Abstract:
The first storage area stores original data of an update target that is to be updated by a host. The controller divides data to be written over the original data of the update target stored in the first storage area into a plurality of pieces of update data and thereby distributes the plurality of pieces of update data for each of successive addresses. The second storage area stores the plurality of update data distributed by the controller. The third storage area stores information in which an update area address, which is an address of the first storage area to be overwritten by the plurality of pieces of update data of the original data of the update target, is associated with a storage destination address, which is an address of the second storage area that has stored the plurality of pieces of update data.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-074825, filed on Mar. 29, 2013, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a storage control device. 
       BACKGROUND 
       [0003]    A storage device is a device that has a large number of harddisk drives and large capacity cache memories and stores data used by, for example, servers. A storage device provides a function of snapshot, which holds an image of data at a particular moment. Snapshot is performed in response to a request from, for example, a server. As a snapshot function, there is for example a Copy-On-Write (referred to as COW hereinafter) method. Snapshot does not perform a process of copying copy source data as it is. Snapshot performs a process of obtaining meta information related to copy source data, resulting in an advantage that it only requires a short period of time for backup. However, when copy source data is updated with the meta information having been backed up, data at the moment of taking a snapshot is overwritten, making it impossible to refer to the data later. Accordingly, in a storage device that uses snapshot of the COW method, when data is to be updated after obtaining a snapshot, data blocks including the data before the update are saved to a different address. The storage device obtains meta information corresponding to the area in which the data blocks are saved, and writes data to be updated over the saved data. 
         [0004]    A storage device using snapshot of the COW method refers to copy source data as master slices, and manages them in units called chunks. Also, a storage device includes a COW slice that holds data to be used for snapshot. In snapshot, when data has been updated, the storage device stores data obtained by updating the data of a master slice in a data area of a COW slice. For this storing, the storage device stores, in a COW slice and as meta information, mapping information that associates a (physical) address at which data of a master slice was stored and a (logical) address of a COW slice that is a saving destination. 
         [0005]    Accompanying the increase in the scale of storage devices and systems, the amount of mapping information for managing chunks of COW slices tend to increase. An increase in the amount of mapping information leads to an increase in the number of accesses to COW slices, which also increases the usage amount of memories in a storage device. This results in deterioration in the performance of snapshot. It is desirable that the performance of snapshot be maintained even when the amount of mapping information has increased. 
         [0006]    As a technique of maintaining the performance of snapshot, a technique is known that distributes loads of generating snapshots between a host and a storage device (see Patent Document 1 for example).
   Patent Document 1: Japanese Laid-open Patent Publication No. 2004-118413   
 
       SUMMARY 
       [0008]    According to an aspect of the embodiments, a storage control device comprising: a first storage area configured to store original data of an update target that is to be updated by a host; a controller configured to divide data to be written over the original data of the update target stored in the first storage area into a plurality of pieces of update data and thereby to distribute the plurality of pieces of update data for each of successive addresses; a second storage area configured to store the plurality of update data distributed by the controller; and a third storage area configured to store information in which an update area address, which is an address of the first storage area to be overwritten by the plurality of pieces of update data of the original data of the update target, is associated with a storage destination address, which is an address of the second storage area that has stored the plurality of pieces of update data. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0010]    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 invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  illustrates an example that explains a storage device; 
           [0012]      FIG. 2  illustrates an example of mapping information; 
           [0013]      FIG. 3  illustrates an example of a configuration of a storage device; 
           [0014]      FIG. 4  explains an example of a meta slice in detail; 
           [0015]      FIG. 5A  illustrates an example of the integration of write requests to hold the same cache page; 
           [0016]      FIG. 5B  illustrates an example of the integration of write requests to hold the same cache page; 
           [0017]      FIG. 6A  illustrates an example of the integration of write requests to hold successive physical addresses; 
           [0018]      FIG. 6B  illustrates an example of the integration of write requests to hold successive physical addresses; 
           [0019]      FIG. 6C  illustrates an example of the integration of write requests to hold successive physical addresses; 
           [0020]      FIG. 7A  illustrates an example of a process of snapshot; 
           [0021]      FIG. 7B  illustrates an example of a process of snapshot; 
           [0022]      FIG. 8  illustrates an example of a process related to a write request of data across a plurality of chunks; 
           [0023]      FIG. 9A  explains an example of integrating write requests to a data slice; 
           [0024]      FIG. 9B  explains an example of integrating write requests to a data slice; 
           [0025]      FIG. 10A  illustrate an example of down recovery of history information; 
           [0026]      FIG. 10B  illustrate an example of down recovery of history information; 
           [0027]      FIG. 11  illustrates an example of prefetch of meta data; 
           [0028]      FIG. 12  is a flowchart that explains an example of a process performed by a control unit that has received an I/O request; 
           [0029]      FIG. 13  is a flowchart explaining an example in which the control unit refers to mapping information; 
           [0030]      FIG. 14  is a flowchart that explains an example of a process related to copying of data; 
           [0031]      FIG. 15  is a flowchart that explains a process in which the control unit writes meta information to a meta slice; 
           [0032]      FIG. 16  illustrates a flowchart explaining an example of integrating write requests to hold the same cache page; 
           [0033]      FIG. 17  illustrates a flowchart explaining an example of integrating write requests to successive physical addresses; 
           [0034]      FIG. 18  illustrates a flowchart explaining an example of integrating write requests to a data slice; and 
           [0035]      FIG. 19  illustrates a flowchart explaining an example of down recovery of history information. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    Hereinafter, detailed explanations will be given for the present embodiment by referring to the drawings. 
