the content of relocation by the management unit  213  will be described later. In addition, a specific example of relocation by the disk control unit  121  will be described later with reference to  FIGS. 8 and 9 . 
     [2] Operation of Storage System of Embodiment 
     In the storage device  10 , access information (access frequency) of a data area is determined on a data block by data block basis through the function of the storage hierarchization control of the disk control unit  121 . To date, with the data obtained on a data block by data block basis through the function of the storage hierarchization control of the disk control unit  121 , it has been unclear as to which access mode (sequential access or random access) the data within each data block has. In the embodiment, however, the block information obtained by the disk control unit  121  is associated with the access mode information obtained by the virtual driver  212  on the server  20 , making it possible to determine the access mode of data blocks included, in each file. 
     A data block that has been determined to be a sequential access data block is relocated such that the order of the data blocks included in the file is retained. If data blocks that form a sequential access file are distributed between the SSD  111  and the SAS disk  112 , free spaces are provided respectively in front of and after such data blocks to facilitate the sequential relocation. However, if free spaces are simply provided in front of and after a data block that has been relocated to the SAS disk  112 , the utilization efficiency of the SAS disk  112  degrades. Thus, the management unit  213  may make a prediction as to whether a data block is to be relocated to a space in front of or after a relocated data block on the basis of the frequency of relocation, and if the possibility of such relocation is high, the management unit  213  may set the spaces in front of and after the relocated data block to be free spaces. 
     Hereinafter, the operation of the storage system  1  of the embodiment will be described with reference to  FIGS. 5 to 9 . 
     [2-1] Operation of Storage System 
     The operation of the storage system  1  will be described with reference to the flowchart (steps S 1  to S 9 ) illustrated in  FIG. 5 . 
     As described above, with the storage hierarchization control of the disk control unit  121 , the frequency of access to each data block can be determined, but it is not possible to determine the access mode of each data block. Meanwhile, in the storage system  1  of the embodiment, the virtual driver  212  is established on the server  20 , and the virtual driver  212  and the storage hierarchization control function of the disk control unit  121  are made to cooperate by the management unit  213 , making it possible to determine the access mode of the each data block. 
     In the server  20 , if the application  211  issues an R/W command to a target file and the virtual driver  212  receives the R/W command (YES route in step S 1 ), the virtual driver  212  refers to the file access mode table  221  in the memory  22  through the management unit  213 . The virtual driver  212  then determines whether or not the application name of the application  211  that has issued the R/W command this time and the file name of the target file to be accessed by the application  211  are registered in the file access mode table  221  (step S 2 ). 
     If the application name and the file name are both registered (YES route in step S 2 ), the storage system  1  skips the processes in steps S 3  and S 4  and proceeds to the process in step S 5 . Meanwhile, if at least one of the application name and the file name is not registered (NO route in step S 2 ), the virtual driver  212  determines and obtains the application name of the application  211 , the file name of the target file, and the access mode of the target file (step S 3 ). The procedure for determining the access mode in step S 3  will be described later with reference to  FIG. 6 . 
     The virtual driver  212  then provides the management unit  213  with the obtained application name, file name, and access mode (see arrow A 1  in  FIG. 3 ) and registers the application name, the file name, and the access mode into the file access mode table  221  through the management unit  213  (step S 4 ; see arrow A 4  in  FIG. 3 ). In the case where the application name is registered but the file name is not registered in the file access mode table  221 , the virtual driver  212  provides the management unit  213  with the file name and the access mode. 
     Thereafter, in step S 5 , the disk control unit  121  obtains the block information that includes the storage location (physical location) of the data block in the target file which the application  211  is accessing and the access information (access frequency) of that data block (step S 5 ; see arrow A 2  in  FIG. 3 ). 
     The disk control unit  121  then provides the management unit  213  of the server  20  with the obtained block information and access information (see arrow A 3  in  FIG. 3 ) and registers the block information and the access information into the file access mode table  221  through the management unit  213  (step S 6 ; see arrow A 4  in  FIG. 3 ). Here, as described above, the storage location and the access frequency, which have been provided by the disk control unit  121 , are associated with the access mode information by the application name or the file name and registered. The file access mode table  221  stored in the memory  22  is updated as evaluation of the access frequency or the like is repeated, and the management unit  213  instructs the disk control unit  121  to relocate a data block in accordance with the updated file access mode table  221  as follows. 
