Abstract:
Provided is a storage subsystem configuration management method for use in a computer system, comprising: obtaining storage configuration information and hardware resource use information; determining, based on the obtained hardware resource use information, a configuration of a storage subsystem so that a load is not concentrated on a specific hardware resource; transmitting a configuration change instruction to make a change to the determined configuration to the storage subsystem; and making a configuration change based on the configuration change instruction received from the management computer through the first interface.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese patent application 2006-356387 filed on Dec. 28, 2006, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND 
   This invention relates to a technique of managing performance of a storage system including virtualized storage extents, and more particularly, to a technique for equalizing loads on hardware resources. 
   A storage area network (SAN) is known as a technique of connecting one or more external storage devices and one or more computers to each other. The storage area network is in particular effective in a case where a plurality of computers share one large-scale storage device. In a storage system including the storage area network, it is possible to add or delete a storage device or a computer with ease, which means that the storage system excels in extensibility. 
   In general, as an external storage device connected to the SAN, a disk array device is often used. The disk array device is a device equipped with many magnetic storage devices represented by hard disk drives. 
   The disk array device uses a redundant array of independent disks (RAID) technique to manage several magnetic storage devices as one RAID group. The RAID group forms one or more logical storage extents. The computers connected to the SAN carry out processing for inputting/outputting data into/from the storage extents. When data is recorded in the storage extents, the disk array device records redundant data in the magnetic storage devices configuring the RAID group. By recording the redundant data in this manner, even when one of the magnetic storage devices has failed, it becomes possible to restore data. 
   To the SAN, it is possible to connect different kinds of storage subsystems. Therefore, it is required for an operation manager to exercise management while giving consideration to device characteristics, which increases a burden on him/her. 
   A virtual storage technique provides a host computer with a storage capacity equipped for a separately existing storage subsystem by virtualizing the storage capacity so that it is possible to deal with the storage capacity as a resource stored in a virtualized storage subsystem. Therefore, it becomes possible to collectively manage resources on different kinds of storage subsystems having different characteristics, which makes it possible to alleviate a management burden (refer to JP 2005-011277 A). 
   Further, in conventional SAN operation, when a storage volume is mounted to a file system operated by a host computer, it is required to statically allocate a corresponding physical disk capacity in advance. In addition, a vast number of steps including system halt, are required for capacity addition and volume creation and deletion. 
   A thin provisioning technique provides a host computer with a storage volume in virtual units instead of allocating a physical disk capacity in advance. In addition, when writing from the host has occurred, an extent is dynamically allocated from a storage resource pool. Therefore, it is only necessary that the storage resource pool defined in advance is configured using a capacity that is small as compared with a virtual volume, which improves capacity use efficiency. Still in addition, in pool capacity addition, no influence is exerted on the host computer, so operation is simplified and it becomes possible to alleviate management burden (refer to JP 2003-015915 A). 
   In this specification, a storage extent provided to a host computer with the virtual storage technique is referred to as the “external storage extent” and a storage extent provided to the host computer with the thin provisioning technique is referred to as the “virtual storage extent”, thereby distinguishing these storage extents from each other. 
   SUMMARY 
   In a storage system into which the thin provisioning technique has been introduced, an attempt is made to equally distribute a load among storage volumes registered in a storage resource pool. When there is an input into a virtual volume, the storage system sequentially allocates extents from the storage volumes dynamically configuring the pool with a virtual volume function. By distributing the allocated extents among a plurality of hardware resources in this manner, an input/output load is distributed. 
   Depending on a manner of allocation of the storage volumes to the resource pool, however, there is a fear that the load will be concentrated on certain hardware resources. For instance, when the storage volumes configuring the resource pool belong to the same RAID group, even when different storage volumes are allocated, the same hardware, that is, the same RAID group is allocated, so the concentration of the load occurs. 
   In a like manner, in a configuration to which the virtual storage technique has been applied, there is a fear that even when an attempt is made to equally distribute a load among storage volumes configuring a resource pool, the load will be concentrated on a communication interface on a different device side. 
   This invention has been made in the light of the problems described above and has an object to suppress degradation of performance due to concentration of a load on a specific hardware resource in a storage subsystem to which a thin provisioning technique or a virtual storage technique has been applied. 
   A representative aspect of this invention is as follows. That is, there is provided a storage subsystem configuration management method for use in a computer system, the computer system having a storage subsystem, a host computer connected to the storage subsystem through a network, and a management computer that is capable of accessing the storage subsystem and the host computer, in which: the storage subsystem includes a first interface that is connected to the network, a first processor that is connected to the first interface, a first memory that is connected to the first processor, and a storage device that stores data read and written by the host computer; the host computer includes a second interface that is connected to the network, a second processor that is connected to the second interface, and a second memory that is connected to the second processor; the management computer includes a third interface that is connected to the network, a third processor that is connected to the third interface, and a third memory that is connected to the third processor; the storage subsystem includes a virtual storage resource pool configured by at least one virtual storage extent; and the virtual storage extent is allocated with at least one of a storage resource of the storage device and a storage resource of an external storage device provided by an external storage subsystem connected to the storage subsystem equipped with the storage device, and is provided to the host computer as a storage extent from and into which data is read and written, the storage subsystem configuration management method comprising: obtaining, by the management computer, storage configuration information, which includes at least one of information about a configuration of a RAID group configured by the storage device and information about a configuration of an interface connected to the external storage device, from the storage subsystem through the third interface; obtaining, by the management computer, hardware resource use information, which includes at least one of information about use of the storage resource provided to the host computer through the virtual storage extent and information about use of the interface connected to the external storage device, based on the obtained storage configuration information; determining, by the management computer, based on the obtained hardware resource use information, a configuration of the storage subsystem so that a load is not concentrated on a specific hardware resource; transmitting, by the management computer, a configuration change instruction to make a change to the determined configuration to the storage subsystem; and making, by the storage subsystem, a configuration change based on the configuration change instruction received from the management computer through the first interface. 
   According to an embodiment of this invention, it becomes possible to suppress degradation of performance of a storage subsystem due to concentration of a load on a specific hardware resource. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
       FIG. 1  is a block diagram showing a configuration of a storage area network according to an embodiment of this invention; 
       FIG. 2  is a block diagram showing a configuration of the storage subsystem according to the embodiment of this invention; 
       FIG. 3  is a block diagram showing a configuration of the host computer according to the embodiment of this invention; 
       FIG. 4  is a block diagram showing a configuration of the management computer according to the embodiment of this invention; 
       FIG. 5  is a diagram showing a logical configuration of the storage system according to the embodiment of this invention; 
       FIG. 6  is a configuration diagram showing an example of a configuration of the RAID group configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 7  is a configuration diagram showing an example of a configuration of the storage extent configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 8A  is a configuration diagram showing an example of a configuration information stored in the storage subsystem in the case where the storage units identified by the storage unit identification information are virtual storage units according to the embodiment of this invention; 
       FIG. 8B  is a configuration diagram showing an example of a configuration information stored in the storage subsystem in the case where the storage units identified by the storage unit identification information are provided in the storage subsystem according to the embodiment of this invention; 
       FIG. 9  is a configuration diagram showing an example of a configuration of the external storage map information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 10  is a configuration diagram showing an example of a configuration of the virtual storage resource pool configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 11  is a configuration diagram showing an example of a configuration of the virtual storage unit configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 12  is a configuration diagram showing an example of a configuration of the data I/O interface configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 13  is a configuration diagram showing an example of a configuration of the management interface configuration information stored in the storage subsystem according to the embodiment of this invention; 
       FIG. 14  is a configuration diagram showing an example of a configuration of the RAID group mapping information stored in the management computer according to the embodiment of this invention; 
       FIG. 15  shows an example of a configuration of the external storage communication path mapping information stored in the management computer according to the embodiment of this invention; 
       FIG. 16  is a flowchart showing a procedure of processing for updating the storage configuration information held in the management computer according to the embodiment of this invention; 
       FIG. 17  is a flowchart showing a procedure of processing for updating the RAID group mapping information according to the embodiment of this invention; 
       FIG. 18  is a flowchart showing a procedure of processing for updating the external storage communication path mapping information according to the embodiment of this invention; 
       FIG. 19  is flowchart showing a former half portion of processing for additionally registering a new storage extent in a virtual storage resource pool according to the embodiment of this invention; 
       FIG. 20  is flowchart showing a latter half portion of processing for additionally registering a new storage extent in a virtual storage resource pool according to the embodiment of this invention; 
       FIG. 21  is a configuration diagram showing a storage extent selection screen that is an example of the virtual storage extent addition operation receiving interface provided by the virtual storage configuration change interface program according to the embodiment of this invention; 
       FIG. 22  is a diagram showing a logical configuration in which a load is distributed through the virtual storage extent selection and addition processing according to the embodiment of this invention; 
       FIG. 23  is a flowchart showing a procedure of processing for additionally registering a new storage extent in a virtual storage resource pool according to the first modification of the embodiment of this invention; 
       FIG. 24  is a flowchart showing a procedure of processing for searching for a storage extent so that loads on the data I/O interfaces connected with the external storage subsystem are distributed according to the first modification of the embodiment of this invention; 
       FIG. 25  is a diagram showing a logical configuration in which a load is distributed through the virtual storage extent selection and addition processing according to the first modification of the embodiment of this invention; 
       FIG. 26  is a flowchart showing a procedure of processing for equalizing the registration numbers when there are variations in virtual storage extent registration numbers of storage extents configuring the RAID groups, according to the second modification of the embodiment of this invention; 
       FIG. 27  is a diagram showing a logical configuration in which a configuration under a state where a load is concentrated as shown in  FIG. 5  is reconstructed so that the load is distributed, in accordance with the processing procedure according to the second modification of the embodiment of this invention; 
       FIG. 28  is flowchart showing a former half portion of a processing for equalizing the numbers of external connection paths connected to the data I/O interfaces on the external storage subsystem side according to the third modification of the embodiment of this invention; 
       FIG. 29  is flowchart showing a latter half portion of processing for equalizing the numbers of external connection paths connected to the data I/O interfaces on the external storage subsystem side according to the third modification of the embodiment of this invention; and 
       FIG. 30  a diagram showing a logical configuration in which a configuration under a state, in which a load is concentrated as shown in  FIG. 5 , is reconstructed so that the load is distributed in accordance with the processing procedure according to the third modification of the embodiment of this invention; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, an embodiment of this invention will be described with reference to the accompanying drawings. 