         [0037]      FIG. 1  illustrates an example that explains a storage device. A storage device  200  illustrated in  FIG. 1  includes a control unit  201 , a meta slice  202 , a data slice  203 , and a master slice  204 . The control unit  201  controls processes of snapshot in response to a request from a server that serves as a host connected to the storage device. A request from a server is reported when data in the server is updated. The data slice  203  stores the original data of an update target to be updated by a host. Also, the data slice  203  is a result of backing up data held by a host at a particular moment, and the master slice  204  is a group of pieces of data that is a copy source of snapshot. Respective pieces of data in the master slice  204  according to an embodiment are managed in a chunk size of 4 KB. In the storage device  200  illustrated in  FIG. 1 , a COW slice used in the COW method is divided into the data slice  203  and the meta slice  202 . The meta slice  202  is an area for holding meta information used for snapshot processes such as a header chunk, a meta chunk, or the like. The data slice  203  is a data area for holding update data of snapshot. The meta slice  202  includes mapping information used for making accesses. Mapping information is stored in a cache and is used on an as-needed basis. The control unit  201  refers to mapping information in response to an I/O (Input/Output) request, which is a write request from a host. By referring to mapping information stored in a cache, it is possible to perform processes at higher speeds than in cases where mapping information is referred to from the meta slice, which leads to a higher performance for snapshot. An I/O request includes information of data to be updated, the size of the data to be updated, and the address number of a master slice to which updated data is written. The data slice  203  illustrated in  FIG. 1  does not include a slice in middle that causes interruptions, such as a meta slice. Accordingly, the control unit  201  can write slices of the data slice  203  continuously. This make it possible to reduce a seek time used for writing to the data slice  203  and also makes it possible to make I/O requests in an integrated manner. 
         [0038]    A storage device of a scale-out type uses the chunk size of 1 MB because of its large volume. However, a large chunk size will lead to a decrease in the copying speed of snapshot. Accordingly, the copying speed can be improved by reducing the chunk size. For example, by reducing the chunk size to 4 KB, the copying speed of snapshot is improved. However, when a chunk size is reduced, the amount of mapping information used for managing chunks increases, making it difficult to keep the mapping information in an on-memory state, which leads to the necessity of employing various methods of caching. Specific values of chunk sizes are examples and do not limit the scope of the invention. 
         [0039]      FIG. 2  illustrates an example of mapping information. Mapping information is information that associates an address of a master slice and an address of a COW slice. The mapping information in  FIG. 2(A)  is mapping information used by a storage device that uses a COW method. It is illustrated in the mapping information in  FIG. 2(A)  that the data at address 8 in the master slice for example is held at address 2 in the COW slice. Note that the addresses in the master slice are physical addresses while the addresses in the COW slice are logical addresses. When there is less mapping information, it is kept in an on-memory state, and high-speed address conversion is possible by using hash search. However, because an increase in mapping information makes it difficult to keep mapping information in an on-memory state, a method in which cache is utilized is employed in the embodiments. In order to increase the amount of information that can be cached, the embodiments employ a configuration that holds only addresses in a COW slice. This configuration facilitates searches for an associated address in a COW slice from an address in a master slice. 
         [0040]    The mapping information illustrated in  FIG. 2(B)  is mapping information according to an embodiment. The example illustrated in  FIG. 2(B)  illustrates the result of replacing the mapping information in  FIG. 2(A)  with mapping information according to an embodiment. The pieces of mapping information illustrated in  FIG. 2(B)  are arranged successively in the order of the addresses in the master slice and hold the address information of the COW slice. Because the addresses in the master slice start with zero, address 2 in the COW slice for example, which is data at address 8 in the master slice, is held at 9. The mapping information according to an embodiment is information that associates an address in the master slice and an address in the data slice, and addresses in the data slice are stored in the order of successively arranging the addresses in the master slice. By generating mapping information as illustrated in  FIG. 2(B) , the size of data used for mapping information can be reduced. Also, by using pieces of mapping information in the order of addresses in the master slice, times of address conversion from addresses in the master slice into the addresses in the COW slice can be reduced. The control unit  201  reads the mapping information according to an embodiment to a cache so as to use it. Further, by generating a plurality of cache pages, the cache hit rate can be increased, leading to a higher performance for snapshot. Also, the number 0 (zero) stored in the mapping information illustrated in  FIG. 2(B)  indicates that nothing is assigned. Actual memories are not assigned to an area that stores the number 0. This can reduce a memory area for mapping information. It is desirable to use a thin provisioning mechanism for a meta slice in a storage device so that mapping information that has not been written is not assigned to a memory area. 
