Patent Publication Number: US-9838043-B2

Title: Storage system, information processor, and computer-readable recording medium having stored therein program for generating parity

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent application No. 2015-065500, filed on Mar. 27, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is directed to a storage system, an information processor, and a computer-readable recording medium having stored therein a program for generating a parity. 
     BACKGROUND 
     A distributed storage system has been known which arranges data and a parity in different nodes. Since the Erasure Code in a distributed storage system arranges data and a parity in different nodes, data updating accompanies a distributed transaction. As one of the known techniques for such a storage system, each nodes records post-updating image data in the form of a journal in preparation for possible node failure, so that the reliability of the transaction can be improved. 
     Accompanying drawing  FIG. 7  denotes a process of writing a journal in a traditional storage system; and FIG.  8  denotes a process of applying the journal in the same system. 
     A storage system (distributed storage system)  100   a  of  FIGS. 7 and 8  provides a memory region to a non-illustrated host device. The storage system  100   a  includes multiple (three in the illustrated example) data nodes  1   a  (data nodes # 1 -# 3 ) and multiple (two in the illustrated example) parity nodes  2   a  (parity nodes # 1  and # 2 ). This means that  FIGS. 7 and 8  illustrate an example that the stripe of the Erasure Code consists of three data chunks and two parity chunks. Each data node  1   a  is communicably connected to each parity node  2   a  via, for example, a Local Area Network (LAN) cable. 
     Hereinafter, each individual data node is specified by “data node # 1 ”, “data node # 2 ”, or “data node # 3 ”, but an arbitrary data node is represented by “data node  1   a ”. Likewise, each individual parity node is specified by “parity node # 1 ” or “parity node # 2 ”, but an arbitrary parity node is represented by “parity node  2   a”.    
     A data node  1   a  stores therein data received from an external device such as the non-illustrated host device, and includes a Central Processing Unit (CPU)  11   a , a file store  13   a , a journal disk  14   a , and a non-illustrated memory. The data nodes # 1 -# 3  have the same functional configuration,  FIGS. 7 and 8  omit illustration of the functional configurations of the data nodes # 1  and # 3 . 
     The CPU  11   a  is a processor that executes various controls and calculations, and achieves various functions by executing an Operating System (OS) and a program stored in the non-illustrated memory. 
     The file store  13   a  is a known device that readably and writably stores therein data received by the data node  1   a , and is exemplified by a Hard Disk Drive (HDD) or a Solid State Drive (SSD). 
     The journal disk  14   a  is a known device that readably and writably stores therein a journal, which is a record of data received by the data node  1   a , and is exemplified by an HDD or an SDD. 
     A parity node  2   a  is a node that stores therein a parity of data stored in the data node  1   a , and includes a CPU  21   a , a file store  23   a , a journal disk  24   a , and a non-illustrated memory. Since the parity nodes # 1  and # 2  have the same functional configuration,  FIGS. 7 and 8  omit illustration of the functional configuration of the parity node # 1 . 
     The CPU  21   a  is a processor that executes various controls and calculations, and achieves various functions by executing an Operating System (OS) and a program stored in the non-illustrated memory. 
     The file store  23   a  is a known device that readably and writably stores therein a parity of data received by the data node  1   a , and is exemplified by an HDD or an SSD. 
     The journal disk  24   a  is a known device that readably and writably stores therein a journal of a parity to be stored in the file store  23   a , and is exemplified by an HDD or an SDD. 
     Hereinafter, description will now be made in relation to a process of writing a journal and a process of applying the journal in a traditional storage system with reference to  FIGS. 7 and 8 . For simplification of the description to be made by referring to  FIGS. 7 and 8 , a process by “the CPU  11   a  of the data node # 2 ” is referred to as “a process by the data node # 2 ”, and likewise a process by “the CPU  21   a  of the parity node # 2 ” is referred to as “a process by the parity node # 2 ”. 
     The data node # 2  receives updating data “7” from a non-illustrated host device (see symbol B 1  in  FIG. 7 ). 
     The data node # 2  reads pre-updating data “4” from the file store  13   a  (see symbol B 2  in  FIG. 7 ). 
     The data node # 2  calculates the difference between the updating data and the pre-updating data (see symbol B 3  in  FIG. 7 ). In the example of  FIG. 7 , the data node # 2  calculates the value “3” to be the difference data by subtracting the pre-updating data “4” from the updating data “7”. 
