Abstract:
Disclosed herewith is a scalable disk array controller inter-connection network to be employed for a disk array system in which a plurality of disk array controllers connected to each another are expected to be operated as one disk array controller. The inter-connection network enables such disk array controllers to be added/removed without service interruption while the system reliability is kept as is. Each of the plurality of disk array controllers comprises a channel IF unit; a disk IF unit; a cache memory unit; a shared memory unit; means for connecting the channel IF unit /disk IF unit and the cache memory unit; and means for connecting the channel IF unit /disk IF unit and the shared memory unit. The inter-connection network comprises a plurality of switches to be increased in a scalable manner. Each of the switches is inter-connected with other switches with use of a redundant path having a separated physical route. In addition, each of the switches has an output destination table used to set redundant paths and switches between paths after the inter-connection network is modified due to the addition/removal of the above switches. The disk array controller becomes cost-scalable because the disk array controller units can be added/removed to/from the system as required without service interruption while the system reliability is kept as is.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. application Ser. No. 10/067332 filed on Feb. 7, 2002, U.S. application Ser. No. 10/090767 filed on Mar. 6, 2002 and U.S. application Ser. No. 10/222831 filed on Aug. 19, 2002, the disclosure of which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a controller of a disk array apparatus configured by a plurality of magnetic disk drives used for storing data respectively.  
           [0004]    2. Description of the Related Art  
           [0005]    [Patent Document 1] 
           [0006]    JP-A No. 256003/2001  
           [0007]    Large-scale corporations such as banks, securities firms, telephone companies are now trying to reduce the costs of the operation, maintenance, and management of their computer systems and storage systems by integrating those computers and storages in a data center respectively while they have been distributed to some places so far.  
           [0008]    In order to meet such the demands, large high-end disk array controllers are beginning to support the channel interfaces with more than one hundred host computers (connectivity) and the storages, each having a large capacity of more than several hundreds of terabytes.  
           [0009]    On the other hand, along with the expectation that the storage area network (SAN) will spread more in the near future, small-scale disk array controllers are also coming to be required of the same enhanced high reliability as that of the large high-end disk array controllers.  
           [0010]    As a result, each of the disk array controllers as described above has now been configured with scalability to cope with any of small-scale and large-scale configurations with the same enhanced highly reliable architecture. And, one of the methods for enabling such the disk array controller is to integrate a plurality of disk array controllers so as to operate them as one system.  
           [0011]    [0011]FIG. 2 shows an outline of the disk array controller disclosed by JP-A No. 256003/2001 as an example of the conventional techniques for those disk array controllers. According to this conventional technique, the disk array controller comprises a plurality of disk array control units, each including a plurality of channel IF units  11  used for the data transfer between a plurality of host computers  50  and the plurality of disk array control units  1 ; a plurality of disk IF units  12  used for the data transfer between a plurality of magnetic disk drives  5  and the plurality of disk array control units  2 ; a plurality of cache memory units  14  for storing data read/to be written from/in the plurality of magnetic disk drives  5  temporarily; and a plurality of shared memory units  13  for storing control information related to the disk array control units  1  (ex., information related to the controlling of the data transfer between the channel IF units  11 /disk IF units  12  and the cache memory units  14 , and management information of the data to be stored in the magnetic disk drives  5 ). The shared memory units  13  and the cache memory units  14  can be accessed from every channel IF unit  11  and every disk IF unit  12  over the plurality of disk array control units  1 . In this configuration of the disk array controller, inter-connection networks  10  and  20  are used for the connection between the channel IF units  11 /disk IF units  12  and the shared memory units  13 , as well as between the channel IF units  11 /disk IF units  12  and the cache memory units  14  over the plurality of disk array control units  1 .  
           [0012]    [0012]FIG. 3 shows a detailed internal block diagram of a conventional disk array control unit (DKC)  1 . Each disk array control unit  1  includes a plurality of SM-SWs (shared memory switches)  110 , and a plurality of SM paths between units  141 , through which the units  141  are connected to the inter-connection network  10 . And, each disk array control unit  1  includes a plurality of CM-SWs (cache memory switches)  111 , as well as a plurality of CM paths between units  142 , through which the units  142  are connected to the inter-connection network  20 .  
           [0013]    As described above, the conventional disk array controller thus comes to be able to provide any of small to super large scale configurations with scalability, since a plurality of disk array control units are connected to each another through the inter-connection networks.  