         [0041]      FIG. 3  illustrates an example of a configuration of a storage device. A storage device  300  is connected to a server  340 , and stores data used by the server  340 . The storage device  300  includes a control device  310 , a control device  320 , and a storage device  330 . The control device  320  includes a control unit  321 , a Central Processing Unit (CPU)  322 , a memory  323 , a Virtual Disk (VDISK)  324 , and a Hard Disk Drive (HDD)  325 . A plurality of the control units  321  may be in a storage system. The control unit  321  controls a storage device related to the control device  320 , and is an example of the control unit  201  illustrated in  FIG. 1 . The CPU  322  executes processes of the control unit  321 . Various data used by the CPU  322  is stored in the memory  323  and the HDD  325 . The VDISK  324  is a virtual storage area obtained by integrating physical storage areas in the storage device  330 , and is controlled by the control unit  321 . The VDISK  324  includes a meta slice  202 , a data slice  203 , and a master slice  204 . The control device  310  includes a management unit  311 , a CPU  312 , a memory  313 , a HDD  314 , a control unit  315 , a VDISK  316 , and a management database (DATABASE)  317 . The control device  310  includes the management unit  311  and the management DB  317  in order to manage the control device  320 . The management unit  311  exists in one control device in the storage system, and manages assignment information of hardware of all control devices and information of states by using the management DB  317 . The control device  310  is a control device that controls the control device  320 . The CPU  312  executes processes of the management unit  311  and the control unit  315 . Various data used by the CPU  312  is stored in the memory  313  and the HDD  314 . The VDISK  316  is a virtual storage area that uses the storage device  330 , and is controlled by the control unit  315 . The storage device  330  includes a plurality of hard disks, each of which is identified by a LUN (Logical Unit Number). A LUN is a number for identifying each hard disk. The storage device in  FIG. 3  includes LUNs  331   a  through  331   n , and the LUNs  331   b  through  331   m  are assigned to the VDISK  324  while the LUNs  331   a  and  331   n  are assigned to the VDISK  316 . A VDISK uses a thin provisioning mechanism. The control unit  201  illustrated in  FIG. 1  may be the control unit  315 . Also, the meta slice  202 , the data slice  203 , and the master slice  204  may be included in the VDISK  316 . 
         [0042]      FIG. 4  illustrates an example of a meta slice in detail. The meta slice  202  is a group of data having a chunk size of 8 KB. A chunk size may be changed and does not limit the size of data. The control unit  201  refers to the meta slice  202  so as to control snapshot. The area of chunk 0 in the meta slice  202  stores header information related to the meta slice. The area of chunk 1 in the meta slice  202  stores mapping information related to the meta slice. The area of chunk 2 in the meta slice  202  stores history information (which will be explained later in  FIG. 10 ) that is used for snapshot and prefetch information (which will be explained later in  FIG. 11 ). Chunk 3 and subsequent chunks in the meta slice  202  store addresses, which are mapping information used in the present embodiment. 
       &lt;Integration of Requests to Write to Meta Slice&gt; 
       [0043]    The control unit  201  receives a write request from the server  340 . A write request includes data and meta information used for updating data. The control unit  201  divides a received request into a prescribed size and performs processes. The control unit  201  accesses a data slice and a meta slice in response to respective divisional I/O requests. When, for example, the control unit  201  writes data to successive physical addresses, the writings can be performed efficiently and the number of accesses to the data slice or the meta slice can also be reduced by executing I/O requests in an integrated manner at one time. Accordingly, the storage device according to an embodiment integrates I/O requests to hold the same cache page for I/O requests to a meta slice. Further, the storage device according to an embodiment integrates I/O requests to successive physical addresses for I/O requests to a meta slice. 
         [0044]    By integrating I/O requests to hold information of the same cache page and by further integrating I/O requests to successive physical addresses, the control unit  201  can reduce the number of accesses to a cache. The occurrence of a process of determining the address of each access is one cause of a time taken for a snapshot process. In a method according to the present embodiment, a plurality of I/O requests related to the same cache page are integrated into one I/O request, and further it is possible to improve the snapshot performance because they are integrated into an I/O request to successive physical addresses. Also, because an I/O request includes additional information of the size of data to be written, the address number of a master slice to which data is to be written, information related to a cache page, and the like, the control unit performs integration by using these pieces of information. 
         [0045]      FIG. 5  illustrates an example of the integration of write requests to hold the same cache page.  FIG. 5  illustrates an example of the integration of write requests by using FIG.  5 A( 1 ) through FIG.  5 B( 7 ). The example in  FIG. 5  will be explained by using queues  401   a  through queues  401   b , jobs  402   a  through  402   e , and pieces of additional information  403   a  through  403   f . The queue  401   a  processes jobs sequentially in the order of the jobs  402   a ,  402   b , . . . ,  402   e . Jobs and additional information associated with jobs are I/O requests divided by the control unit. In FIG.  5 A( 1 ), the situation is that in which the jobs  402   a  through  402   e  are waiting for a process of the queue  401   a . The jobs  402   a  through  402   e  have the pieces of additional information  403   a  through  403   e  that correspond to the jobs  402   a  through  402   e , respectively. In the example illustrated in  FIG. 5 , the additional information  403   b  and the additional information  403   d  have information of the same cache page. 