     Specifically, the management unit  213  refers to the file access mode table  221  (see arrow A 5  in  FIG. 3 ) and determines which data block is to be relocated (step S 7 ). Here, the management unit  213  determines a data block that is located in the SSD  111  and has a low frequency of access to be the data block to be relocated to the SAS disk  112 . Meanwhile, the management unit  213  determines a data block that is located in the SAS disk  112  and has a high frequency of access to be the data block to be relocated to the SSD  111 . In addition, the management unit  213  refers to the file access mode table  221  (see arrow A 5  in  FIG. 3 ) and determines the access mode (sequential access or random access) of the data block to be relocated. 
     In addition, the management unit  213  determines the content of relocation in the hierarchized disk  11  on the basis of the block information (storage location) of the data block to be relocated, which has been determined to be a block to be relocated on the basis of the frequency of access thereto, and the access mode, which has been determined by the management unit  213 , of the data block to be relocated. In other words, the management unit  213  determines or grasps the storage location of the data block to be relocated by referring to the file access mode table  221 . As a result of grasping the storage location, if the data block to be relocated to the SAS disk  112  is located on the SSD  111 , the management unit  213  determines the content of relocation such that the target data block is relocated to the SAS disk  112 . In addition, as a result of grasping the storage location, if the data block to be relocated to the SSD  111  is located on the SAS disk  112 , the management unit  213  determines the content of relocation such that the target data block is relocated to the SSD  111 . Meanwhile, as a result of grasping the storage location, if the data block to be relocated to the SSD  111  is already located on the SSD  111 , the management unit  213  retains the current arrangement of the target data block. Similarly, as a result of grasping the storage location, if the data block to be relocated to the SAS disk  112  is already located on the SAS disk  112 , the management unit  213  retains the current arrangement of the target data block. 
     The management unit  213  instructs the disk control unit  121  of the storage device  10  to relocate the data block in accordance with the content of relocation determined as described above (step S 8 ; see arrow A 6  in  FIG. 3 ). 
     The disk control unit  121  of the storage device  10  then relocates the data block in the hierarchized disk  11  in accordance with the content of data block relocation instructed by the management unit  213  (step S 9 ). 
     [2-2] Access Mode Determination Procedure 
     In step S 3  of  FIG. 5 , the virtual driver  212  determines the access mode of the target file through the procedure illustrated in  FIG. 6 .  FIG. 6  is a flowchart (steps S 11  to S 13 ) for describing the procedure for determining the access mode (access characteristics) of the target file in the embodiment. 
     The virtual device  212  determines whether or not the R/W command seeks the R/W position in the target file through an index (step S 11 ). If the R/W command does not seek the R/W position through an index, that is, when the R/W command reads the target file from the beginning thereof (NO route in step S 11 ), the virtual device  212  determines that the target file is a sequential access file (step S 12 ). Meanwhile, if the R/W command seeks the R/W position through an index (YES route in step S 11 ), the virtual device  212  determines that the target file is a random access file (step S 13 ). 
     [2-3] Relocation Procedure 
     Subsequently, with reference to the flowchart (steps S 21  to S 26 ) illustrated in  FIG. 7 , the procedure for relocating a data block from the SSD  111  to the SAS disk  112  (processes in steps S 7  to S 9  of  FIG. 5 ) in the embodiment will be described in further detail. 
     First, the management unit  213  refers to the file access mode table  221  and evaluates I/O characteristics (access information/access frequency) of the data block in the target file accessed by the application  211  (step S 21 ). Thus, the management unit  213  determines the data block that is located in the SSD  111  and has a low frequency of access to be the data block to be relocated to the SAS disk  112 . 
     In addition, the management unit  213  refers to the file access mode table  221  and determines the access mode of the data block to be relocated, that is, whether the data block is a sequential access data block or a random access data block (step S 22 ). 