     The data node # 2  writes updating data (post-updating data) “7” into the journal disk  14   a  (see symbol B 4  in  FIG. 7 ). 
     The data node # 2  forwards the calculated difference data “3” to the parity node # 2  (see symbol B 5  in  FIG. 7 ). 
     The parity node # 2  receives the difference data “3” from the data node # 2  (see symbol B 6  in  FIG. 7 ). 
     The parity node # 2  reads a pre-updating parity from the file store  23   a  (see symbol B 7  in  FIG. 7 ). 
     The parity node # 2  applies the difference to the pre-updating parity (see symbol B 8  in  FIG. 7 ). In the example of  FIG. 7 , the data node # 2  calculates a post-updating parity “5” by adding the difference data “3” to the pre-updating parity “2”. 
     The parity node # 2  writes the calculated post-updating parity “5” into the journal disk  24   a  (see symbol B 9  in  FIG. 7 ). 
     Next, the data node # 2  reads a journal from the journal disk  14   a  for applying the journal (see symbol B 10  in  FIG. 8 ). 
     Then, the data node # 2  writes the read journal into the file store  13   a  (see symbol B 11  in  FIG. 8 ). 
     The parity node # 2  reads a journal from the journal disk  24   a  for applying the journal (see symbol B 12  in  FIG. 8 ). 
     The parity nodes # 2  writes the read journal into the file store  13   a  (see symbol B 13  in  FIG. 8 ). 
     [Patent Literature 1] Japanese Laid-open Patent Publication No. 08-87424 
     [Patent Literature 2] Japanese Laid-open Patent Publication No. 2006-338461 
     In the traditional storage system  100   a , the data node  1   a  and the parity node  2   a  include dedicated journal disks  14   a  and  24   a , respectively. With this configuration, the data node  1   a  writes the post-updating data, as a journal, into the journal disk  14   a  while the parity node  2   a  writes a post-updating parity, as a journal, into the journal disk  24   a . Accordingly, an increase in the number of parity nodes  2   a  accompanies an increase in the data volume of the journals, which needs more disk volume. Furthermore, the storage system  100   a  endures increased Input/Output (I/O) overhead. 
     SUMMARY 
     With the above in view, there is provided a storage system including a first information processor that stores therein data, a second information processor that is communicably connected to the first information processor and that stores therein a parity of the data, and a superordinate device that is communicably connected to the first information processor. The first information processor includes a first memory device that stores therein the data, a difference generator that generates difference data representing a difference between updating data received from the superordinate device and the data stored in the first memory device before updating, a second memory device stores therein the difference data generated by the difference generator, and a data transmitter that transmits the difference data stored in the second memory device to the second information processor. The second information processor includes a third memory device that stores therein the parity, a data receiver that receives the difference data transmitted from the data transmitter, and a parity difference applier that generates a post-updating parity that is to be written into the third memory device by applying the difference data received by the data receiver to the parity stored in the third memory device before the updating. 
     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. 
     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 
         FIG. 1  is a diagram schematically illustrating the functional configuration of a storage system according a first embodiment; 
         FIG. 2  is a diagram schematically illustrating the functional configuration of a CPU included in a data node of the first embodiment; 
         FIG. 3  is a diagram schematically illustrating the functional configuration of a CPU included in a parity node of the first embodiment; 
         FIG. 4  is a sequence diagram denoting a process of writing a journal and a process of applying the journal in a storage system of the first embodiment; 
         FIG. 5  is a diagram denoting a process of writing a journal in a storage system of the first embodiment; 
         FIG. 6  is a diagram is a diagram denoting a process of applying a journal in a storage system of the first embodiment; 
         FIG. 7  is a diagram denoting a process of writing a journal in a traditional storage system; and 
         FIG. 8  is a diagram is a diagram denoting a process of applying a journal in a traditional storage system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, description will now be made in relation to a storage system, an information processor, and a computer-readable recording medium having stored therein a program for generating a parity with reference to accompanying drawings. However, the embodiment to be detailed below is are merely example and does not intend to exclude another modification and application of techniques that are not referred in this description. In other words, various changes and modification can be suggested without departing from the gist of the embodiment. 
     The accompanying drawings may include other elements and functions in addition to those appearing in the drawings. 
     Like reference numbers designate the same or the substantially same parts or elements in the accompanying drawings, so repetitious description is omitted here. 
     (A) First Embodiment 
     (A-1) System Configuration: 
       FIG. 1  schematically illustrates the functional configuration of a storage system according to a first embodiment. 