         [0046]    FIG.  5 A( 1 ): The control unit  201  checks additional information associated with each job in the order from the job  402   e , which is the last job among jobs waiting for a process of the queue  401   a.    
         [0047]    FIG.  5 A( 2 ): The control unit  201  finds a job that has information of the same cache page. In the example illustrated in  FIG. 5 , the additional information  403   b  and the additional information  403   d  have information of the same cache page. 
         [0048]    FIG.  5 A( 3 ): The control unit  201  generates additional information dedicated to jobs having information of the same cache page. In  FIG. 5  A( 3 ), the additional information  403   f , dedicated to the additional information  403   d , is generated. 
         [0049]    FIG.  5 A( 4 ): The control unit  201  performs setting so that when a subsequent job having information of the same cache page is executed, the first job having information of the same cache page is executed parallelly. In FIG.  5 A( 4 ), the job  402   b  is assigned to the queue  401   b . The additional information  403   d  is stored in a prescribed queue. 
         [0050]    FIG.  5 B( 5 ): The control unit  201  sets the additional information  403   f  as the additional information of the job  402   d.    
         [0051]    FIG.  5 B( 6 ): The control unit  201  executes unsynchronized I/O. 
         [0052]    FIG.  5 B( 7 ): The control unit  201  collects pieces of additional information of respective terminated jobs. Also, the additional information  403   f  generated in FIG.  5 A( 3 ) is discarded. 
         [0053]      FIG. 6  illustrates an example of the integration of write requests to hold successive physical addresses.  FIG. 6  illustrates an example of the integration of write requests to successive physical addresses by using FIG.  6 A( 1 ) through FIG.  6 C( 9 ). The example in  FIG. 6  will be explained by using queues  411   a  through  411   c , jobs  412   a  through  412   f , and pieces of additional information  413   a  through  413   h . The queue  411   a  processes jobs sequentially in the order of the jobs  412   a , . . . ,  412   e . Jobs and additional information associated with the jobs are I/O requests divided by the control unit. The process of integrating write requests to successive physical addresses in  FIG. 6  are executed parallelly to the process of integrating write requests to hold the same cache page illustrated in  FIG. 5 . By processing I/O requests to successive physical addresses in an integrated manner, the control unit  201  can reduce the number of accesses to a cache, improving the snapshot performance. FIG.  6 A( 1 ) illustrates an example after the process illustrated in FIG.  5 B( 5 ). In FIG.  6 A( 1 ), the jobs  412   a  through  412   e  are waiting for the process of the queue  411   a . The job  412   f  is set to be executed by the queue  411   b  parallelly to the process of the job  412   d . The additional information  413   d  is an example of additional information generated in FIG.  5 A( 3 ). The additional information  413   f  is additional information stored in a prescribed queue. The jobs  412   a  through  412   e  have pieces of additional information corresponding to the jobs  412   a  through  412   e , respectively. In the example illustrated in  FIG. 6 , the additional information  413   b  and the additional information  413   f  have information of successive physical addresses. 
         [0054]    FIG.  6 A( 1 ): The control unit  201  checks additional information associated with each job in the order from the job  412   a , which is the first job among jobs waiting for a process of the queue  411   a.    
         [0055]    FIG.  6 A( 2 ): The control unit  201  finds a job that has information of successive physical addresses. In the example illustrated in  FIG. 6 , the additional information  413   b  and the additional information  413   f  have information of successive physical addresses. 
         [0056]    FIG.  6 A( 3 ): The control unit  201  generates additional information dedicated to jobs having information of successive physical addresses. In FIG.  6 A( 3 ), the additional information  413   h , dedicated to the additional information  413   b  and the additional information  413   d , is generated. 
         [0057]    FIG.  6 B( 4 ): The control unit  201  performs setting so that when the first job having information of successive physical addresses is executed, a subsequent job having information of successive physical addresses is executed parallelly. In FIG.  6 A( 4 ), the job  412   d  is assigned to the queue  411   c . The additional information  413   b  is stored in a prescribed queue. 
         [0058]    FIG.  6 B( 5 ): The control unit  201  deletes the additional information  413   d  of the job  412   d , and sets the job  412   f  as a job subsequent to the job  412   d.    
         [0059]    FIG.  6 B( 6 ): The control unit  201  sets the additional information  413   h  as additional information of the job  412   b.    
         [0060]    FIG.  6 C( 7 ): The control unit  201  executes unsynchronized I/O. 
         [0061]    FIG.  5 C( 8 ): The control unit  201  collects pieces of additional information of respective terminated jobs. Also, the additional information  413   f  is discarded. 
         [0062]    The control unit  201  may execute one of the process of integrating jobs having the same cache page and the process of integrating jobs having information of successive physical addresses, and may also execute both of them. 