     The management unit  213  then determines the content of relocation in the hierarchized disk  11  in a manner similar to that in step S 8  of  FIG. 5  on the basis of the information on the data block to be relocated (i.e., storage location of the data block) and the access mode of the data block. Then, the management unit  213  stores, in the memory  22 , data arrangement as mapping information used when the data block to be relocated is moved (relocated) from the SSD  111  to the SAS disk  112  in accordance with the determined content of relocation (step S 23 ). 
     Thereafter, the management unit  213  determines whether or not a specific evaluation period has passed (step S 24 ), and if the evaluation period has not passed (NO route in step S 24 ), the management unit  213  repeats the processes in steps S 21  to S 24  described above. At that time, the management unit  213  refers to the file access mode table  221  updated through the processes in steps S 1  to S 4  of  FIG. 5 . 
     Meanwhile, if the evaluation period has passed (YES route in step S 24 ), the management unit  213  instructs the disk control unit  121  to relocate the data block in accordance with the mapping information stored in the memory  22  (step S 25 ). The disk control unit  121  then moves (relocates) the data block to be relocated from the SSD  111  to the SAS disk  112  in accordance with the content of relocation instructed by the management unit  213  (step S 26 ). 
     Note that the management unit  213  determines a data block that is located in the SAS disk  112  and has a high frequency of access to be the data block to be relocated to the SSD  111 . As described above, in the SSD  111 , whether the data blocks contained in a sequential access file are located sequentially or fragmented and arranged randomly does not make a difference in terms of access performance. Thus, when a data block is relocated from the SAS disk  112  to the SSD  111 , the management unit  213  instructs the disk control unit  121  to relocate the target data block to any given free space, in the. SSD  111  without determining the access mode of the target data block. The disk control unit  121  thus relocates the target data block on the SAS disk  112  to any given free space in the SSD  111  in accordance with such an instruction from the management unit  213 . 
     [2-4] Method for Determining Data Block Relocation Content 
     The method for determining the content of relocation by the management unit  213  in step S 8  of  FIG. 5  or in step S 23  of  FIG. 7  will now be described. Here, in particular, the method for determining the content of relocation for relocating data blocks that have been determined to be sequential access data blocks while retaining the sequentiality thereof will be described. Note that a file for which the access mode indicates sequential access will be referred to as a sequential file, and a file for which the access mode indicates random access will be referred to as a random file, hereinafter. 
     First, the method for determining the content of relocation in the storage system  1  of the embodiment will be described in brief, hereinafter. When relocating data blocks included in a sequential file, the management unit  213  determines the content of relocation such that the data blocks are arranged sequentially on the basis of the information obtained by the virtual driver  212 . However, if some of the data blocks included in the file are moved and the entire file is not relocated sequentially, the management unit  213  provides free spaces in front of and after the data blocks that have been relocated sequentially. Thus, when data blocks are to be relocated thereafter, the management unit  213  can locate the data blocks sequentially using the free spaces. Meanwhile, when data blocks included in a random file are to be relocated, the management unit  213  determines the content of relocation so as to relocate the data blocks in the random file to areas excluding the area for the data blocks in the sequential file and the free spaces. In addition, the management unit  213  may release the free spaces that have been provided in front of and after the partial data blocks when relocating the sequential file in the following cases. Specifically, the management unit  213  reevaluates the frequency of access through the storage hierarchization control function of the disk control unit  121  when relocating a random file. If the evaluation result remains the same, the management unit  213  determines that a data block in a sequential file is not to be relocated to the free space and releases the free space to allow a data block in a random file to be relocated thereto. Note that, as described above, if spaces in front of and after the relocated data block are simply set to be free spaces, the utilization efficiency of the disk degrades. Therefore, the management unit  213  may make a prediction as to whether a data block is to be relocated to a space in front of or after a relocated data block on the basis of the frequency of relocation, and if the possibility of such relocation is high, the management unit  213  may provide free spaces in front of and after the relocated data block. 
     In the storage system  1  of the embodiment, when a data block is relocated from the SSD  111  to the SAS disk  112 , the management unit  213  determines the content of relocation, and the disk control unit  121  relocates the target data block, in a manner indicated by the following items (A1) to (A4). 