     FIG. 1  shows a configuration of a storage area network according to the embodiment of this invention. The storage area network is configured by a data I/O network and management networks  600 . 
   The data I/O network includes storage subsystems  100 , host computers  300 , and network switches  400 . The host computers  300  and the storage subsystems  100  are connected to each other through the network switches  400 , thereby enabling mutual data input/output. The data I/O network is illustrated using thick lines in  FIG. 1 . The data I/O network is a network based on a conventional technique such as Fibre Channel or Ethernet. 
   Each management network  600  is a network based on a conventional technique such as Fibre Channel or Ethernet. The storage subsystems  100 , the host computers  300 , and the network switches  400  are connected to a management computer  500  through the management network  600 . 
   Each host computer  300  runs an application, such as a database or a file server, and carries out input/output of data into/from storage extents. Each storage subsystem  100  is equipped with a storage device, such as a magnetic disk drive or a semiconductor storage device, and provides data storage extents. Each network switch  400  is a device that connects the host computer  300  and the storage subsystem  100  to each other and is a Fibre Channel switch, for instance. 
   It should be noted here that in the embodiment of this invention, a form in which the management network  600  and the data I/O network are independent of each other is used, but a form in which a single network having both of a function of the management network  600  and a function of the data I/O network is provided may be used instead. 
     FIG. 2  shows a configuration of the storage subsystem  100  according to the embodiment of this invention. The storage subsystem  100  includes data I/O interfaces  140 , management interfaces  150 , a storage controller  190 , a program memory  1000 , data I/O cache memories  160 , and magnetic disk drives  120 . The data I/O interfaces  140 , the management interfaces  150 , the program memory  1000 , the data I/O cache memories  160 , and the magnetic disk drives  120  are connected to one another through the storage controller  190 . 
   The data I/O interfaces  140  connect to the network switch  400  through the data I/O network. The management interfaces  150  connect to the management computer  500  through the management network  600 . It should be noted that the number of the data I/O interfaces  140  and the number of the management interfaces  150  are arbitrary. Also, it is not required to configure the data I/O interfaces  140  independently of the management interfaces  150 , and management information may be inputted/outputted through the data I/O interfaces  140  and shared with the management interfaces  150 . 
   The storage controller  190  is equipped with a processor that controls the storage subsystem  100 . The data I/O cache memories  160  are temporary storage extents for accelerating input/output with respect to storage extents by the host computer  300 . The data I/O cache memories  160  are generally configured using volatile memories but may be configured using nonvolatile memories or magnetic disk drives instead. It should be noted that there is no limitation on the number and capacities of the data I/O cache memories  160 . The magnetic disk drives  120  store data that the host computer  300  reads/writes. 
   The program memory  1000  stores programs and control information that are necessary for processing carried out in the storage subsystem  100 . The program memory  1000  is configured using a magnetic disk drive or a volatile semiconductor memory. 
   The program memory  1000  stores RAID group configuration information  1001 , storage extent configuration information  1002 , logical unit configuration information  1003 , external storage map information  1004 , virtual storage resource pool configuration information  1005 , virtual storage unit configuration information  1006 , data I/O interface configuration information  1007 , management interface configuration information  1008 , and a virtual storage unit management program  1009 . 
   The RAID group configuration information  1001  stores correspondences between RAID groups and magnetic disk drives configuring the RAID groups. This will be described later with reference to  FIG. 6 . 
   The storage extent configuration information  1002  stores correspondences between logical storage extents and the RAID groups. This will be described later with reference to  FIG. 7 . 
   The logical unit configuration information  1003  stores correspondences among communication interfaces, storage units that are units of storage resources accessible from the host computer  300 , and the logical storage extents. This will be described later with reference to  FIG. 8 . 
   The external storage map information  1004  stores correspondences among external storage extents, the communication interfaces, and the storage units. This will be described later with reference to  FIG. 9 . Each external storage extent is a storage extent of an external storage subsystem that is provided to the host computer  300  as a storage extent of the storage subsystem  100 . 
   The virtual storage resource pool configuration information  1005  stores correspondences among virtual storage resource pools, virtual storage extents included in the virtual storage resource pools, and the storage extents. This will be described later with reference to  FIG. 10 . Each virtual storage extent is a virtual storage extent provided to the host computer  300  with a thin provisioning technique. Each virtual storage resource pool is configured by a plurality of virtual storage extents. 
   The virtual storage unit configuration information  1006  stores correspondences between virtual storage units and the virtual storage resource pools. This will be described later with reference to  FIG. 11 . 
   The data I/O interface configuration information  1007  stores information about the data I/O interfaces  140  with which the storage subsystem  100  is provided. This will be described later with reference to  FIG. 12 . 
   The management interface configuration information  1008  stores information about the management interfaces  150  with which the storage subsystem  100  is provided. This will be described later with reference to  FIG. 13 . 
   The virtual storage unit management program  1009  is a program that manages a configuration of a storage system configured with the virtual storage technique or the thin provisioning technique. 
     FIG. 3  shows a configuration of the host computer  300  according to the embodiment of this invention. The host computer  300  includes data I/O interfaces  340 , management interfaces  350 , an input interface  370 , an output interface  375 , a processor unit  380 , a magnetic disk drive  320 , and data I/O cache memories  360 . 
   The data I/O interfaces  340 , the management interfaces  350 , the input interface  370 , the output interface  375 , the processor unit  380 , the magnetic disk drive  320 , and the data I/O cache memories  360  are connected to one another through a communication bus  390 . The host computer  300  has a hardware configuration achievable with a general-purpose computer (Personal Computer). 
   The data I/O interfaces  340  connect to the network switch  400  through the data I/O network and inputs/outputs data. The management interfaces  350  connect to the management computer  500  through the management network  600  and inputs/outputs management information. It should be noted that the number of the data I/O interfaces  340  and the number of the management interfaces  350  are arbitrary. Also, it is not required to configure the data I/O interfaces  340  independently of the management interfaces  350  and management information may be inputted/outputted through the data I/O interfaces  340  and shared with the management interfaces  350 . 