       &lt;Method of Writing Snapshot&gt; 
       [0063]      FIG. 7  illustrate an example of a process of snapshot.  FIG. 7A  illustrates an example of a process of snapshot of the COW method.  FIG. 7  illustrate an example of a case where a request has been made by the server to write data b and data c. In this example, data b and data c are data for updating the chunk of data a in a master slice  102 . A process of snapshot of the COW method will be explained by using (1) through (3) in  FIG. 7A .  FIG. 7A  includes a control unit  101 , a master slice  102 , and a COW slice  103 . As exemplified in (1) of  FIG. 7A , when the server has made a write request, the control unit  101  stores, in a memory, data b and data c for updating. The control unit  101  reads, from the master slice  102 , data a, which is a data chunk of the target to which data b and data c are to be written. The control unit  101  stores data a in a memory. In (2) in  FIG. 7A , the control unit  101  writes read data a to the COW slice  103 . In (3) in  FIG. 7A , the control unit  101  writes data b and data c on data a in the COW slice so as to generate data a′. 
         [0064]      FIG. 7B  illustrates an example of a process of snapshot according to an embodiment. In  FIG. 7B , constituents similar to those in  FIG. 1  are denoted by similar numbers. A process of snapshot according to an embodiment will be explained by using (4) through (6) in  FIG. 7B . As exemplified in (4) in  FIG. 7B , when a write request has been made by a server, the control unit  201  stores data b and data c in a memory. The control unit  201  reads, from the master slice  204 , data a, to which data b and data c are to be written. The control unit  201  stores data a in a memory. In (5) in  FIG. 7B , the control unit  201  writes data b and data c on data a so as to generate data a′. In (6) in  FIG. 7B , the control unit  201  writes data a′ to the data slice  203 . In the snapshot in  FIG. 7A , an I/O request of writing to a COW slice occurs three times, i.e., writing of data a, writing of data b, and writing of data c. In the snapshot in  FIG. 7B , an I/O request of writing to a data slice occurs once, i.e., the writing of data a′. Accordingly, by performing writing in a method of the snapshot illustrated in  FIG. 7B , the number of accesses to the data slice is reduced, increasing the processing speed of snapshot. 
       &lt;Reduction of Read Processes Performed by Control Unit&gt; 
       [0065]      FIG. 8  illustrates an example of a process related to a write request of data across a plurality of chunks.  FIG. 8  illustrates an example of a case where there is a write request to data across chunks 1 through 3. Write requests to chunks 1 and 3 are requests to update part of the data of chunks 1 and 3. A write request to chunk 2 is a request to update the entire data of chunk 2. In the method of the snapshot illustrated in  FIG. 7B , the control unit reads the data of chunk 1 through chunk 3 from the master slice. Next, the control unit writes the data of chunks 1 through 3 to the COW slice. Thereafter, the control unit writes the data for updating over the data of chunks 1 through 3 that was written to the COW slice. 
         [0066]    In snapshot according to an embodiment, the control unit  201  does not read data of a chunk in which data is updated entirely. In this example, the control unit  201  does not read data in chunk 2. Data in chunk 2 is updated entirely when the control unit  201  updates the data. Accordingly, it is not necessary for the control unit  201  to perform a process of reading the data in chunk 2. By avoiding a reading process on a chunk in which data is updated entirely as above, unnecessary reading processes can be reduced, leading to a higher performance for snapshot. 
       &lt;Integration of Write Requests to Data Slice&gt; 
       [0067]    When the control unit  201  has received a write request from the server  340 , it divides the received request into a prescribed size, and performs a process. The control unit  201  accesses a data slice in accordance with the respective divisional I/O requests. When, for example, the control unit  201  writes data to successive physical addresses, this process will be performed more efficiently and the number of accesses to a data slice can be reduced by executing all I/O requests at one time in an integrated manner. Accordingly, the storage device according to an embodiment processes I/O requests to successive physical addresses in an integrated manner for I/O requests to a data slice. By integrating I/O requests to successive physical addresses, the control unit  201  can reduce the number of accesses to a cache, leading to a higher performance for snapshot. Also, because an I/O request includes additional information on the size of data to be written, the address number of a master slice to which data is to be written, information related to a cache page, and the like, the control unit performs integration by using these pieces of information. 
         [0068]      FIG. 9  illustrate examples of processes of integrating requests to write data to a data slice.  FIG. 9  illustrates examples of integration of write requests to a data slice by using FIG.  9 A( 1 ) through FIG.  9 B( 7 ). The examples in  FIG. 9  will be explained by using queues  421   a  through  421   b , jobs  422   a  through  422   e , and additional information  413   a  through  413   f . The queue  421   a  processes jobs sequentially in the order of the jobs  422   a , . . . ,  422   e . Jobs and additional information associated with the jobs are I/O requests divided by the control unit. By processing data to be written to successive physical address in an integrated manner, the control unit  201  can reduce the number of accesses to a cache, leading to a higher performance for snapshot. In FIG.  9 A( 1 ), the jobs  422   a  through  422   e  are waiting for the process of the queue  421   a . The jobs  422   a  through  422   e  include pieces of additional information  423   a  through  423   e  corresponding to the jobs  422   a  through  422   e , respectively. In the example illustrated in  FIG. 9 , the additional information  423   b  and the additional information  423   d  have information of the address of the same master slice. 
         [0069]    FIG.  9 A( 1 ): The control unit  201  checks additional information associated with each job in the order from the job  422   a , which is the first job among jobs waiting for a process of the queue  421   a.    