           [0014]    However, the conventional technique does not mention details of how to configure any of such the inter-connection networks while the technique can cope with any of small to super large scale configurations with scalability by connecting plurality of disk array control units through the inter-connection networks configured by switches. And, how to form such a scalable inter-connection network is very important to realize scalable disk array controllers, whether they are small or super-large in scale. In other words, a small-scale inter-connection network should be used when the number of the connected disk array control units is not so large while a large-scale inter-connection network should be used when the number of the connected disk array control unit is large. When an inter-connection network is to be scaled up/down, the scale-down/up should be made without service interruptions while the controller&#39;s reliability is kept. In addition, the unit module for configuring a scalable inter-connection network should be fabricated with as less hardware items as possible.  
           [0015]    Under such circumstances, it is an object of the present invention to provide a scalable disk array control unit inter-connection network to realize scalable disk array controllers.  
           [0016]    More concretely, it is an object of the present invention to provide a disk array control unit inter-connection network to/from which scalable disk array control units can be added/removed without service interruptions while the network reliability is kept so as to provide scalable disk array controllers.  
           [0017]    It is another object of the present invention to provide an implementation method of a cost-scalable disk array control unit inter-connection network with less hardware items.  
         SUMMARY OF THE INVENTION  
         [0018]    In order to achieve the above objects, the disk array controller of the present invention is provided with an inter-connection network for connecting a plurality of disk array control units, each including a channel IF unit; a disk IF unit; a cache memory unit; a shared memory unit; means for connecting the channel IF unit/disk IF unit and the cache memory unit; and means for connecting the channel IF unit/disk IF unit and the shared memory unit so as to read/write data. The inter-connection network is configured by a plurality of switches to be added in a scalable manner and each of the switches has an output path table used to form a mesh connection path with use of a redundant path having a separated physical route to other switches and set a redundant path or change the path to another after the modification of the inter-connection network. Each of the switches shares a physical link with other switches as far as possible to form paths to other switches to reduce the necessary number of physical switch ports.  
           [0019]    Furthermore, in order to shorten the cable length for each inter-connection between switches of the scalable connection network, those two types of racks are disposed alternately. One for housing a plurality of the disk array control units. The other for housing the plurality of disk array control units and the plurality of switches.  
           [0020]    In another aspect, the disk array controller of the present invention comprises a plurality of disk array control units (DKC), a plurality of switching units (GSW) connecting the plurality of DKCs, and a path for connecting each of the connection units to another. Each of the disk array control units includes a first interface having an interface with a host computer, a second interface unit having an interface with a first memory unit, a second memory for storing data read/to be written from/in the first memory temporarily, and a transfer path for connecting the first/second interface unit to the second memory. Each of the connection units includes a plurality of disk ports connected to the first or second interface, an inter-unit port connected to a path used to connect each of the connection units to another to transfer data, and management means for changing the correspondence between a connection unit assumed as a final device for receiving transfer data and a unit for outputting the data.  
           [0021]    As an example, the management means has a managing table for managing the correspondence between each connection unit for receiving data and a port between units for outputting the data. For example, the managing table should preferably manage four connection units, and the managing table should preferably have at least five ports between units.  
           [0022]    The correspondence between a destination unit and an output port changes according to the number of connected connection units. As an example, the managing table may be updated according to the number of connection units connected to each another.  