     (A1) Case where the access mode of the data block to be relocated indicates sequential access and a plurality of sequential data blocks are present on the SAS disk  112  to which the data block is to be relocated. The management unit  213  determines the content of relocation such that the plurality of sequential data blocks are sequentially relocated to the SAS disk  112  and instructs the disk control unit  121  accordingly. Thus, the disk control unit  121  sequentially relocates the plurality of sequential data blocks to the SAS disk  112  in the hierarchized disk  11  of the storage device  10 . 
     (A2) Case where the access mode of the data block to be relocated indicates sequential access and another data block to be located in front of or after that data block is not present on the SAS disk  112  to which the data block is to be relocated. The management unit  213  determines the content of relocation such that a free space for the other data block is to be provided in front of or after the data block relocated to the SAS disk  112  and instructs the disk control unit  121  accordingly. Thus, the disk control unit  121  provides the free space for the other data block in front of or after the data block relocated to the SAS disk  112  in the hierarchized disk  11  of the storage device  10 . 
     (A3) Case where the other data block is not relocated to the free space even after a specific period of time elapses or relocation processing of a data block is carried out a specific number of times after the free space is provided. 
     The management unit  213  determines the content of relocation such that the free space is released and a data block other than the other data block is relocated to the free space and instructs the disk control unit  121  accordingly. Thus, the disk control unit  121  relocates a data block other than the other data block to the free space. 
     (A4) Case where the access mode of the data block to be relocated indicates random access. The management unit  213  determines the content of relocation such that the data block is relocated in accordance with the arrangement of data blocks for which the access mode indicates sequential access and instructs the disk control unit  121  accordingly. Thus, the disk control unit  121 , taking the area for the sequential access data blocks and the free space into consideration, relocates the random access data block to an area excluding the area for the sequential access data blocks and the free space. 
     A specific example of relocation by the disk control unit  121  will now be described with reference to  FIGS. 8 and 9 .  FIGS. 8 and 9  illustrate a specific example of relocating data blocks in the storage system  1 . In  FIGS. 8 and 9 , four blocks S 1  to S 4  indicate sequential data blocks included in a sequential file A, and the four data blocks S 1  to S 4 , which are to be arranged sequentially in this order, form the single sequential file A. A block labeled “free” indicates a free space that has a capacity of a single data block. 
       FIG. 8  illustrates a case where the four data blocks S 1  to S 4  included in the sequential file A located on the SSD  111  and four data blocks R included in a random file B located on the SSD  111  are to be relocated in the SAS disk  112 . In such a case, the management unit  213  determines the access mode of each data block, that is, whether each data block is a sequential access data block or a random access data block, and then defragmentation relocation of the data blocks from the SSD  111  to the SAS disk  112  is carried out. Specifically, the four data blocks S 1  to S 4  included in the sequential file A are relocated so as to be arranged sequentially in the SAS disk  112 . The four data blocks R included in the random file B are each relocated to a given free space aside from the areas where the data blocks S 1  to S 4  are located in the SAS disk  112 . 
     In the case of the upper SAS disk  112  illustrated in  FIG. 9 , as in the example illustrated in  FIG. 8 , the entire data blocks included in the sequential file A and the random file B on the SSD  111  are to be moved (relocated) and are relocated to the SAS disk  112 . In this case, relocation is carried out in a manner similar to that described with reference to  FIG. 8 . 
     In the case of the lower SAS disk  112  illustrated in  FIG. 9 , some of the data blocks included in the sequential file A and the random file B on the SSD  111  are to be moved (relocated) and are relocated to the SAS disk  112 . In this case, the two data blocks S 3  and S 4  included in the sequential file A and the two data blocks R included in the random file B are to be moved (relocated) and are relocated to the SAS disk  112 . In this case as well, the management unit  213  determines the access mode of each data block, that is, whether each data block is a sequential access data block or a random access data block, and then defragmentation relocation of the data blocks from the SSD  111  to the SAS disk  112  is carried out. Specifically, the two data blocks S 3  and S 4  included in the sequential file A are relocated so as to be arranged sequentially in the SAS disk  112 . In addition, since there is a possibility that the sequential data blocks S 1  and S 2  are relocated in front of the data blocks S 3  and S 4  through later relocation processing, free spaces for two data blocks are provided. The two data blocks R included in the random file B are each relocated to a given free space excluding the area where the data blocks S 3  and S 4  are located and the free spaces for the two data blocks provided in front of the data blocks S 3  and S 4  in the SAS disk  112 . 