         [0070]    FIG.  9 A( 2 ): The control unit  201  finds a job that has information of the same master slice. In the example illustrated in FIG.  5 ( 2 ), the additional information  423   b  and the additional information  423   d  have information of the address number of the same master slice. 
         [0071]    FIG.  9 A( 3 ): The control unit  201  generates additional information dedicated to jobs having information of the address of the same master slice. In FIG.  9 ( 3 ), the additional information  423   f , dedicated to the additional information  423   b  and the additional information  423   d , is generated. 
         [0072]    FIG.  9 A( 4 ): The control unit  201  performs setting so that when the first job having information of the address of the same master slice is executed, a subsequent job is executed parallelly. In FIG.  9 ( 4 ), the job  422   d  is assigned to the queue  421   b . The additional information  423   b  is stored in a different queue as additional information whose process has been terminated. 
         [0073]    FIG.  9 B( 5 ): The control unit  201  sets the additional information  423   f  as additional information of the job  422   b.    
         [0074]    FIG.  9 B( 6 ): The control unit  201  executes unsynchronized I/O. 
         [0075]    FIG.  9 B( 7 ): The control unit  201  collects pieces of additional information of respective terminated jobs. Also, the additional information generated in FIG.  9 ( 3 ) is discarded. 
       &lt;Regarding Down Recovery&gt; 
       [0076]    The meta slice  202  according to an embodiment stores history information, which is information representing a proceeding level of a data writing process, i.e., information representing which of the addresses in a data slice the data writing process has proceeded to. History information further stores information representing a proceeding level of a process of writing data to a meta slice. When a writing process to a data slice has been terminated and thereafter a writing process to a meta slice is to be started, the control unit  201  updates history information. When a failure has occurred while the control unit  201  is updating history information, preventing the history update, there is a possibility that the history information and the actual state will not correspond. It is desirable that history information be able to be recovered even when a failure has occurred. Also, history information is updated also when the control unit  201  is halted. 
         [0077]      FIG. 10  illustrate an example of down recovery of history information. In  FIG. 10 , elements similar to those in  FIG. 1  are denoted by the same numbers.  FIG. 10  illustrate an example in which writing of meta information for data 7 to a meta slice has failed because of a failure despite the fact that the control unit  201  has already written data 1 through data 9 to the data slice. Note that the writing of meta information of data 1 through data 6 and data 8 and data 9 to the meta slice  202  has succeeded.  FIG. 10(A)  illustrates an example of a storage device when the control unit  201  has been activated after the occurrence of a failure. The data slice  203  in  FIG. 10(A)  has stored data 1 through data 9. The meta slice  202  in  FIG. 10(A)  has stored meta information of data 1 through data 6 and data 8 and data 9. In the area specified by address 3 in the meta slice  202 , meta information of data 1 and data 4 has been stored. In the area specified by address 4 in the meta slice  202 , meta information of data 2 and data 3 has been stored. In the area specified by address 5 in the meta slice  202 , meta information of data 8 and data 9 has been stored. In the area specified by address 6 in the meta slice  202 , meta information of data 5 and data 6 has been stored. The area specified by address 7 in the meta slice  202  is an area for storing meta information of data 7. 
         [0078]    History information includes the number of data chunks that have already been written to a data slice, the number of meta chunks that are being updated to a meta slice, and address information of meta chunks that are being updated to a meta slice. The number of data chunks that have already been written to a data slice in history information is information representing a proceeding level of a data writing process to a data slice. The number of meta chunks that are being updated to a meta slice in history information and address information of meta chunks that are being updated into a meta slice are information representing a proceeding level of a writing process to a meta slice. The example of history information illustrated in  FIG. 10(   a ) illustrates a case where the number of data chunks that have already been written to a data slice is four, the number of meta chunks that are being updated to a meta slice is three, and the address information of meta chunks that are being updated to a meta slice holds 5, 6, and 7. In the data slice  203 , despite the fact that the writing of data 1 through 9 has succeeded, the number of data chunks that have already been written is four, resulting in a non-correspondence between history information and the actual status. Accordingly, the control unit  201  recovers the history information. Also, when the control unit  201  of a storage device has been halted normally, zero is stored as the number of meta chunks that are being updated. Accordingly, the control unit  201  performs recovery when the number of meta chunks being updated in history information obtained upon the activation is not zero. 
         [0079]    The control unit  201  lists pieces of meta information of data that has been written to a data slice successfully. At this moment, history information indicates that data 1 through data 4 have been written to a data slice successfully. Accordingly, information included in history information is not included in the list. The control unit  201  determines that pieces of data that have been written to a data slice successfully are data 5, data 6, data 8, and data 9. The control unit  201  searches a meta slice for data for which the writing to the meta slice has failed because of a failure, by using the address information of meta chunks that are being updated to a meta slice. The control unit  201  determines that data 5 and data 6, which were written before the data for which writing failed, are pieces of data that have been written successfully. The control unit  201  writes in the history information that pieces of data up to data 6 have been processed normally.  FIG. 10(B)  illustrates an example of a storage device after history information is recovered. The control unit  201  has changed the number of data chunks that have already been written in the history information to six. By contrast, the control unit  201  does not determine that data 8 and data 9, which are written later than data 7, have been written normally, and determines them to be invalid data. Accordingly, meta information related to data 8 and data 9 are deleted from the meta slice  202 . The control unit  201  restarts backup from data for which writing failed. 