     A storage system  100  illustrated in  FIG. 1  provides a memory region to a non-illustrated host device (superordinate device), and functions as a distributed storage system that distributedly stores data into multiple data nodes  1 . The storage system  100  includes multiple (three in the illustrated example) data nodes  1  (data nodes # 1 -# 3 ; first information processors) and multiple (two in the illustrated example) parity nodes  2  (parity nodes # 1  and # 2 ; second information processors). This means that  FIG. 1  illustrates an example that the stripe of the Erasure Code consists of three data chunks and two parity chunks. Each data node  1  is communicably connected to each parity node  2  via, for example, a LAN cable. 
     Hereinafter, each individual data node is specified by “data node # 1 ”, “data node # 2 ”, or “data node # 3 ”, but an arbitrary data node is represented by “data node  1 ”. Likewise, each individual parity node is specified by “parity node # 1 ” or “parity node # 2 ”, but an arbitrary parity node is represented by “parity node  2 ”. 
     A data node  1  stores therein data received from an external device such as the non-illustrated host device, and includes a CPU  11 , a memory  12 , a file store (first memory device)  13 , and a journal disk (second memory device)  14 . 
     The file store  13  is a known device that readably and writably stores therein data received by the data node  1 , and is exemplified by an HDD or an SSD. 
     The journal disk  14  is a known device that readably and writably stores therein a journal (updating history, updated difference), which is a record of data received by the data node  1   a , and is exemplified by an HDD or an SDD. 
     The memory  12  is a memory device including a Read Only Memory (ROM), and a Random Access Memory (RAM). In the ROM of the memory  12 , a program such as Basic Input/Output System (BIOS) is written. A software program stored in the memory  12  is appropriately read by the CPU  11 , which then executes the program. The RAM of the memory  12  is used as a primary recording memory and a working memory. 
     The CPU  11  is a processor that executes various controls and calculations, and achieves various functions by executing an OS and a program stored in the memory  12 . Specifically, as will be detailed below by referring to  FIG. 2 , the CPU  11  functions as a data receiver  111 , a file store reader  112 , a difference generator  113 , a journal writer  114 , a journal reader  115 , a difference applier (data difference applier)  116 , a file store writer  117 , and a data transmitter  118 . 
     A program (program for generating a parity) that achieves the functions of the data receiver  111 , the file store reader  112 , the difference generator  113 , the journal writer  114 , the journal reader  115 , the difference applier  116 , the file store writer  117 , and the data transmitter  118  is provided in the form of being recorded in a tangible and non-transitory computer-readable storage medium, such as a flexible disk, a CD (e.g., CD-ROM, CD-R, and CD-RW), a DVD (DVD-ROM, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, and HD DVD), a Blu-ray disk, a magnetic disk, an optical disk, and an magneto-optical disk. A computer reads the program from the recording medium using a non-illustrated medium reader and stores the read program in an internal or external storage device for future use. Alternatively, the program may be recorded in a recording device (recording medium), such as a magnetic disk, an optical disk, or a magneto-optical disk, and may be provided from the recording device to the computer via a communication path. 
     Further alternatively, in achieving the functions of the data receiver  111 , the file store reader  112 , the difference generator  113 , the journal writer  114 , the journal reader  115 , the difference applier  116 , the file store writer  117 , and the data transmitter  118 , the program stored in an internal memory device (corresponding to the memory  12  in this embodiment) is executed by the microprocessor (corresponding to the CPU  11  in this embodiment) of the computer. For this purpose, the computer may read the program stored in the recording medium and execute the program. 
     A parity node  2  is a node that stores therein a parity of data stored in the data node  1 , and includes a CPU  21 , a memory  22 , and a file store  23  (third memory device). 
     The file store  23  is a known device that readably and writably stores therein a parity of data to be stored in the data node  1 , and is exemplified by an HDD or an SSD. 
     The memory  22  is a memory device including a ROM and a RAM. In the ROM of the memory  22 , a program such as the BIOS is written. A software program stored in the memory  22  is appropriately read by the CPU  21 , which then executes the program. The RAM of the memory  22  is used as a primary recording memory and a working memory. 
     The CPU  21  is a processor that executes various controls and calculations, and achieves various functions by executing an OS and a program stored in the memory  22 . Specifically, as will be detailed below by referring to  FIG. 3 , the CPU  21  functions as a data receiver  211 , a file store reader  212 , a difference applier (parity difference applier)  213 , and a file store writer  214 . 