   The input interface  370  connects to a device, such as a keyboard and a mouse, which is used by an operator to input information. The output interface  375  connects to a device, such as a general-purpose display, which outputs information for the operator. The processor unit  380  carries out various kinds of computations and corresponds to a CPU or a processor. The magnetic disk drive  320  stores software such as an operating system and applications. 
   The data I/O cache memories  360  are configured using volatile memories or the like and accelerate data input/output into/from the magnetic disk drive  320 . The data I/O cache memories  360  are generally implemented using volatile memories but may be configured using nonvolatile memories or magnetic disk drives. It should be noted that there is no limitation on the number and capacities of the data I/O cache memories  360 . 
     FIG. 4  shows a configuration of the management computer  500  according to the embodiment of this invention. The management computer  500  includes a data I/O interface  540 , a management interface  550 , an input interface  570 , an output interface  575 , a processor unit  580 , a magnetic disk drive  520 , a program memory  5000 , and a data I/O cache memory  560 . 
   The data I/O interface  540 , the management interface  550 , the input interface  570 , the output interface  575 , the processor unit  580 , the magnetic disk drive  520 , the program memory  5000 , and the data I/O cache memory  560  are connected to one another through a communication bus  590 . The management computer  500  has a hardware configuration achievable with a general-purpose computer (PC) and the function of each portion is the same as that of the host computer shown in  FIG. 3 . 
   The program memory  5000  stores storage configuration information  5010 , RAID group mapping information  5011 , external storage communication path mapping information  5012 , a configuration information update program  5013 , a virtual storage configuration change program  5014 , and a virtual storage configuration change interface program  5015 . 
   The storage configuration information  5010  is a collection of control information that the storage subsystem  100  holds separately. Therefore, in the storage configuration information  5010 , a plurality of pieces of control information, whose number is equal to the number of the storage subsystems  100  that are management targets, are recorded. Also, the RAID group configuration information  1001  and the like included in the storage configuration information  5010  are the same as the information stored in each storage subsystem  100 . 
   The RAID group mapping information  5011  stores the numbers of virtual storage extents registered in RAID groups. This will be described later with reference to  FIG. 14 . 
   The external storage communication path mapping information  5012  stores the numbers of connection paths of external storage devices connected to the data I/O interfaces  140  of the storage subsystem  100 . This will be described later with reference to  FIG. 15 . 
   The configuration information update program  5013  collects the control information from each storage subsystem  100  that is a management target and updates the storage configuration information  5010  at predetermined timings. The virtual storage configuration change program  5014  instructs the storage subsystem  100  to change the configuration of the virtual storage subsystem. The virtual storage configuration change interface program  5015  provides a user interface for instructing to change the configuration of the virtual storage subsystem. 
   Here, a storage system according to the embodiment of this invention, to which the virtual storage technique and the thin provisioning technique have been applied, will be described. 
     FIG. 5  shows a logical configuration of the storage system according to the embodiment of this invention. The storage system shown in  FIG. 5  includes a storage subsystem  100 A that is directly accessed by the host computer  300  and a connected external storage subsystem  100 B that provides the storage subsystem  100 A with external storage extents. 
   The storage subsystem  100 A receives access from the host computer  300  through data I/O interfaces  140 A. Also, the storage subsystem  100 A connects with data I/O interfaces  140 C of the external storage subsystem  100 B through data I/O interfaces  140 B. 
   The storage subsystem  100 A stores RAID groups  11  (RG- 01 , RG- 02 , and RG- 03 ). In the RAID groups  11 , logical storage extents  12  are defined. For instance, in the RAID group “RG- 02 ”, logical storage extents “LD- 02 ” and “LD- 03 ” are defined. 
   Meanwhile, logical storage extents  12  (LD- 11  to LD- 15 ) provided by the external storage subsystem  100 B are associated with storage units  17  (LU- 11  and the like) of the data I/O interfaces  140  “PT- 11 ” and “PT- 12 ”. The storage units  17  are associated with external storage extents  13  of the storage subsystem  100 A. 
   Each virtual storage resource pool  14  includes virtual storage extents  15  (VD- 01  to VD- 03  or VD- 11  to VD- 14 ). Each virtual storage extent  15  is associated with a logical storage extent  12  or an external storage extent  13 . Also, a virtual storage unit  16  (VU- 01  or VU- 02 ) is defined for each virtual storage resource pool  14 . The virtual storage unit  16  is provided to the host computer  300  as storage extents through a corresponding data I/O interface  140 A. 
   In the configuration shown in  FIG. 5 , the virtual storage extents  15  “VD- 02 ” and “VD- 03 ” configuring the virtual storage resource pool “PL- 01 ” belong to the same RAID group but are registered as different virtual storage extents  15 . In this case, the virtual storage unit management program  1009  equally allocates the virtual storage extents “VD- 01 ”, “VD- 02 ”, and “VD- 03 ”. Therefore, actually allocated hardware resources are concentrated on the RAID group “RG- 02 ”. In this case, there arises a problem that a load is not distributed as expected. 
   In addition, when the storage controller  190  of the storage subsystem  100  distributes a load by equally allocating the virtual storage extents “VD- 11 ”, “VD- 12 ”, “VD- 13 ”, and “VD- 14 ” configuring the virtual storage resource pool “PL- 02 ” through execution of the virtual storage unit management program  1009 , the data I/O interface  140  “PT- 11 ” on an external storage subsystem  100 B side has only one path that is a path to “LD- 12 ” but the data I/O interface  140  “PT- 12 ” has three paths that are paths to “LD- 13 ”, “LD- 14 ”, and “LD- 15 ”, so the load is concentrated on “PT- 12 ”. Therefore, there arises a problem that the load is not distributed as expected. 
   Next, control information corresponding to the storage system shown in  FIG. 5  and stored in the storage subsystem  100  and the management computer  500  will be described. 
     FIG. 6  shows an example of a configuration of the RAID group configuration information  1001  stored in the storage subsystem  100  according to the embodiment of this invention. As described above, the RAID group configuration information  1001  stores correspondences between the RAID groups and the magnetic disk drives configuring the RAID groups. 
   The RAID group configuration information  1001  includes RAID group identification information  10011  and magnetic disk drive identification information  10012 . 
   The RAID group identification information  10011  is identifiers that uniquely identify the RAID groups  11  configured in the storage subsystem  100 . The magnetic disk drive identification information  10012  is identifiers that uniquely identify the magnetic disk drives  120  configuring the RAID groups  11  identified by the RAID group identification information  10011 . For instance, the RAID group “RG- 01 ” is configured by the magnetic disk drives “HD- 01 ”, “HD- 02 ”, “HD- 03 ”, and “HD- 04 ”. 
     FIG. 7  shows an example of a configuration of the storage extent configuration information  1002  stored in the storage subsystem  100  according to the embodiment of this invention. As described above, the storage extent configuration information  1002  stores correspondences between the logical storage extents and the RAID groups. The storage extent configuration information  1002  includes storage extent identification information  10021  and RAID group identification information  10022 . 
   The storage extent identification information  10021  is identifiers that uniquely identify the logical storage extents  12  provided by the storage subsystem  100 . The RAID group identification information  10022  is identifiers that uniquely identify the RAID groups  11  that provide the logical storage extents  12 . 
     FIG. 8A  and  FIG. 8B  each show an example of a configuration of the logical unit configuration information  1003  stored in the storage subsystem  100  according to the embodiment of this invention. As described above, the logical unit configuration information  1003  stores correspondences between the communication interfaces, the storage units that are units of the storage resources accessible from the host computer  300 , and the storage extents. 
   The logical unit configuration information  1003  includes communication interface identification information  10031 , storage unit identification information  10032 , and storage extent identification information  10033 . 
   The communication interface identification information  10031  is identifiers that uniquely identify the data I/O interfaces  140 . The storage unit identification information  10032  is identifiers that uniquely identify the storage units. As described above, the storage units are units of the storage resources accessible from the host computer  300  connected to the storage subsystem  100  and correspond to volumes mounted to a file system that the host computer  300  operates. The storage extent identification information  10033  is identifiers that uniquely identify the logical storage extents  12  provided by the storage subsystem  100 . 
     FIG. 8A  shows the logical unit configuration information  1003 A stored in the storage subsystem  100  according to the embodiment of this invention in the case where the storage units identified by the storage unit identification information  10032  are virtual storage units. In this case, it is impossible to directly identify storage extents corresponding to logical units, so it is not required to record values in the storage extent identification information  10033 . 