           [0023]    These and other objects and methods for solving conventional problems will become more apparent as the description proceeds with the following embodiment and appended drawings as described below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a block diagram of a disk array controller in an embodiment of the present invention;  
         [0025]    [0025]FIG. 2 is a block diagram of a conventional disk array controller;  
         [0026]    [0026]FIG. 3 is another block diagram of the conventional disk array controller;  
         [0027]    [0027]FIG. 4 is still another block diagram of the conventional disk array controller;  
         [0028]    [0028]FIG. 5 is another block diagram of the disk array controller in the embodiment of the present invention;  
         [0029]    [0029]FIG. 6 is still another block diagram of the disk array controller in the embodiment of the disk array controller of the present invention;  
         [0030]    [0030]FIG. 7 is a detailed internal block diagram of the disk array control unit shown in FIG. 1;  
         [0031]    [0031]FIG. 8 is a block diagram of the disk array controller to which a plurality of the disk array control units shown in FIG. 7 are connected;  
         [0032]    [0032]FIG. 9 is a detailed internal block diagram of the disk array control unit shown in FIG. 5;  
         [0033]    [0033]FIG. 10 is a block diagram of the disk array controller to which a plurality of the disk array control units shown in FIG. 9 are connected;  
         [0034]    [0034]FIG. 11 is a detailed internal block diagram of the disk array control unit shown in FIG. 6;  
         [0035]    [0035]FIG. 12 is a block diagram of the disk array controller to which a plurality of disk array control units shown in FIG. 11 are connected;  
         [0036]    [0036]FIG. 13 is another block diagram of the disk array controller to which a plurality of the disk array control units shown in FIG. 7 are connected;  
         [0037]    [0037]FIG. 14 illustrates how the disk array control units of the present invention are housed in racks;  
         [0038]    [0038]FIG. 15 illustrates how a plurality of racks in which disk array control units are housed respectively are connected to each another in the embodiment of the present invention;  
         [0039]    [0039]FIG. 16 illustrates the scalability of the disk array control unit racks and switch cards in the embodiment of the present invention;  
         [0040]    [0040]FIG. 17 illustrates how switch cards are housed in the disk array control unit racks in the embodiment of the present invention;  
         [0041]    [0041]FIG. 18 illustrates how switch cards are housed in the disk array control unit racks in the embodiment of the present invention; and  
         [0042]    [0042]FIG. 19 is an internal block diagram of the GSW  2 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]    Hereunder, an embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0044]    As shown in FIG. 1, a disk array controller  100  is configured so that it is connected to a plurality of disk array control units  1  through an inter-connection network consisting of the number of connecting switches  2  decided by the number of the controller  100 . A disk array control unit  1  comprises a plurality of channel interface (IF) units  11  connected to a plurality of host computers  50 ; a plurality of disk interface (IF) units  12  connected to a plurality of magnetic disk drives  5 ; a plurality of shared memory units  13 ; and a plurality of cache memory units  14 . An inter-connection network consisting of a plurality of connecting switches  2  to connect a plurality of disk array control units  1 - 1  is used for the connection between the channel IF units  11 /disk IF units  12  and the shared memory units  13 , and another inter-connection network, consisting of a plurality of connecting switches  2  to connect a plurality of disk array control units  1 - 1 , is used for the connection between the channel IF units  11 /disk IF units  12  and the cache memory units  14 . These two networks used to connect a plurality of disk array control units may be configured independently of each other or united into one.  
         [0045]    [0045]FIG. 4 shows an example of how the connecting switches  2  are disposed in an inter-connection network. In FIG. 4, four disk array control units  1  (hereinafter, to be described as the DKC: disk array controller) are connected to a connecting switch  2  (hereinafter, to be described as the GSW: Global Connection Switch) and four GSWs  2  (GSWs  2 - 0  to  2 - 3 ) are connected to each another to configure a disk array controller. Up to  16  DKCs can be connected to one another in one disk array controller. The DKCs  1 - 0  to  1 - 3  are connected to the GSW  2 - 0 , the DKCs  1 - 4  to  1 - 7  are connected to the GSW  2 - 1 , the DKCs  1 - 8  to  1 - 11  are connected to the GSW  2 - 2 , and the DKCs  1 - 12  to  1 - 15  are connected to the GSW  2 - 3 , respectively. The GSWs  2 - 0  to  2 - 3  are disposed like a matrix and connected to each another. Each GSW  2  uses five ports (port  0  to port  5 ) to connect itself to other GSWs  2 . The five ports are the minimum number of ports required to secure two redundant paths physically separated between each GSW  2  and other GSWs  2 . Each GSW  2  is connected to its adjacent GSWs through inner physical links ( 3 - 2 ,  3 - 4 ,  3 - 6 , and  3 - 8 ) and outer physical links ( 3 - 1 ,  3 - 3 ,  3 - 5 , and  3 - 7 ) and both GSW  2 - 0  and GSW  2 - 2  and both GSW  2 - 1  and GSW  2 - 3  positioned diagonally are connected through the diagonal physical links ( 3 - 9  and  3 - 10 ).  
         [0046]    Next, a description will be made concretely for how to set a path for the connection between adjacent GSWs  2  with reference to FIG. 5. For the connection from GSW  2 - 0  to GSW  2 - 1 , a path  4 - 01 A is set in the physical link  3 - 1  and a path  4 - 01 B is set in the physical link  3 - 2 . For the connection from GSW  2 - 1  to GSW  2 - 0 , a path  4 - 10 A is set in the physical link  3 - 1  and a path  4 - 10 B is set in the physical link  3 - 2 . Because physically separated two paths (redundant paths) are used such way, a path, when its operation is disabled due to a failure, is switched over to another, thereby avoiding service interruptions. Usually, the two paths are used in the following two ways. (1) Only one of the two paths is used in normal operations and it is switched over to another when a trouble occurs in the path. (2) Both of the paths are used to achieve the load balance and when the operation of either of the paths is disabled due to a trouble, it is closed to stop the load balance and only the normal path is used. Although such the two paths are also set between adjacent GSWs just like between GSW  2 - 0  and GSW  2 - 1  described above, the description will be omitted here.  