     A program (program for generating a parity) that achieves the functions of the data receiver  211 , the file store reader  212 , the difference applier  213 , and the file store writer  214  is provided in the form of being recorded in a tangible and non-transitory computer-readable storage medium, such as a flexible disk, a CD (e.g., CD-ROM, CD-R, and CD-RW), a DVD (DVD-ROM, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, and HD DVD), a Blu-ray disk, a magnetic disk, an optical disk, and an magneto-optical disk. A computer reads the program from the recording medium using a non-illustrated medium reader and stores the read program in an internal or external storage device for future use. Alternatively, the program may be recorded in a recording device (recording medium), such as a magnetic disk, an optical disk, or a magneto-optical disk, and may be provided from the recording device to the computer via a communication path. 
     Further alternatively, in achieving the functions of the data receiver  211 , the file store reader  212 , the difference applier  213 , and the file store writer  214 , the program stored in an internal memory device (corresponding to the memory  22  in this embodiment) is executed by the microprocessor (corresponding to the CPU  21  in this embodiment) of the computer. For this purpose, the computer may read the program stored in the recording medium and execute the program. 
       FIG. 2  schematically illustrates the functional configuration of a CPU included in each data node of this embodiment. 
     The data receiver  111  receives data (updating data) from an external device such as a non-illustrated host device. Specifically, the data receiver  111  of each of the data nodes # 1 -# 3  segments data that the corresponding node received from the external device into chunks in a unit of several bytes (e.g., four bytes) and receives the segmented chunks. Storing data into the file store  13  of a data node  1 , calculating of a parity in a parity node  2 , and storing of a parity into the file store  23  are carried out in units of a segmented chunk. 
     The file store reader  112  reads data stored in the file store  13 . Specifically, the file store reader  112  reads, when the data receiver  111  receives updating data, pre-updating data stored in a region to be updated of the file store  13 . The file store reader  112  further reads pre-updating data stored in the file store  13  at the timing of applying a journal that is to be detailed below. 
     The difference generator  113  generates difference information. Specifically, the difference generator  113  generates difference information representing a difference between the updating data received by the data receiver  111  and the pre-updating data read from the file store  13  by the file store reader  112 . As described above, since a parity is calculated in units of a segmented chunk in the first embodiment, the difference generator  113  performs a subtraction of the bit length on the Galois Field using the updating data (post-updating image) and pre-updating data (pre-updating image) along the following expression.
 
Δ D=D   ai   −D   bi   [Expression 1]
 
     Here, the term D ai  represents a post-updating image; the term D bi  represents a pre-updating image; and the negative sign “−” represents the subtraction on the Galois Field. Consequently, the difference generator  113  defines the difference segmented in units of several bytes. 
     The journal writer  114  writes data into a journal. Specifically, the journal writer  114  writes the difference data generated by the difference generator  113  to be a journal into the journal disk  14 . 
     The data node  1   a  of the traditional system illustrated in  FIGS. 7 and 8  writes a post-updating image to be a journal. In contrast, the journal writer  114  of the data node  1  of this embodiment writes a Galois updating difference calculated by the difference generator  113  to be the journal. 
     The journal reader  115  reads data from a journal. Specifically, the journal reader  115  reads the difference data written by the journal writer  114  from the journal disk  14  at a timing of applying a journal. Here, examples of the timing of applying a journal is a timing at which the void volume of the journal disk  14  runs short or when a predetermined time has elapsed since the previous applying of a journal. 
     The difference applier  116  generates post-updating data to be written into the file store  13 . Specifically, the difference applier  116  applies (for example, adds) the difference data that the journal reader  115  has read from the journal disk  14  to the pre-updating data that the file store reader  112  has read from the file store  13 . Consequently, the difference applier  116  generates a post-updating data. 
     The file store writer  117  writes data into the file store  13 . Specifically, the file store writer  117  writes the post-updating data generated by the difference applier  116  into the file store  13 . 
     The data transmitter  118  transmits data to the parity node  2 . Specifically, the data transmitter  118  transmits the difference data read by the journal reader  115  from the journal disk  14  to the parity node  2 . 
     The traditional data node  1   a  described by referring to  FIGS. 7 and 8  forwards the calculated updating difference to the parity node  2   a  after the completion of the process in the data node  1   a  (or in parallel with the process of writing of the journal). In contrast, the data transmitter  118  in the data node  1  of this embodiment forwards the updating difference read from the journal disk  14  to the parity node  2  at any timing until the journal is invalidated. 