     FIG. 8B  shows the logical unit configuration information  1003 B stored in the storage subsystem  100  according to the embodiment of this invention in the case where the storage units identified by the storage unit identification information  10032  are provided in the storage subsystem  100 . 
     FIG. 9  shows an example of a configuration of the external storage map information  1004  stored in the storage subsystem  100  according to the embodiment of this invention. The external storage map information  1004  includes external storage extent identification information  10041 , communication interface identification information  10042 , and storage unit identification information  10043 . 
   The external storage extent identification information  10041  is identifiers that uniquely identify the external storage extents  13 . The communication interface identification information  10042  is identifiers that uniquely identify the data I/O interfaces  140 . The storage unit identification information  10043  is identifiers that uniquely identify the storage units. 
   The external storage extents  13  are virtual storage extents that are not actually equipped for the storage subsystem  100 , as described above. Therefore, requests for input/output into/from the external storage extents  13  are transferred to the storage units stored in the externally connected storage subsystem through the data I/O interfaces  140 . 
   Also, when a data I/O interface  140  and a storage unit are identified, it is possible to identify a storage extent, in which data is stored, based on the logical unit configuration information  1003 B shown in  FIG. 8B . Therefore, the external storage map information  1004  stores correspondences between the identifiers of the external storage extents and the storage extents. 
     FIG. 10  shows an example of a configuration of the virtual storage resource pool configuration information  1005  stored in the storage subsystem  100  according to the embodiment of this invention. 
   The virtual storage resource pool configuration information  1005  includes virtual storage resource pool identification information  10051 , virtual storage extent identification information  10052 , and storage extent identification information  10053 . 
   The virtual storage resource pool identification information  10051  is identifiers that uniquely identify the virtual storage resource pools  14 . In each virtual storage resource pool, a plurality of virtual storage extents are registered. 
   The virtual storage extent identification information  10052  is identifiers that uniquely identify the virtual storage extents  15  included in the virtual storage resource pools  14 . 
   The storage extent identification information  10053  is identifiers that uniquely identify the storage extents corresponding to the virtual storage extents  15  identified by the virtual storage extent identification information  10052 . There are a case where the storage extent identification information  10053  is associated with the logical storage extents  12  provided by the storage subsystem  100  and a case where the storage extent identification information  10053  is associated with the external storage extents  13 . 
     FIG. 11  shows an example of a configuration of the virtual storage unit configuration information  1006  stored in the storage subsystem  100  according to the embodiment of this invention. The virtual storage unit configuration information  1006  defines the virtual storage units  16  included in the virtual storage resource pools  14 . 
   The virtual storage units  16  are virtual storage units operated by the virtual storage unit management program  1009  stored in the storage subsystem  100  and are dynamically allocated with physical storage resources from the storage extents registered in the virtual storage resource pools  14 . 
   The virtual storage unit configuration information  1006  includes virtual storage unit identification information  10061  and virtual storage resource pool identification information  10062 . 
   The virtual storage unit identification information  10061  is identifiers that uniquely identify the virtual storage units  16 . The virtual storage resource pool identification information  10062  is identifiers that uniquely identify the virtual storage resource pools  14 . 
     FIG. 12  shows an example of a configuration of the data I/O interface configuration information  1007  stored in the storage subsystem  100  according to the embodiment of this invention. 
   The data I/O interface configuration information  1007  includes data I/O interface identification information  10071 , communication interface bandwidths  10072 , and storage subsystem identification names  10073 . 
   The storage subsystem identification names  10073  are identification names that uniquely identify the storage subsystems  100 . The data I/O interface identification information  10071  is identifiers that uniquely identify the data I/O interfaces  140 . The communication interface bandwidths  10072  are bandwidths of the data I/O interfaces  140 . 
     FIG. 13  shows an example of a configuration of the management interface configuration information  1008  stored in the storage subsystem  100  according to the embodiment of this invention. 
   The management interface configuration information  1008  includes storage subsystem identification names  10081  and management interface identification information  10082 . 
   The storage subsystem identification names  10081  are identification names that uniquely identify the storage subsystems  100 . The management interface identification information  10082  stores information that identifies the management interfaces  150  provided for the storage subsystems  100 . In the management interface configuration information  1008  shown in  FIG. 13 , IP addresses of the management interfaces  150  are recorded as the management interface identification information  10082 . 
     FIG. 14  shows an example of a configuration of the RAID group mapping information  5011  stored in the management computer  500  according to the embodiment of this invention. 
   The RAID group mapping information  5011  includes RAID group identification information  50111  and virtual storage extent registration numbers  50112 . 
   The RAID group identification information  50111  is identifiers that uniquely identify the RAID groups  11 . The virtual storage extent registration numbers  50112  are the numbers of the virtual storage extents  15 , out of the logical storage extents  12  stored in the RAID groups  11 , which are registered in the virtual storage resource pools  14 . 
     FIG. 15  shows an example of a configuration of the external storage communication path mapping information  5012  stored in the management computer  500  according to the embodiment of this invention. 
   The external storage communication path mapping information  5012  includes communication interface identification information  50121 , communication interface bandwidths  50122 , and external storage extent connection path numbers  50123 . 
   The communication interface identification information  50121  is identifiers that uniquely identify the data I/O interfaces  140 . The communication interface bandwidths  50122  store communication performance of the data I/O interfaces  140 . The external storage extent connection path numbers  50123  store the numbers of the storage units, out of the storage units connected to the data I/O interfaces  140 , which are provided as the external storage extents  13 . 
   The control information stored in the storage subsystem  100  and the management computer  500  are described above. Hereinafter, processing carried out in the storage subsystem  100  and the management computer  500  in the embodiment of this invention will be described. 
     FIG. 16  is a flowchart showing a procedure of processing for updating the storage configuration information held in the management computer  500  according to the embodiment of this invention. The storage configuration information held in the management computer  500  is updated at predetermined timings through execution of the configuration information update program  5013  by the processor unit  580  of the management computer  500 . 
   The processor unit  580  of the management computer  500  issues a storage configuration information transmission request message to the storage subsystem  100  managed by the management computer  500  (S 101 ). 
   Upon receiving the storage configuration information transmission request from the management computer  500 , the storage controller  190  of the storage subsystem  100  transmits every piece of configuration information stored in the program memory  1000  (S 102 ). 
   Upon receiving the storage configuration information from the storage subsystem  100 , the processor unit  580  of the management computer  500  updates the storage configuration information stored in the program memory  5000  (S 103 ). 
   Next, the processor unit  580  of the management computer  500  carries out the following processing for every external storage extent recorded in the external storage extent identification information  10041  of the external storage map information  1004  (S 104 ). 
   The processor unit  580  of the management computer  500  first searches for an external storage subsystem equipped with an external storage extent that is a processing target (S 105 ). More specifically, first, the processor unit  580  of the management computer  500  obtains a data I/O interface  140  recorded for the processing target external storage extent in the communication interface identification information  10042  of the external storage map information  1004 . Next, the processor unit  580  of the management computer  500  obtains a record coinciding with the obtained data I/O interface  140  from the data I/O interface configuration information  1007  based on the obtained data I/O interface  140 . Then, the processor unit  580  of the management computer  500  stores a storage subsystem identification name  10073  in the obtained record in the program memory  5000 . 
   The processor unit  580  of the management computer  500  judges whether configuration information of the storage subsystem  100  found as a result of the processing of Step S 105  has been obtained (S 106 ). When the configuration information is yet to be obtained (result of the judgment in Step S 106  is “No”), the processor unit  580  of the management computer  500  proceeds to processing of Step S 107 . 
   The processor unit  580  of the management computer  500  obtains a corresponding management interface  150  communication address recorded in the management interface identification information  10082  of the management interface configuration information  1008 . Then, the processor unit  580  of the management computer  500  issues a storage configuration information transmission request message to the external storage subsystem having the communication address like in the processing of Step S 101  (S 107 ). 
   Upon receiving the storage configuration information transmission request, the storage controller  190  of the external storage subsystem transmits every piece of configuration information stored in the program memory  1000  (S 108 ). 
   Upon receiving the storage configuration information, the processor unit  580  of the management computer  500  updates the storage configuration information stored in the program memory  5000  (S 109 ). In addition, the processor unit  580  of the management computer  500  stores the identification name of the storage subsystem  100  in the memory and stores that the configuration information of the storage subsystem  100  has been obtained. 