       &lt;Prefetch of Meta Data&gt; 
       [0080]      FIG. 11  illustrates an example of prefetch of meta data. In the present embodiment, a higher performance for snapshot is expected by treating part of mapping information in a cache. However, mapping information that uses a cache doest not cause effects on the performance of snapshot except for a case when there is a cache hit. In a sequential access, a plurality of pieces of mapping information are obtained. A process of reading mapping information of a meta slice onto a cache has to be executed for each writing request to a data slice until cache hits are detected for all successive pieces of mapping information. An increase in writing processes and reading processes deteriorates the performance of snapshot. By prefetching mapping information has a high possibility of being referred to in the cache of meta information, the rate of cache hits is increased, and the performance deterioration of snapshot is moderated. 
         [0081]    The control unit  201  includes at least as many physical addresses as a threshold specifies, and determines that data obtained by integrating I/O requests to successive physical addresses is a sequential access. Data obtained by integrating I/O requests to successive physical addresses includes information related to a cache page used by the control unit. The control unit determines whether or not a cache has information of the cache page that is expected to be used. When the cache does not have the information of the cache page expected to be used, the control unit performs prefetch, in which the cache page is read to a cache first. In  FIG. 11 , elements similar to those in  FIG. 1  are denoted by the same numbers. The storage device illustrated in  FIG. 11  includes a cache area  210  from which the meta slice  202  reads data. The master slice  204  is a group of pieces of data including chunk 0 through chunk n, and  FIG. 11  illustrates an example of a case where sequential accesses are made to chunk 0 through chunk 8. The mapping information included in the meta slice  202  illustrated in  FIG. 11  includes cache pages 00 through 08. Data in chunks 0 through 5 used by the sequential accesses use cache page 04, and data in chunks 6 through 8 are assumed to use cache page 05. The cache area  210  illustrated in  FIG. 11  is assumed to hold cache pages 02 through 04. When sequential accesses are made to chunks 0 through 5, the control unit  201  performs prefetch of reading beforehand, onto the cache area  210 , cache page 05, which is expected to be used next. By performing prefetch, it is possible to read, onto a cache area, information having a high possibility of being referred to next in the cache area  210 , improving the cache hit rate and enhancing the performance of snapshot. It is also possible to obtain statistical information of accesses so as to delete cache pages that are used less frequently for a cache page in a cache area. 
         [0082]      FIG. 12  is a flowchart that explains an example of a process performed by the control unit that has received an I/O request. The control unit  201  receives an I/O request from the server (step S 101 ). The control unit  201  determines whether or not the data size of the I/O request received from the server is equal to or greater than a threshold (step S 102 ). The control unit  201  divides the I/O request (step S 103  and YES in step S  102 ). The control unit  201  determines whether or not data to be written by a divisional I/O request is being used currently (step S 104 , NO after the process in S 103  and S 102 ). The control unit  201  sets a divisional I/O request as a subsequent queue (step S 106 ). When the process in step S  106  is terminated, the control unit  201  makes the process return to S 104 . The control unit  201  refers to the mapping information (step S 105 , and NO in step S  104 ). Step S  105  will be described in detail in the flowchart illustrated in  FIG. 13 . 
         [0083]      FIG. 13  is a flowchart explaining an example in which the control unit refers to the mapping information. The control unit  201  determines whether or not the address of the master slice to which the I/O request writes data is included in a cache (step S 201 ). The control unit  201  reads the mapping information onto the cache area from the meta slice (step S 202  and NO in step S  201 ). The control unit  201  determines whether or not addresses of a chunk of the data slice have been assigned to the addresses of the master slice that is to be updated by the I/O request (step S 203  and YES after step S  202  or in step S  201 ). The control unit  201  converts the address of the master slice of the I/O request into the address of the chunk of the data slice (step S 204  and YES in step S  203 ). The control unit  201  treats, as “being used”, the addresses of the master slice to be used as the I/O request (step S 205 ). The control unit  201  performs processes related to copying of data (step S 206 ). Step S  206  will be described in detail in the flowchart illustrated in  FIG. 14 . 
         [0084]      FIG. 14  is a flowchart that explains an example of a process related to copying of data. The control unit  201  determines whether or not the data to be updated by the I/O request is a partial copy in each chunk (step S 301 ). The process in step S 301  corresponds to the explanations in  FIG. 8 . Chunks that are determined to be NO in step S 301  are chunks whose entire data is an update target as represented by chunk 2 in  FIG. 8 , and it is not necessary to copy chunk 2 onto the control unit  201 . Meanwhile, partial copies are chunks 1 and 3 in  FIG. 8 , and the chunks are copied onto the control unit  201 . The control unit  201  reads the chunks onto the control unit  201  (step S 302  and YES in step S 301 ). The control unit  201  integrates I/O requests obtained in the dividing in step S  103  illustrated in  FIG. 12  (step S 303  and NO in step S 301 ). The integration process in step S 303  is the processes explained in  FIG. 5 ,  FIG. 6 , and  FIG. 9 . The control unit  201  updates the update data in the I/O request in the control unit, and writes the updated data to the data slice  203  (step S 304 ). The control unit  201  performs a writing process of mapping information (step S 305 ). The writing process of mapping information will be explained in detail in  FIG. 15 . 