         [0047]    Next, a description will be made for how to set a path for the connection between diagonally positioned GSWs with reference to FIG. 6. For the connection from GSW  2 - 0  to GSW  2 - 2 , a path  4 - 02 A is set in the inner physical links  3 - 8  and  3 - 6  and a path  4 - 02 B is set in the diagonal physical link  3 - 9 . For the connection from GSW  2 - 2  to GSW  2 - 0 , a path  4 - 20 A is set in the inner physical links  3 - 2  and  3 - 4  and a path  4 - 20 B is set in the diagonal physical link  3 - 9 . For the connection from GSW  2 - 1  to GSW  2 - 3 , a path  4 - 13 A is set in the inner physical links  3 - 4  and  3 - 6  and a path  4 - 13 B is set in the diagonal physical link  3 - 10 . And, for the connection from GSW  2 - 3  to GSW  2 - 1 , a path  4 - 31 A is set in the inner physical links  3 - 2  and  3 - 8  and a path  4 - 31 B is set in the diagonal physical link  3 - 10 . For the connection between diagonal GSWs  2 , one of the redundant two paths takes a 1-hop connection while the other path takes a 2-hop connection through another GSW  2 .  
         [0048]    [0048]FIG. 7 shows another example of the connection between diagonal GSWs  2 . The path-topology is the same in both FIGS. 6 and 7. In FIGS. 6 and 7, the circumferential direction of the 2-hop path is set oppositely from each other. Such the path setting is achieved by setting a relationship between a desired output destination GSW  2  and its corresponding output port in an output destination table (to be described later) formed in the subject GSW  2 .  
         [0049]    This completes the description for how to set a path when four GSWs  2  are used in a disk array controller (maximum configuration). Next, how to increase the number of GSWs  2  in a small scale configuration in a scalable manner will be described. Preferably, the current services should not be interrupted at any scale-up/scale-down operation. And, it should also be avoided to change the reliability during the scale-up/down operation.  
         [0050]    When the number of DKCs  1  are four or less, only one GSW  2  (GSW  2 - 0  in this example) is installed. When the number of DKCs  1  is 5 and over to 9 (excluded), GSW  2 - 1  is connected to the GSW  2 - 0  additionally. FIG. 8 shows a configuration of the disk array controller in which two GSWs (GSW  2 - 0  and GSW  2 - 1 ) are used. In this connection, a bi-directional path is set in the inner physical link  3 - 1  and in the outer physical link  3 - 2  respectively, thereby redundant paths can be set to mutual addresses. In other words, paths  4 - 01 A and  4 - 10 A are set in the outer physical link  3 - 1  and paths  4 - 01 B and  4 - 10 B are set in the inner physical link  3 - 2 , respectively.  
         [0051]    When the number of DKCs  1  increases to 9 and over to 13 (excluded), GSW  2 - 3  is connected to GSW  2 - 0  and GSW  2 - 1  additionally. In this connection, for the connection between adjacent GSW  2 - 1  and GSW  2 - 2 , a bi-directional path is set in each of the inner physical link  3 - 3  and the outer physical link  3 - 4 , thereby redundant paths are set to the mutual addresses.  
         [0052]    [0052]FIG. 9 shows how a path is set for the connection between diagonal GSW  2 - 0  and GSW  2 - 2 . In other words, for the connection from GSW  2 - 0  to GSW  2 - 2 , a path  4 - 02 C is set in each of the outer physical links  3 - 1  and  3 - 3  and another path  4 - 02 B is set in the diagonal physical link  3 - 9 . And, for the connection from GSW  2 - 2  to GSW  2 - 0 , a path  4 - 20 A is set in each of the inner physical links  3 - 2  and  3 - 4  and another path  4 - 20 B is set in the diagonal physical link  3 - 9 , thereby redundant paths are set to the mutual addresses. The paths  4 - 02 C and  4 - 20 B may be set oppositely to each other. In any ways, a redundant path is secured for every destination, so that the topology is able to continue service under single point of failure.  