   When the configuration information obtainment and update processing described above has been completed, the processor unit  580  of the management computer  500  recalculates the RAID group mapping information  5011  (S 111 ). In a like manner, the processor unit  580  of the management computer  500  recalculates the external storage communication path mapping information  5012  (S 112 ). It should be noted that the detailed procedure of the processing of Step S 111  will be described below with reference to  FIG. 17  and the detailed procedure of the processing of Step S 112  will be described below with reference to  FIG. 18 . 
     FIG. 17  is a flowchart showing a procedure of processing for updating the RAID group mapping information  5011  according to the embodiment of this invention. 
   The processor unit  580  of the management computer  500  first performs initialization by deleting every piece of information recorded in the RAID group mapping information  5011  (S 201 ). 
   The processor unit  580  of the management computer  500  repeats the following processing for every RAID group  11  stored in the RAID group identification information  10011  of the RAID group configuration information  1001  (S 202 ). 
   The processor unit  580  of the management computer  500  repeats the following processing for the RAID groups  11  recorded in the RAID group identification information  10022  of the storage extent configuration information  1002  (S 203 ). 
   The processor unit  580  of the management computer  500  judges whether a storage extent recorded in the storage extent identification information  10021  for a processing target RAID group  11  is recorded in the storage extent identification information  10053  of the virtual storage resource pool configuration information  1005  (S 204 ). When the storage extent is registered in a virtual storage resource pool  14  (result of the judgment in Step S 204  is “Yes”), the processor unit  580  of the management computer  500  adds “1” to a corresponding virtual storage extent registration number  50112 . 
     FIG. 18  is a flowchart showing a procedure of processing for updating the external storage communication path mapping information  5012  according to the embodiment of this invention. 
   The processor unit  580  of the management computer  500  performs initialization by deleting every piece of information recorded in the external storage communication path mapping information  5012  (S 301 ). 
   The processor unit  580  of the management computer  500  carries out the following processing for every external storage extent recorded in the external storage extent identification information  10041  of the external storage map information  1004  (S 302 ). 
   The processor unit  580  of the management computer  500  adds “1” to an external storage extent connection path number  50123  of a data I/O interface  140  recorded in the communication interface identification information  10042  of the external storage map information  1004  (S 303 ). 
   The processor unit  580  of the management computer  500  further searches the data I/O interface configuration information  1007  and obtains a bandwidth of the corresponding data I/O interface  140 . Then, the processor unit  580  of the management computer  500  stores the obtained performance value as a communication interface bandwidth  50122  (S 304 ). 
     FIG. 19  and  FIG. 20  are flowcharts showing a procedure of processing for additionally registering a new storage extent in a virtual storage resource pool  14  according to the embodiment of this invention. In this processing, the processor unit  580  of the management computer  500  newly adds a virtual storage extent  15  to a virtual storage resource pool  14  and allocates a storage extent provided by a RAID group  11 . In this case, in order to distribute loads of input/output processing on the RAID groups  11 , the processor unit  580  of the management computer  500  selects a RAID group with a less number of storage extents allocated to the virtual storage resource pool  14 . By selecting the storage extent in this manner, it becomes possible to equally distribute access among the respective RAID groups. It should be noted that the storage extent may be selected by giving consideration to performance of a magnetic disk drive that provides the storage extent. 
   The processor unit  580  of the management computer  500  outputs an interface for receiving an operation to add a virtual storage extent to the output interface  575  by executing the virtual storage configuration change interface program  5015  (S 401 ). The interface for receiving the virtual storage extent addition operation will be described later with reference to  FIG. 21 . 
   The processor unit  580  of the management computer  500  receives input of a virtual storage resource pool  14 , to which a virtual storage extent is to be registered, from a user through the interface for receiving the virtual storage extent addition operation (S 402 ). 
   The processor unit  580  of the management computer  500  searches for every RAID group  11 , whose virtual storage extent registration number  50112  coincides with the minimum value, by referring to the RAID group mapping information  5011  through execution of the virtual storage configuration change program  5014  (S 403 ). In the processing of Step S 403 , when the minimum value of the virtual storage extent registration numbers  50112  is “0”, for instance, the processor unit  580  of the management computer  500  obtains every RAID group whose value in the field coincides with “0”. 
   The processor unit  580  of the management computer  500  carries out the following processing for every RAID group  11  found as a result of the processing of Step S 403  (S 404 ). 
   The processor unit  580  of the management computer  500  searches the storage extent configuration information  1002  and obtains every storage extent written in an entry whose RAID group  11  recorded in the RAID group identification information  10022  coincides with a processing target RAID group  11  (S 405 ). 
   The processor unit  580  of the management computer  500  outputs every storage extent obtained through the processing described above to the virtual storage extent addition operation receiving interface by executing the virtual storage configuration change interface program  5015  (S 407 ). 
   The processor unit  580  of the management computer  500  receives input of a storage extent to be added to the virtual storage resource pool  14  selected by the user in the processing of Step S 402  through the interface (S 408 ). At least one of the storage extents listed in the interface is selected as the storage extent to be added. 
   The processor unit  580  of the management computer  500  transmits a virtual storage extent addition request message to a storage subsystem  100  whose storage extent is to be added (S 409 ). The virtual storage extent addition request message includes information about the addition target virtual storage resource pool  14  and the storage extent to be registered in the virtual storage resource pool  14 . 
   Upon receiving the virtual storage extent addition request message, the storage controller  190  of the storage subsystem  100  associates the virtual storage extent identification information  10052  with the selected storage extent. In addition, the storage controller  190  of the storage subsystem  100  updates the virtual storage resource pool configuration information  1005  in order to register the selected storage extent in the selected virtual storage resource pool  14  (S 410 ). For instance, when it is requested to register the storage extent “ED- 05 ” in the virtual storage resource pool “PL- 02 ”, the storage controller  190  of the storage subsystem  100  associates the storage extent “ED- 05 ” with the virtual storage extent “VD- 14 ”, generates a record whose contents correspond to this association, and records the generated record in the virtual storage resource pool configuration information  1005 . 
   When the processing of Step S 410  is completed, the storage controller  190  of the storage subsystem  100  transmits a completion notification message corresponding to the virtual storage extent addition request message (S 411 ). 
   Upon receiving the completion notification message, the processor unit  580  of the management computer  500  updates the virtual storage resource pool configuration information  1005  in a like manner (S 412 ). 
     FIG. 21  shows a storage extent selection screen  5021  that is an example of the virtual storage extent addition operation receiving interface provided by the virtual storage configuration change interface program  5015  according to the embodiment of this invention. 
   The storage extent selection screen  5021  includes an area  5022  in which a virtual storage resource pool  14  is selected, an area  5023  in which a storage extent  12  is selected, and a done button  5024 . 
   The area  5022  includes check boxes for selection from among already-existing virtual storage resource pools  14  and a check box for selection of creation of a new virtual storage resource pool. The area  5023  includes check boxes for selection from among storage extents that can be added. The check boxes are checked by an input from an input device such as a pointing device. In addition, by an input of selection of the done button  5024 , the processor unit  580  performs processing for adding a storage extent selected in the area  5023  to a virtual storage resource pool corresponding to a check box selected in the area  5022 . 
   The area  5022  receives selection from among already-existing virtual storage resource pools  14  and receives an instruction to newly create a virtual storage resource pool  14 . The area  5023  displays a list of storage extents that can be added. More specifically, in the area  5023 , storage extents, with which load equalization is achieved, are displayed as storage extents in a first priority group (Priority  1 ). On the other hand, storage extents that have less effect of the load equalization but can be added are displayed as storage extents in a second priority group (Priority  2 ). For instance, the storage extent found as a result of the processing of Step S 405  is displayed in the first priority group. With the storage extent selection screen  5021  displayed in this manner, it becomes possible for an operator to easily select a storage extent from the candidate list so that a load is not concentrated on a specific RAID group or data I/O interface  140 . 
   The storage extent selection screen  5021  starts to be displayed in the processing of Step S 401  shown in  FIG. 19 . The processor unit  580  of the management computer  500  receives selection of a virtual storage resource pool  14  to which a storage extent  12  is to be added (S 402  in  FIG. 19 ). Before the selection of the virtual storage resource pool  14 , it is impossible to designate priorities of storage extents to be added, so the area  5023  is not displayed or is displayed under a selection impossible state. 