         [0085]      FIG. 15  is a flowchart that explains a process in which the control unit writes meta data to the meta slice. The control unit  201  writes history information and information related to prefetch to the meta slice  202  (step S 401 ). The control unit  201  writes the mapping information to the meta slice  202  (step S 402 ). When the process in step S  402  is terminated, the control unit  201  terminates the process. 
         [0086]      FIG. 16  illustrates a flowchart explaining an example of integrating write requests to hold the same cache page. The control unit  201  checks additional information of jobs that have been queued starting from the job queued at the tail (step S 501 ). The control unit  201  determines whether or not there are jobs having the same cache page (step S 502 ). The control unit  201  generates additional information dedicated to jobs that have been queued on the subsequent side from among jobs having the same cache page (step S 503  and YES in step S 502 ). The control unit  201  performs setting so that jobs that have been queued on the prior side are executed when jobs that have been queued on the subsequent side among jobs having the same cache page are executed (step S 504 ). The control unit  201  sets the dedicated additional information to jobs that have been queued on the subsequent side (step S 505 ). The control unit  201  executes non-synchronized I/O (step S 506  and NO in step S 502 ). The control unit  201  collects additional information of all jobs (step S 507 ). The control unit  201  terminates the process of integrating write requests to hold the same cache page. 
         [0087]      FIG. 17  illustrates a flowchart explaining an example of integrating write requests to successive physical addresses. The control unit  201  checks additional information of jobs that have been queued starting from the job queued at the top (step S 601 ). The control unit  201  determines whether or not there are jobs having successive physical addresses (step S 602 ). The control unit  201  generates additional information dedicated to jobs having successive physical addresses (step S 603  and YES in step S 602 ). The control unit  201  performs setting so that jobs that have been queued on the subsequent side having successive and parallel physical addresses are executed when the top job from among jobs having successive physical addresses is executed (step S 604 ). The control unit  201  determines whether or not subsequent jobs having successive physical addresses hold jobs that are executed further parallelly (step S 605 ). The control unit  201  queues jobs executed parallelly to jobs further subsequent to the jobs having successive physical addresses (step S 606  and YES in step S 605 ). The control unit  201  sets the dedicated additional information as the top job among jobs having information of successive physical addresses (step S 607  and NO in step S 605 ). The control unit  201  executes non-synchronized I/O (step S 608  and NO in step S 602 ). The control unit  201  collects additional information of all jobs (step S 609 ). The control unit  201  terminates the process of integrating write requests to successive physical addresses. 
         [0088]      FIG. 18  illustrates a flowchart explaining an example of integrating write requests to a data slice. The control unit  201  checks additional information of jobs that have been queued starting from the job queued at the top (step S 701 ). The control unit  201  determines whether or not there are jobs having the address of the same master slice (step S 702 ). The control unit  201  generates additional information dedicated to the jobs having information of the address of the same master slice (step S 703  and YES in step S 702 ). The control unit  201  performs setting so that jobs that have been queued on the subsequent side are executed when jobs queued on the front side from among jobs having the information of the address of the same master slice is executed (step S 704 ). The control unit  201  sets the dedicated additional information to jobs queued on the front side (step S 705 ). The control unit  201  executes non-synchronized I/O (step S 706  and NO in step S 702 ). The control unit  201  collects additional information of all jobs (step S 707 ). The control unit  201  terminates the process of integrating write requests to the data slice. 
         [0089]      FIG. 19  illustrates a flowchart explaining an example of down recovery of history information. The control unit  201  reads history information from a meta slice when the control unit  201  is activated (step S 801 ). The control unit  201  obtains information of the number of data chunks that are included in the history information and that have already been written and the number of meta chunks that are being updated (step S 802 ). The control unit  201  determines whether or not the number of meta chunks that are being updated is zero (step S 803 ). The control unit  201  lists pieces of meta information of data for which the writing to the data slice succeeded (step S 804  and NO in step S 803 ). The control unit  201  extracts data for which the writing to the meta slice failed due to a failure (step S 805 ). The control unit  201  determines data that has been written before data for which the writing failed as data for which the writing succeeded normally (step S 806 ). The control unit  201  recovers the history information by using the data for which the writing succeeded normally (step S 807 ). The control unit  201  deletes, from the meta slice, meta information written after data for which the writing failed from among the pieces of meta information listed in S 804  (step S 808 ). The control unit  201  writes the history information to the meta slice (step S 809 ). The control unit  201  obtains the information of the number of data chunks to which the writing has already been performed (step S 810  and YES in step S 803 ). 
         [0090]    As explained above, according to the methods of embodiments, the performance of snapshot can be maintained even when the amount of mapping information has increased accompanying the increase in scale of storage devices and systems. 
         [0091]    All examples and conditional language provided herein are intended for the pedagogical purpose of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification related to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have 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.