     [3] According to the Storage System  1  of the Embodiment, the Following Effects Can be Obtained. 
     Associating the block information obtained by the disk control unit  121  with the access mode information obtained by the virtual driver  212  on the server  20  makes it possible to determine the access mode of the data blocks included in each file. Thus, the data blocks can be relocated on the basis of the result of determining the access mode of the data blocks, and thus degradation in performance of sequentially accessing data moved from the SSD  111  to the SAS disk  112  can be suppressed. In addition, data fragmentation caused by relocating the data blocks in the hierarchized disk  11  can be suppressed, making it possible to suppress a negative influence caused by fragmentation or the like. 
     Furthermore, the access mode of each target file used by the application  211  is determined by the virtual driver  212 . Thus, the access mode of each file can be reliably determined as to whether the file is a sequential access file or a random access file at a section close to an operating system (OS). However, using the virtual driver  212  simply for determining the access mode may cause the access speed to be reduced. Thus, the virtual driver  212  continues with the determination of the access mode only for a specific period of time after the application  211  starts accessing the storage device  10  and stops the determination of the access mode after the specific period of time elapses. Accordingly, reduction in the access speed can be suppressed. 
     In addition, the virtual driver  212  collects access mode information only of the files that are not registered in the file access mode table  221 . Thus, the block arrangement in the hierarchized disk  11  can be optimized without increasing a load on the disk access. 
     If the data blocks included in a sequential file are distributed between the SSD  111  and the SAS disk  112 , providing free spaces in front of and after such data blocks makes it possible to relocate data blocks sequentially with ease. 
     Here, the management unit  213  makes a prediction as to whether a data block is to be relocated in front of or after a relocated data block on the basis of the frequency of relocation, and if the possibility of such relocation is high, the management unit  213  sets free spaces in front of and after the relocated data block. Thus, free spaces can be set in front of and after the data block relocated to the SAS disk  112  without reducing utilization efficiency of the SAS disk  112 . 
     If another sequential data block is not relocated to the free space even after a specific period passes or relocation processing of data blocks is carried out a specific number of times after the free spaces are provided in front of and after the relocated data block, the management unit  213  releases the free spaces. Thus, the management unit  213  can relocate data blocks to the free spaces, and the area on the SAS disk  112  can be used effectively. 
     [4] Other 
     In the embodiment the single server  20  is provided for the single storage device  10 . Alternatively, the storage system  1  may be configured such that multiple servers access the storage device  10 . In such a case, at least one of the servers may be configured to function as the server  20  described above, and various pieces of information obtained by the server  20  may be shared among the multiple servers. Thus, an effect similar to that of the embodiment described above can be obtained in each of the servers by the use of the single server  20 . 
     All or part of the functions of the disk control unit  121 , the virtual driver  212 , and the management unit  213  described above can be realized by a computer (including a CPU, an information processing device, and various terminals) that functions as a copy processing unit  24  executing a specific program (storage control program). 
     In addition, the application program may be provided in the form of a program recorded on a computer readable recording medium such as a flexible disk, a CD (CD-ROM, CD-R, CD-RW, etc.), a DVD (DVD-ROM, DVD-RAM, DVD−R, DVD−RW, DVD+R, DVD+RW, etc.) and a Blu-ray Disc. In such a case, the computer reads the program from the recording medium, transfers the program to an internal storage device or an external storage device, and stores the program therein for use. 
     Here, the computer conceptually includes hardware and an OS and refers to hardware that operates under the control of the OS. In addition, in a case in which the OS is unnecessary and the hardware is operated solely through a program, the hardware itself corresponds to the computer. The hardware includes at least a microprocessor such as a CPU and a unit configured to read a computer program recorded in the recording medium. The stated program includes program codes that cause the computer as described above to realize the functions of the disk control unit  121 , the virtual driver  212 , and the management unit  213 . In addition, part of these functions may be realized not through the program but through the OS. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.