   Upon receiving the selection of the virtual storage resource pool  14 , the processor unit  580  of the management computer  500  extracts storage extents that are addition targets for the selected virtual storage resource pool  14 . Then, in the processing of Step S 407 , the processor unit  580  of the management computer  500  displays the area  5023  of the storage extent selection screen  5021  or sets the area  5023  under a selection possible state and displays the storage extents that can be added. 
   It should be noted here that the screen displayed in the processing of Step S 401  and the screen displayed in the processing of Step S 407  may be the same as described above or may be different screens. 
     FIG. 22  shows a configuration in which a load is distributed through the virtual storage extent  15  selection and addition processing according to the embodiment of this invention. 
   Referring to  FIG. 5 , as described above, in spite of the fact that the storage extents “LD- 02 ” and “LD- 03 ” are included in the same RAID group, they are registered as different virtual storage extents “VD- 02 ” and “VD- 03 ” in the virtual storage resource pool “PL- 01 ”. Therefore, there is a fear that an input/output load will be concentrated on “RG- 02 ”. 
   On the other hand, according to the embodiment of this invention, a storage extent belonging to a RAID group, whose storage extent registration number is minimum, is allocated through the processing of Step S 403  in  FIG. 19 . Therefore, at a point when the virtual storage extents “VD- 01 ” and “VD- 02 ” are registered in  FIG. 5 , when the virtual storage extent  15  “VD- 03 ” is newly added, it becomes possible to associate not the storage extent “LD- 03 ” (dotted line in  FIG. 22 ) but the storage extent “LD- 04 ” (thick line in  FIG. 22 ) belonging to a different RAID group with the virtual storage extent “VD- 03 ”. By selecting the storage extent to be registered in the virtual storage resource pool  14  in this manner, it becomes possible to avoid a situation in which performance is degraded due to concentration of a load on the specific RAID group “RG- 02 ”. 
   According to the embodiment of this invention, when a storage extent is added to a virtual storage resource pool  14 , it becomes possible to present storage extents configuring RAID groups  11 , whose numbers of storage extents registered in the virtual storage resource pool  14  are minimum, as storage extents with high priorities on the selection screen. Therefore, it becomes possible to select a storage extent so that a load is not concentrated on a specific RAID group with ease, which makes it possible to add a storage extent without degrading overall processing performance of a storage subsystem. 
   (First Modification) 
   In the embodiment of this invention described above, when a new storage extent is added to a virtual storage resource pool  14 , focus is made on the number of storage extents registered in the virtual storage resource pool  14 , and addition target storage extents are presented as storage extents with high priorities so that loads on RAID groups are distributed. 
   In a first modification of the embodiment of this invention, an external storage extent is selected so that loads on the data I/O interfaces  140  on an external storage subsystem side are distributed. 
   It should be noted here that system configurations shown in  FIG. 1  to  FIG. 4  in this first modification are the same as those of the embodiment and description thereof will be omitted. In a like manner, the description of the same processing as in the embodiment will be omitted. 
     FIG. 23  is a flowchart showing a procedure of processing for additionally registering a new storage extent in a virtual storage resource pool  14  according to the first modification of the embodiment of this invention. 
   The processor unit  580  of the management computer  500  executes the virtual storage configuration change interface program  5015 , thereby outputting an interface for receiving an operation to add a virtual storage extent to the output interface  575  (S 413 ). The interface for receiving the virtual storage extent addition operation is the same as that of the storage extent selection screen  5021  shown in  FIG. 21 . 
   The processor unit  580  of the management computer  500  receives input of a virtual storage resource pool in which a virtual storage extent is to be registered (S 414 ). 
   The processor unit  580  of the management computer  500  executes the virtual storage configuration change program  5014 , thereby searching for every data I/O interface  140 , whose external storage extent connection path number  50123  is minimum value, by referring to the external storage communication path mapping information  5012  (S 415 ). 
   The processor unit  580  of the management computer  500  carries out the following processing for every data I/O interface  140  found as a result of the processing of Step S 415  (S 416 ). 
   The processor unit  580  of the management computer  500  searches the logical unit configuration information  1003  and obtains every storage extent written in an entry whose data I/O interface  140  recorded in the communication interface identification information  10031  coincides with a processing target data I/O interface  140  (S 417 ). 
   The processor unit  580  of the management computer  500  outputs every storage extent obtained as a result of the processing described above to an interface for storage extent selection (S 419 ). As the interface, it is possible to use the storage extent selection screen  5021  shown in  FIG. 21 . In this case, as candidates for a storage extent to be added, besides the storage extent of the external storage subsystem found as a result of the processing described above, storage extents in the storage subsystem may be displayed at the same time. By selecting the storage extent to be added from among both of the storage extents in the storage subsystem and the external storage subsystem in this manner, it becomes possible to select a configuration with more flexibility. 
   Following this, the processor unit  580  of the management computer  500  receives selection of a storage extent, and the processing by the user and the management computer  500  continues to the processing procedure shown in  FIG. 20 . 
   Here, a modification will be described in which, when there is a performance difference between the communication bandwidths of the data I/O interfaces  140 , a load is distributed by adding a new storage extent to a virtual storage resource pool  14  with consideration given to the communication bandwidths. 
     FIG. 24  is a flowchart showing a procedure of processing for searching for a storage extent so that loads on the data I/O interfaces  140  connected with the external storage subsystem are distributed according to the first modification of the embodiment of this invention. More specifically, instead of the processing of Step S 415  in which every data I/O interface  140 , whose external storage extent connection path number is minimum, is searched for, processing of Steps S 501  to S 503  shown in  FIG. 24  is carried out. 
   Upon receiving an input of a virtual storage resource pool  14  from the user through the processing of Step S 414 , the processor unit  580  of the management computer  500  calculates a ratio between the numbers of external storage extent connection paths defined for respective data I/O interfaces  140  by referring to the communication interface bandwidths  50122  of the external storage communication path mapping information  5012  (S 501 ). For instance, as shown in  FIG. 15 , since the bandwidths of the communication interfaces “PT- 11 ” and “PT- 12 ” are “1 Gbps” and “2 Gbps”, the performance ratio becomes “1:2”. 
   The processor unit  580  of the management computer  500  calculates a ratio between the numbers of already allocated paths with respect to the ratio between the numbers of paths calculated in the processing of Step S 501  by referring to the external storage extent connection path numbers  50123  (S 502 ). For instance, in  FIG. 15 , the ratio between the external storage extent connection path numbers of the communication interfaces “PT- 11 ” and “PT- 12 ” is “1:3”. 
   When a new storage extent is added to a virtual storage resource pool  14  with consideration given to the communication bandwidths of the data I/O interfaces  140  as described above, it is preferable that the performance ratio between the data I/O interfaces  140  and the ratio between the numbers of connected paths be the same. 
   Thus, the processor unit  580  of the management computer  500  obtains a data I/O interface  140  whose difference between the performance ratio and the ratio between the numbers of connected paths is maximum (S 503 ). More specifically, the performance ratio between the data I/O interfaces  140  is “1:2” and the ratio between the numbers of connected paths before addition is “1:3”. Therefore, when the storage extent is added so as to be connected to “PT- 11 ”, the ratio between the numbers of connected paths becomes “2:3”. On the other hand, when the storage extent is added so as to be connected to “PT- 12 ”, the ratio becomes “1:4”. Consequently, when the storage extent is added to “PT- 11 ”, the ratio between the numbers of connected paths after addition comes close to the performance ratio. Accordingly, in the configuration shown in  FIG. 15 , “PT- 11 ” is selected. 
     FIG. 25  shows an example of a configuration in which a load is distributed through the virtual storage extent  15  selection and addition processing according to the first modification of the embodiment of this invention. 
   Referring to  FIG. 5 , as described above, in the virtual storage resource pool “PL- 02 ”, the external storage extents “ED- 02 ”, “ED- 03 ”, “ED- 04 ”, and “ED- 05 ” are registered as virtual storage extents. When an attempt is made to equally distribute a load by equally allocating areas to these virtual storage extents  15 , the data I/O interface  140  “PT- 11 ” on an external storage subsystem side has only one path that is a path to “LD- 12 ” but the data I/O interface  140  “PT- 12 ” has three paths that are paths to “LD- 13 ”, “LD- 14 ”, and “LD- 15 ”, so the load is concentrated on “PT- 12 ”. 