     Alternatively, the data transmitter  118  may transmit the difference data to multiple parity nodes  2  for maintain data redundancy. In this case, the parity nodes  2  execute the process in parallel with one another. 
       FIG. 3  schematically illustrates the functional configuration of the CPU included in the parity node of the first embodiment. 
     The data receiver  211  receives data from the data node  1 . Specifically, the data receiver  211  receives the difference data transmitted from the data transmitter  118  of the data node  1 . 
     The file store reader  212  reads a parity from the file store  23 . Specifically, the file store reader  212  reads, when the data receiver  211  receives the difference data, pre-updating parity stored in the file store  23 . 
     The difference applier  213  generates a post-updating parity to be written into the file store  23 . Specifically, the difference applier  213  applies (for example, adds) the difference data received by the data receiver  211  to the pre-updating parity that the file store reader  212  has read from the file store  23 . Consequently, the difference applier  213  generates the post-updating parity. 
     The file store writer  214  writes a parity into the file store  23 . Specifically, the file store writer  214  writes the post-updating parity generated by the difference applier  213  into the file store  23 . 
     The traditional parity node  2   a  illustrated in  FIGS. 7  an  8  writes a post-updating parity into the file store  23   a  and further generates a post-updating parity by applying the difference data to a pre-updating parity. The parity node  2   a  writes the generated post-updating parity as a journal into the journal disk  24   a , in contrast, the parity node  2  according to this embodiment is not equipped with a journal disk to store a journal, and the file store writer  214  writes a Galois updating difference forwarded from the data node  1  directly into the file store  23 . 
     In the first embodiment, there is no need to apply a journal to the file store  23  of the parity node  2  each time a journal is generated. Specifically, the data node  1  is allowed to forward a journal (difference data) of the sum of several journals in a lump to the parity node  2 , which then applies the forwarded journals to the file store  23  all at once. Hereinafter, the reason for the above will now be detailed. 
     Representing a generation matrix for a linear block code by (a ij ) and representing a data chunk by D i , the parity chunk P i  is represented by the following expression. Here, the symbols i and j are natural numbers; the symbol i represents the parity number and the symbol j represents a value specifying a coefficient depending on data.
 
 P   i   =a   i1   D   1   +a   i2   D   2   + . . . +a   in   D   n   [Expression 2]
 
     Here, representing a Galois difference journal of one or more pieces of data D k  by ΣΔD k , the post-updating image D k ′ of a data chunk is represented by the following expression.
 
 D′   k   =D   k   +ΣΔD   k   [Expression 3]
 
     Accordingly, the post-updating image P i ′ of a parity chunk is represented by the following expression. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     The Galois difference journal ΣΔD k  of a data chunk is recorded as a journal of a parity chunk without being modified, and a product of the journal of a parity chunk and the coefficient (a ik ) is added to the pre-post image of the parity chunk. This calculates the post-updating image P i ′ of the parity chunk. 
     (A-2) Operation: 
     Description will now be made in relation to a process of writing a journal and a process of applying a journal in a storage system of an example of the first embodiment along the sequence diagram  FIG. 4  (steps S 1 -S 13 ) with reference to  FIGS. 5 and 6 . Here, steps S 1 -S 4  of  FIG. 4  and symbols A 1 -A 4  of  FIG. 5  represent a process of writing a journal and steps S 5 -S 13  of  FIG. 4  and symbols A 5 -A 13  of  FIG. 6  represent a process of applying a journal. 
       FIGS. 4-6  describe a process of writing a journal and a process of applying a journal in the data node # 2  and the parity node # 2 . For simplification, illustration of the functional configurations of the data nodes # 1  and # 3  and parity node # 1  is omitted, and illustration of the memory  12  and the memory  22  respectively provided in the data node # 2  and the parity node # 2  is also omitted illustrated. 
     The data receiver  111  of the data node # 2  receives updating data “7” from a non-illustrated host device (see step S 1  of  FIG. 4  and symbol A 1  of  FIG. 5 ). 
     The file store reader  112  reads pre-updating data “4” from the file store  13  (see step S 2  of  FIG. 4  and symbol A 2  of  FIG. 5 ). 