         [0053]    When the number of DKCs  1  increases to 13 and over, GSW  2 - 3  is further connected to GSW  2 - 0 , GSW  2 - 1 , and GWS 2 - 3 . In this case, the controller takes the maximum configuration and the paths may be set as shown in FIGS. 5 and 6. When only the connection between diagonal GSWs is required (the configuration shown in FIG. 9 is changed to that shown in FIG. 6), the path  4 - 02 C must be rerouted as a matter of course. Concretely, when GSW  2 - 3  is added, not only a path is set so as to be connected to GSW  2 - 3 , but also the path  4 - 02 C (connected to GSW  2 - 2  from GSW  2 - 0  through GSW  2 - 1 ) shown in FIG. 9 is changed to the path  4 - 02 A (connected to GSW  2 - 2  from GSW  2 - 0  through GSW  2 - 2 ) shown in FIG. 7. To enable this path rerouting, the output destination table retained in the GSW  2  should be configured beforehand so as to be changeable.  
         [0054]    [0054]FIG. 19 shows an internal block diagram of the GSW  2 . The GSW  2  is configured by GSW input ports  23  used to input data from the DKC  1  or another GSW  2 ; GSW output ports  24  used to output data to the DKC  1  or another GSW  2 ; input buffers  21  used to temporarily store data inputted from the GSW input ports  23  to make error checks; selectors  22  used to select the GSW input port  23  or GSW output port  24 ; and an output destination table  20 . The output destination table  20  can be referred to with respect to every input data.  
         [0055]    The output destination table  20  stores output target GSWs  2 , output destination port numbers corresponding to those GSWs, and VALID flags, each used for denoting whether the subject set port number is valid or invalid.  
         [0056]    Hereunder, how to change the set items in the output destination table  20  will be described with reference to FIG. 10. The configuration of the output destination table  20  of the GSW  2 - 0  is shown at the left side ( 20 - 1 ) in FIG. 10. In FIG. 10, it is premised that GSWs are disposed as shown in FIG. 9. For the connection to the adjacent GSW  2 - 1 , the port  0  (link  3 - 1 ) and the port  1  (link  3 - 2 ) in FIG. 4 are used. For the connection to the diagonal GSW  2 - 2 , the port  0  (link  3 - 1 ) and the port  2  (link  3 - 9 ) in FIG. 4 are used. The VALID flag for each of the ports is set at “1” (valid).  
         [0057]    The output destination table  20  stores rerouting paths to reach GSW  2 - 2  through GSW  2 - 3 . At this time, however, because the adjacent GSW  2 - 3  is not used, all the VALID flags for the adjacent connections to GSW  2 - 3  are set at “0” (invalid). On the other hand, the configuration  20 - 1  of the output destination table  20 , when GSW  2 - 3  is added (as shown in FIGS. 5 and 6), is shown at the right side in FIG. 10. At this time, the VALID flags of the ports  3  (link  3 - 8 ) and  4  (link  3 - 7 ) are set at “1” (valid) respectively in correspondence with the connection of the newly added adjacent GSW  2 - 3 . The VALID flag of the port  0  (link  3 - 1 ) used for the connection to the diagonal GSW  2 - 2  is updated so that the port  3  (link  3 - 8 ) is used for the diagonal connection.  
         [0058]    [0058]FIG. 11 shows how GSWs are added and the path setting is changed in accordance with the updating of the output destination table  20  shown in FIG. 10 (only the path set for the connection between GSW  2 - 0  and GSW  2 - 2  that are diagonally positioned to each other is shown here). More concretely, the port  3  (link  3 - 8 ) setting is validated to prevent service interruptions, then the current port  0  (link  3 - 1 ) setting is invalidated a certain time later. In this way, service interruption can be avoided by updating the output destination table  20  when any GSW  2  is added to the controller. To decrease the number of GSWs  2 , the above procedure is reversed; service interruptions are also avoided in the same way. The output destination table  20  can be set/updated through a management terminal (not shown) connected to (or part of) a disk array controller. It is also possible for each GSW  2  to set its output destination table  20  autonomously when its connection is recognized.  
         [0059]    Next, a description will be made for a configuration of the disk array controller preferred for increasing the number of GSWs  2  in another embodiment of the present invention.  