   In the first modification of the embodiment, through the processing of Step S 415  in  FIG. 23 , a storage extent to be connected to a communication interface whose external storage extent connection path number is minimum is allocated. Therefore, in  FIG. 5 , at a point when the virtual storage extents “VD- 11 ”, “VD- 13 ”, and “VD- 14 ” are registered, when the virtual storage extent  15  “VD- 12 ” is newly added, it becomes possible to associate not the storage extent “ED- 03 ” (dotted line in  FIG. 25 ) but the storage extent “ED- 01 ” (thick line in  FIG. 25 ) connected to the communication interface “PT- 11 ” with the virtual storage extent “VD- 12 ”. By selecting the storage extent to be registered in the virtual storage resource pool  14  in this manner, it becomes possible to avoid a situation in which processing performance is degraded due to concentration of the load on the specific data I/O interface “PT- 12 ”. 
   According to the first modification of the embodiment, an external storage extent is selected so that loads on the data I/O interfaces  140  on the external storage subsystem side are distributed, so it becomes possible to prevent a situation in which the data I/O interfaces  140  become a bottleneck and performance is deteriorated. 
   In addition, according to the first modification of the embodiment, the numbers of paths connected to respective data I/O interfaces  140  are determined based on the ratio between the bandwidths of the interfaces, so it becomes possible to equalize loads based on the bandwidth ratio. 
   In the aforementioned embodiment of this invention and first modification of the embodiment, a technique has been proposed with which, when a virtual storage extent  15  is newly added to a virtual storage resource pool  14 , a storage extent for distributing a load appropriately is selected. However, there is a possibility that when the configuration is changed in course of system operation due to deletion of a virtual storage resource pool  14 , deletion of a storage extent, or the like, the load is not distributed appropriately. In view of this problem, in a second modification of the embodiment, a technique will be described with which, when a system is changed to a configuration in which a load is concentrated on a specific RAID group  11 , the system is reconstructed to have a configuration in which the load is equally distributed. 
   (Second Modification) 
     FIG. 26  is a flowchart showing a procedure of processing for equalizing the registration numbers when there are variations in virtual storage extent registration numbers of storage extents configuring the RAID groups  11 , according to the second modification of the embodiment of this invention. It should be noted that this processing is achieved through execution of the virtual storage configuration change program  5014  by the processor unit  580  of the management computer  500 . 
   The processor unit  580  of the management computer  500  carries out the following processing for every virtual storage extent  15  recorded in the storage extent identification information  10053  of the virtual storage resource pool configuration information  1005  (S 601 ). It should be noted that when there exists a performance difference between the storage extents  12  provided in the storage subsystem  100  and the external storage extents  13 , it is also possible to achieve load equalization while limiting targets of the processing of Step S 601  by setting only the storage extents  12  in the storage subsystem  100  as the targets. 
   Next, the processor unit  580  of the management computer  500  searches for an entry of the storage extent configuration information  1002  whose storage extent recorded in the storage extent identification information  10021  coincides with a storage extent corresponding to a virtual storage extent  15  that is a processing target. Then, the processor unit  580  of the management computer  500  obtains a RAID group  11  from the RAID group identification information  10022  in the found entry. 
   The processor unit  580  of the management computer  500  searches for an entry of the storage extent configuration information  1002  whose RAID group  11  recorded in the RAID group identification information  10022  coincides with the obtained RAID group  11 . Then, the processor unit  580  of the management computer  500  carries out the following processing for a storage extent recorded in the storage extent identification information  10021  in the found entry (S 602 ). 
   Here, a case where the virtual storage extent  15  “VD- 02 ” has been selected as the processing target of Step S 601  will be described in a concrete manner. The virtual storage resource pool configuration information  1005  shows that a storage extent corresponding to the virtual storage extent  15  “VD- 02 ” is “LD- 02 ”. Therefore, the processor unit  580  of the management computer  500  first obtains an entry of the storage extent configuration information  1002  whose value of the storage extent identification information  10021  is “LD- 02 ”. 
   The value of the RAID group identification information  10022  in the obtained entry is “RG- 02 ”, so the processor unit  580  of the management computer  500  further searches for an entry of the storage extent configuration information  1002  whose value of the RAID group identification information  10022  coincides with “RG- 02 ”. As a result, the storage extents “LD- 02 ” and “LD- 03 ” are found and a loop starting from Step S 602  is carried out for these storage extents. 
   Following this, the processor unit  580  of the management computer  500  judges whether the storage extent (hereinafter referred to as “storage extent ( 2 )”) set as a processing target in Step S 602  and the storage extent (hereinafter referred to as “storage extent ( 1 )”) that corresponds to the virtual storage extent set as the processing target in Step S 601  are registered in the same virtual storage resource pool  14  by referring to the virtual storage resource pool configuration information  1005  (S 603 ). In the processing of Step S 603 , it is only necessary that entries, whose storage extent identification information  10053  corresponds to the storage extent ( 1 ) and the storage extent ( 2 ), are searched for and is judged whether values of the virtual storage resource pool identification information  10051  recorded in the found entries coincide with each other. 
   When the storage extent ( 1 ) and the storage extent ( 2 ) are registered in the same virtual storage resource pool  14  (result of the judgment in Step S 603  is “Yes”), the processor unit  580  of the management computer  500  transmits a message requesting integration of the storage extent ( 1 ) and the storage extent ( 2 ) to the storage subsystem  100  (S 604 ). The integration of storage extents means provision of the storage extents as one storage extent to the outside by allocating the same virtual storage extent as virtual storage extents corresponding to the storage extents. 
   Upon receiving the request to integrate the storage extent ( 1 ) and the storage extent ( 2 ), the storage subsystem  100  searches for an entry of the virtual storage resource pool configuration information  1005  whose storage extent identification information  10053  coincides with the storage extent ( 2 ), and updates a value of the virtual storage extent identification information  10052  in the found entry to a value of the virtual storage extent identification information  10052  of the storage extent ( 1 ) (S 605 ). For instance, when the storage extents “LD- 02 ” and “LD- 03 ” described above are integrated, it is only necessary that the value of the virtual storage extent identification information  10052  in an entry of the virtual storage resource pool configuration information  1005 , whose storage extent identification information  10053  coincides with “LD- 03 ”, is updated to “VD- 02 ” that is the virtual storage extent  15  of “LD- 02 ”. 
   When the processing of Step S 605  is completed, the storage subsystem  100  transmits a completion notification to the management computer  500  (S 606 ). 
   Upon receiving the integration completion notification from the storage subsystem  100 , the management computer  500  updates the storage configuration information (S 607 ). 
     FIG. 27  shows an example of a storage configuration in which a configuration under a state where a load is concentrated as shown in  FIG. 5  is reconstructed so that the load is distributed, in accordance with the processing procedure according to the second modification of the embodiment of this invention. 
   In the second modification of the embodiment, the processor unit  580  of the management computer  500  extracts storage extents, whose virtual storage extents are to be integrated, through the processing of Step S 603  in  FIG. 26 , and integrates the extracted storage extents through the processing of Step S 605 . More specifically, referring to  FIG. 27 , the logical storage extent “LD- 03 ” configuring the virtual storage extent “VD- 03 ” is integrated into the virtual storage extent “VD- 02 ” corresponding to the storage extent “LD- 02 ” belonging to the same RAID group (changed from a configuration indicated by a dotted line to a configuration indicated by a thick line in  FIG. 27 ). By integrating the virtual storage extents in this manner, it becomes possible to prevent a situation in which extent allocation is concentrated on the specific RAID group “RG- 02 ”. 
   According to the second modification of the embodiment, a plurality of virtual storage extents  15  allocated to the same RAID group  11  are integrated into one virtual storage extent  15 , so a situation is prevented in which a load is concentrated on a specific RAID group  11 , which makes it possible to suppress degradation of performance of the storage subsystem. 
   (Third Modification) 
   Next, a third modification of the embodiment will be described in which when there occurs unevenness in the numbers of external connection paths connected to the data I/O interfaces  140  on the external storage subsystem side, a configuration is changed so that the numbers are equalized. 
     FIG. 28  and  FIG. 29  are flowcharts showing a procedure of processing for equalizing the numbers of external connection paths connected to the data I/O interfaces  140  on the external storage subsystem side according to the third modification of the embodiment of this invention. Processing carried out at the management computer  500  is achieved through execution of the virtual storage configuration change program  5014 . 