     The difference generator  113  calculates the difference between the updating data received by the data receiver  111  and the pre-updating data read by the file store reader  112  (see step S 3  of  FIG. 4  and symbol A 3  of  FIG. 5 ). In the example of  FIG. 5 , the difference generator  113  calculates the difference data to be the value “3” obtained by subtracting the pre-updating data “4” from the updating data “7”. 
     The journal writer  114  writes the difference data “3” generated by the difference generator  113 , as the journal, into the journal disk  14  (see step S 4  of  FIG. 4  and symbol A 4  of  FIG. 5 ). 
     The journal reader  115  reads the difference data (journal) “3” written by the journal writer  114  from the journal disk  14  at the timing of applying a journal (see step S 5  of  FIG. 4  and symbol A 5  of  FIG. 6 ). 
     The file store reader  112  reads the pre-updating data “4” from the file store  13  (see step S 6  of  FIG. 4  and symbol A 6  of  FIG. 6 ). 
     The difference applier  116  applies the difference data read by the journal reader  115  to the pre-updating data read by the file store reader  112  (see step S 7  of  FIG. 4  and symbol A 7  of  FIG. 6 ). In the example of  FIG. 6 , the difference applier  116  generates the post-updating data “7” by adding the difference data “3” to the pre-updating data “4”. 
     The file store writer  117  writes the post-updating data “7” generated by the difference applier  116  into the file store  13  (see step S 8  of  FIG. 4  and symbol A 8  of  FIG. 6 ). 
     The data transmitter  118  forwards the difference data “3” read by the journal reader  115  to the parity node # 2  (see step S 9  of  FIG. 4  and symbol A 9  of  FIG. 6 ) in parallel with the process of steps S 6 -S 8  of  FIG. 8  (symbols A 6 -A 8  of  FIG. 6 ). 
     The data receiver  211  of the parity node # 2  receives the difference data “3” forwarded by the parity node # 1  (see step S 10  of  FIG. 4  and symbol A 10  of  FIG. 6 ). 
     The file store reader  212  reads the pre-updating parity “2” from the file store  23  (see step S 11  of  FIG. 4  and symbol A 11  of  FIG. 6 ). 
     The difference applier  213  applies the difference data received by the data receiver  211  to the pre-updating parity read by the file store reader  212  (see step S 12  of  FIG. 4  and symbol A 12  of  FIG. 6 ). In the example of  FIG. 6 , the difference applier  213  generates the post-updating parity “5” by adding the difference data “3” to the pre-updating parity “2”. 
     The file store writer  214  writes the post-updating parity “5” generated by the difference applier  213  into the file store  23  (see step S 13  of  FIG. 4  and symbol A 13  of  FIG. 6 ). 
     The above procedure completes the process of writing a journal and the process of applying the journal. 
     (A-3) Effects: 
     In the data node  1 , the difference generator  113  generates the difference data representing the difference between the updating data received from the superordinate device and the pre-updating data stored in the file store  13 . Then, the journal disk  14  stores the difference data generated by the difference generator  113 . 
     In the parity node  2 , the data receiver  211  receives the difference data transmitted from the data transmitter  118  of the data node  1 . The difference applier  213  generates the post-updating parity to be written into the file store  23  by applying the difference data received by the data receiver  211  to the pre-updating parity stored in the file store  23 . 
     This allows the distributed storage system  100  to have a less disk volume. Specifically, the parity node  2  has no need to have a journal disk, so that the storage resource of the parity node  2  can be more effectively used. The parity node  2  does not have to write a journal and consequently the load on the disk I/O can be lightened when a journal is to be applied. Furthermore, a process of applying a journal to the file store  23  of the parity node  2  can be precisely accomplished. 
     In the data node  1 , the difference applier  116  generates the post-updating data to be written into the file store  13  by applying the difference data stored in the journal disk  14  to the pre-updating data stored in the file store  13 . 
     This can precisely accomplish the process of applying a journal to the file store  13  of the data node  1 . 
     The difference generator  113  generates the difference data through the calculation on the Galois Field using the updating data received from the superordinate device and the pre-updating data stored in the file store  13 . 
     This allows the difference generator  113  to easily calculate the difference. 
     (B) Others 
     The technique disclosed above is not limited to the foregoing embodiment and various changes and modifications can be suggested without departing from the spirit of the above embodiment. The configurations and steps of the above embodiment can be omitted, selected, or combined according to the requirement. 
     The disclosed storage system allows a distributed storage system to have a less disk volume. 
     All examples and conditional language recited herein are intended for the pedagogical purposes 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 relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present inventions 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.