         [0060]    In FIG. 12, four DKCs  1  are housed in each DKC rack  30 . Concretely, DKCs  1 - 0  to  1 - 3  are housed in the DKC rack  30 -A, DKCs  1 - 4  to  1 - 7  are housed in the DKC rack  30 -B, DKCs  1 - 8  to  1 - 11  are housed in the DKC rack  30 -C, and DKCs  1 - 12  to  1 - 15  are housed in the DKC rack  30 -D. The four DKC racks  30  are connected to the GSWs  2 - 0  to  2 - 3  housed in the GSW rack  50  through the cables  80 - 1  to  80 - 4 . A rack back-plane or cable is used for the connection between the ports of the GSWs  2  in the GSW rack  50 . Practically, the GSWs  2  should not be housed in the GSW rack  50 , since their dispositions therein are to be limited; because a plurality of disk units (hereinafter, to be described as DKUs)  60  are connected to each DKC rack  30 , when a plurality of DKC racks  30  are used, they come to be disposed as shown in FIG. 15. Each DKU  60  connected to a DKC rack  30  has many magnetic disks controlled by each DKC  1 . When those DKUs  50  are disposed such way, the connection between a DKC rack  30  and a GSW rack  50  comes to require a longer cable connection than the distance between the two DKC racks  30  in some portions wherever the GSW rack  50  is placed. To avoid this problem, the GSWs  2  are disposed at random in the DKC rack as shown in FIG. 13, thereby the GSW rack  50  can be disposed in any place and the cable length between the DKC racks becomes shorter.  
         [0061]    In FIG. 13, the DKC racks  30 A and  30 D are disposed as shown in FIG. 12 and GSW  2 - 0  and GSW  2 - 1  are loaded in GSW slots  31  in the DKC rack  30 B while GSW  2 - 2  and GSW  2 - 3  are housed in the DKC rack  30 C. The DKC racks  30 B and  30 C are disposed adjacently. And, in another rack, each GSW  2  is connected to another through one of the cables  80 - 5  to  80 - 10 . Then, the DKC rack  30 A is disposed outside the DKC rack  30 B and the DKC rack  30 D is disposed outside the DKC rack  30 C so as to make their cables shorter than the distance between two DKC racks.  
         [0062]    [0062]FIG. 14 shows an example in which every DKC rack is configured so as to be enabled to house GSWs  2 . In this connection, the GSW  2  slots  31  for the DKC racks  30 A and  30 D become empty. An advantage of this configuration that, the configuration just requires development of only one type DKC racks  30 .  
         [0063]    [0063]FIG. 16 shows a relationship between the number of GSWs  1  and the number of DKC racks  30  required as a result of the addition of the DKCs  1 . When the number of DKC racks  30  increases to two, GSWs  2 - 0  and  2 - 1  are housed in the DKC rack  30 B. When the number of DKC racks  30  increases to three, the GSW  2 - 2  is housed in the DKC rack  30 C. In addition, when the number of DKC racks  30  increases to four, the GSW  2 - 3  is housed in the DKC rack  30 C. Such way, GSWs  2  are housed in accordance with the number of DKCs  1  and the number of DKC racks. This is why the present invention can provide a cost-scalable disk array controller.  
         [0064]    Next, a description will be made for an example of how to make the connection between GSWs  2  housed in the DKC racks  30 B and  30 D with reference to FIG. 17. At first, the GSW  2 - 0  and the GSW  2 - 1  housed in the DKC rack  30 B are connected to the GSW  2 - 2  and the GSW  2 - 3  housed in the DKC rack  30 C through cables led out from the end face of the rack  30 C. Then, the GSW  2 - 0  is connected to the DKC 1  housed in the DKC rack  30 A through a cable led out from the end face of the rack  30 A while the GSW  2 - 2  is connected to the DKC 1  housed in the DKC rack  30 D through a cable led out from the end face of a board facing the rack  30 D. In FIG. 17, while cables are used for the connection between the GSWs in the same DKC rack, that is, between GSW  2 - 0  and GSW  2 - 1 , as well as between GSW  2 - 2  and GSW  2 - 3 , a back-plane may also be used for the connection. And, while cables are used for the connection between GSW and DKC in the same DKC rack, a back-plane may also be used for the connection. Such the connection shortens the necessary cable length between DKC racks  30 .  
         [0065]    [0065]FIG. 18 shows another example of the connection between GSWs in a DKC rack and between GSW and DKC in the GSW rack. The connection between GSWs in the same DKC rack, as well as the connection between such a GSW and a DKC in the GSW rack may be done with use of a middle plane as shown in FIG. 18. The GSW 2  card may be a half-size one, so that GSWs  2  are loaded in the slot from both sides. In such a case, the GSW 2  board cost, as well as the number of slots can be reduced compared to the GSW 2  card shown in FIG. 17. The DKC rack  30  thus becomes more compact in size.