     FIG. 28  shows a former half portion of the processing for equalizing the numbers of external connection paths connected to the data I/O interfaces  140  on the external storage subsystem side according to the third modification of the embodiment of this invention, in which an instruction to move a storage unit is issued to the external storage subsystem. 
     FIG. 29  shows a latter half portion of the processing for equalizing the numbers of external connection paths connected to the data I/O interfaces  140  on the external storage subsystem side according to the third modification of the embodiment of this invention, in which an instruction to update the external storage map information is issued to the storage subsystem. 
   The processor unit  580  of the management computer  500  carries out the following processing for a storage subsystem  100  whose identification name is recorded as a storage subsystem identification name  10073  in the data I/O interface configuration information  1007  (S 701 ). 
   The processor unit  580  of the management computer  500  searches the external storage communication path mapping information  5012  and obtains an entry whose external storage extent connection path number  50123  is maximum (S 702 ). A data I/O interface  140  recorded in the communication interface identification information  50121  in the obtained entry is set as the “communication interface ( 1 )”. 
   In a like manner, the processor unit  580  of the management computer  500  searches for a data I/O interface  140 , whose external storage extent connection path number is minimum, and sets the found interface  140  as the “communication interface ( 2 )” (S 703 ). 
   Next, the processor unit  580  of the management computer  500  obtains an entry of the external storage map information  1004  whose communication interface identification information  10042  coincides with the communication interface ( 2 ). Following this, the processor unit  580  of the management computer  500  transmits a request message to move a storage unit recorded in the obtained entry to the communication interface ( 1 ) to the external storage subsystem (S 704 ). 
   Upon receiving the request message, the external storage subsystem  100 B searches for an entry of the logical unit configuration information  1003  corresponding to the designated storage unit. Then, the external storage subsystem  100 B rewrites the communication interface identification information  10031  in the found entry to a value of the communication interface ( 1 ) and, at the same time, updates a value of the storage unit identification information  10032  so that it does not coincide with other storage unit identification information values corresponding to the communication interface ( 1 ) (S 705 ). 
   For instance, referring to  FIG. 15 , the communication interface ( 1 ) is “PT- 11 ” and the communication interface ( 2 ) is “PT- 12 ”. Therefore, the processor unit  580  of the management computer  500  requests to move “LU- 11 ”, that is, the storage extent “LD- 13 ”, out of storage units connected to “PT- 12 ”, to “PT- 11 ” that is the communication interface ( 1 ). 
   The external storage subsystem  100 B updates a value of the communication interface identification information  10031  in an entry of the logical unit configuration information  1003 , whose value of the storage extent identification information  10033  coincides with “LD- 13 ”, to “PT- 11 ”. In addition, the external storage subsystem  100 B updates a value of the storage unit identification information  10032  to “LU- 13 ” that is a value other than already recorded “LU- 11 ” and “LU- 12 ”. In this manner, the movement of the storage unit is completed. 
   The external storage subsystem  100 B stores storage unit identification information after the completion of the movement to the communication interface ( 1 ) in a completion notification message in the processing of Step S 705 , and transmits the message to the management computer  500  (S 706 ). 
   Upon receiving the storage unit movement completion notification message, the processor unit  580  of the management computer  500  updates the configuration information based on the storage unit identification information after the completion of the movement included in the message (S 707 ). 
   Next, the processor unit  580  of the management computer  500  searches the external storage map information  1004  and obtains an entry whose communication interface identification information  10042  and storage unit identification information  10043  coincide with those of the movement target storage unit of the communication interface ( 2 ) whose movement has been instructed in the processing of Step S 704 . Then, the processor unit  580  of the management computer  500  transmits a request message to update a correspondence destination of an external storage extent  13  in the obtained entry to the destination storage unit of the communication interface ( 1 ) designated as the destination through the processing of Step S 704  (S 708 ). 
   Upon receiving the request message, the storage subsystem  100  updates the communication interface identification information  10042  and the storage unit identification information  10043  in the entry of the external storage map information  1004  (S 709 ). 
   In the example described above, the storage unit “LU- 11 ” defined for the communication interface ( 2 ), that is, “PT- 12 ” is the movement target, so the storage subsystem  100  updates an entry for the external storage extent “ED- 03 ” of the external storage map information  1004 . More specifically, the management computer  500  issues, to the storage subsystem  100 , a request to update the communication interface identification information  10042  in the entry to “PT- 11 ” that is the communication interface ( 1 ) and update the storage unit identification information  10043  to “LU- 13 ”. In response to the request, the storage subsystem  100  updates the external storage map information  1004 . 
   Following this, the storage subsystem  100  transmits an update processing completion notification (S 710 ). 
   Upon receiving the completion notification transmitted from the storage subsystem  100 , the processor unit  580  of the management computer  500  updates the configuration information (S 711 ). 
   When the processing described above is completed, the processor unit  580  of the management computer  500  updates the external storage communication path mapping information  5012 . 
   In addition, the processor unit  580  of the management computer  500  searches for an entry, whose value of the communication interface identification information  50121  coincides with the communication interface ( 1 ), and an entry, whose value of the communication interface identification information  50121  coincides with the communication interface ( 2 ), and judges whether values of the external storage extent connection path numbers  50123  in the entries are equal to each other (S 712 ). It should be noted that it is not required to judge that the values completely coincide with each other and it is possible to regard that the values are equal to each other when a difference therebetween is small, for instance, when the difference is “1”. 
   When having judged that the external storage extent connection path number  50123  of the communication interface ( 1 ) and that of the communication interface ( 2 ) are not equal to each other (result of the judgment in Step S 712  is “No”), the processor unit  580  of the management computer  500  returns to the processing of Step S 704 , and repeatedly carries out the processing for moving one of the storage units of the communication interface ( 2 ) to the communication interface ( 1 ). 
   On the other hand, when having judged that the external storage extent connection path number  50123  of the communication interface ( 1 ) and that of the communication interface ( 2 ) are equal to each other (result of the judgment in Step S 712  is “Yes”), the processor unit  580  of the management computer  500  judges whether all of the external storage extent connection path numbers  50123  of the data I/O interfaces  140  equipped to the external storage subsystem are equal to each other (S 713 ). 
   When having judged that all of the external storage extent connection path numbers  50123  of the data I/O interfaces  140  equipped to the external storage subsystem are not equal to each other (result of the judgment in Step S 713  is “No”), the processor unit  580  of the management computer  500  returns to the processing of Step S 702  and repeats the path number equalizing processing for a different data I/O interface  140 . 
   On the other hand, when having judged that all of the external storage extent connection path numbers  50123  of the data I/O interfaces  140  equipped to the external storage subsystem are equal to each other (result of the judgment in Step S 713  is “Yes”), the processor unit  580  of the management computer  500  repeats the equalizing processing for another external storage subsystem (S 714 ). 
     FIG. 30  shows an example of a storage configuration in which a configuration under a state, in which a load is concentrated as shown in  FIG. 5 , is reconstructed so that the load is distributed in accordance with the processing procedure according to the third modification of the embodiment of this invention. 
   In the third modification of the embodiment, the processor unit  580  of the management computer  500  moves a connection path of an external storage extent through the processing of Step S 704  in  FIG. 28 . More specifically, the processor unit  580  of the management computer  500  moves the connection path of the external storage extent “ED- 03 ” from the data I/O interface “PT- 12 ” to “PT- 11 ” (changed from a configuration indicated by a dotted line to a configuration indicated by a thick line in  FIG. 30 ). By equalizing the numbers of external connection paths in this manner (“PT- 11 ” corresponds to two paths to “LD- 12 ” and “LD- 13 ” and “PT- 12 ” corresponds to two paths to “LD- 14 ” and “LD- 15 ”), it becomes possible to avoid a situation in which performance is degraded due to concentration of a load on a specific data I/O interface  140 . 
   According to the third modification of the embodiment, it becomes possible for the processor unit  580  of the management computer  500  to equalize loads on the data I/O interfaces  140  by equalizing the numbers of the external storage extents connected to the data I/O interfaces  140 . Also, the processor unit  580  of the management computer  500  may distribute the connection paths based on the bandwidths of the data I/O interfaces  140 . 
   While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.