Patent Publication Number: US-2003236884-A1

Title: Computer system and a method for storage area allocation

Description:
Background of the Invention  
       [0001] The present invention relates to a computer system and a storage device management method in a computer system.  
       [0002] In recent years, storage devices owned by companies or the like have increased both in number and storage capacity with the increased volume of data handled in companies or the like. Consequently, corporate costs for management of the storage devices have swelled.  
       [0003] As a method for reducing the management cost of storage devices, the use of such approaches as the Storage Area Network (SAN) and the Network Attached Storage (NAS) has been proposed. A SAN is a network where a plurality of computers is connected with a plurality of storage devices by using the Fibre Channel (FC) or the Internet Protocol (IP). A NAS is a storage device connected to the IP network.  
       [0004] Existing technologies will drastically increase the number and varieties of storage devices each computer can use via a network. That is, in a computer system where computers are connected with storage devices via a network, a great number of storage devices having different attributes will be connected to the network. Further, it is possible that different kinds of storage devices, such as SAN and NAS devices, will coexist in a computer system. In such a computer system, it will be important to selectively use storage areas, provided by SAN and NAS devices, according to their characteristics.  
       [0005] Disclosed in Japanese Patent Laid-open No. 2001-142648 is a device allocation method in a SAN for determining which device should be allocated to a host computer by taking into consideration the characteristics of the devices connected to the SAN. Practically, the host computer sends a logical device allocation request, combined with attribute requirements in terms of capacity, performance, etc., to a storage management server for the SAN. On the basis the characteristics of each storage device and the required device attributes, the storage management server selects a storage device for allocation. The storage management server instructs the selected storage device to make itself accessible from the host computer which issued the allocation request. Then, the storage management server updates the storage management information held therein and returns information about the allocated device to the host computer which sent the request. In the same publication, a method for allocating a file system from a NAS device is also disclosed.  
       [0006] In the approach disclosed in Japanese Patent Laid-open No. 2001-142648, however, consideration is not given to such a mixed environment as a computer system connected with different kinds of storage device systems. If the above-mentioned technique is applied to an environment where SAN and NAS devices coexist, the host computer side must determine which storage device system, SAN or NAS, should allocate a storage area before issuing an allocation request to an associated storage management server of the selected storage device system. Practically, however, effective allocation of storage areas is impossible without knowledge of the configurations of both SAN and NAS systems because network bandwidth, storage performance and other characteristics differ depending on the specific system.  
       SUMMARY OF THE INVENTION  
       [0007] Accordingly, it is an object of the present invention to realize such a storage allocation method in a computer system where both SAN and NAS devices exist that a host computer can be allocated an appropriate storage area only by presenting requirements without the necessity of being aware of the types, configurations, etc. of the storage devices connected to the network.  
       [0008] According to the present invention, in a computer system where different kinds of storage devices coexist, a computer, a storage management computer and storage devices work in cooperation with each other as follows.  
       [0009] At first, the computer sends a request to the storage management computer in order to use a storage area of the storage devices connected to the network. Comparing information about the storage devices connected to the network with the requirements included in the request from the computer, the storage management computer selects a storage device having an storage area which meets the requirements.  
       [0010] The storage management computer sends information about the computer to the selected storage device and information about the selected storage device to the computer. Receiving the information about the selected storage device, the computer controls the storage device on the basis of the received storage device information.  
       [0011] The requirements sent in the request from the computer may include information indicating characteristics and reliability requirements which any storage area to be allocated must satisfy.  
       [0012] In addition, as a preferred embodiment, the computer system may be configured in such a manner that if the selected storage device is a storage device connected to the SAN, the storage management computer instructs the storage device to create a storage area reserved for the computer, and if the selected storage device is a NAS, the storage management computer instructs the NAS to create a file system reserved for the computer.  
       [0013] Further, as another preferred embodiment, the computer system may be configured in such a manner that if the selected storage device is a storage device connected to the SAN, the computer creates a file system in a storage area of the selected storage device on the basis of the information sent from the storage management computer, and if the selected storage device is a storage device connected to the IP network, the computer mounts a file system, which is created in the selected storage device, to its own file system on the basis of the information sent from the storage management machine. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is a block diagram of a computer system according to an embodiment of the present invention;  
     [0015]FIG. 2 is a diagram showing a configuration of a file-accessed storage&#39;s configuration management table;  
     [0016]FIG. 3 is a diagram showing a configuration of a block-accessed storage&#39;s configuration management table;  
     [0017]FIG. 4 provides a flowchart of file creation processing;  
     [0018]FIG. 5 provides a flowchart of file creation processing by a storage manager;  
     [0019]FIG. 6 provides a flowchart of logical device allocation processing by a storage subsystem;  
     [0020]FIG. 7 provides a flowchart of file system creation processing; and  
     [0021]FIG. 8 is a diagram showing a configuration of a logical device management table. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0022] An embodiment of the present invention will hereinafter be described. In this embodiment, if a computer receives a file system creation request from an application, it sends a file system creation request, combined with information about the attribute and other requirements which any storage area to be used for the file system must satisfy, to a storage management server, which in turn compares the requirements with the configuration of each storage device and selects a storage device which satisfies the requirements.  
     [0023]FIG. 1 is a diagram showing a system configuration of a computer system according to this embodiment of the present invention.  
     [0024] In this computer system, a plurality of computers  11   a  and  11   b  (hereinafter denoted generically as “host  11 ”) is connected to storage subsystems  12   a  and  12   b  (hereinafter denoted generically as “storage subsystem  12 F) via a FC switch  18 .  
     [0025] In addition, via an IP network  141 , the host  11  is connected with a file server  19  that provides file systems. The file server  19  is connected with a storage subsystem  12   c . The file server  19  and the storage subsystem  12   c  are accommodated in a single cabinet and constitute a NAS  17 . In addition, the host  11 , the storage subsystems  12 F and  12   c , the FC switch  18  and the NAS  17  are connected to a management computer (hereafter denoted as “storage management server”)  13  via an IP network  142 .  
     [0026] The storage management server  13  retains configuration information gathered from the host  11 , the storage subsystems  12 F and  12   c , the FC switch  18  and the NAS  17  via the IP network  142 . In addition, from the FC port address, port WWN and node WWN of each device, the storage management server  13  recognizes the FC network topology indicating the connection states of the individual devices.  
     [0027] The host  11 , the file server  19  and the storage management server  13  are computers each provided with a processor comprising a CPU and a memory, not shown. Each of them provides prescribed processing by making the CPU execute the operating system (OS), application programs and so on stored in the memory.  
     [0028] The storage subsystems  12 F and  12   c  (hereinafter denoted generically as “storage subsystem  12 ”) are storage devices each provided with a plurality of disk units  121 , a disk controller  122 , a plurality of ports  123  connected to the host  11  or the file server  19 , and a network interface  125  connected to the IP network  142 .  
     [0029] The storage subsystem  12  in this embodiment provides physical storage areas of the plurality of disk units  121  to the host  11  as one logical storage area (hereinafter denoted as “logical device”) or a plurality of logical storage areas. Note that it is also possible to provide the whole physical storage area of each disk unit as a single logical device to the host  11 . Also note that there is no correlation between the number of the disk units  121  in the storage subsystem  12  and the number of logical devices provided by the storage subsystem  12 .  
     [0030] In addition, the storage subsystems connected to the SAN and the file server  19  may be replaced by single disk devices. In the following explanation, the use of storage subsystems is assumed.  
     [0031] The port  123  in this embodiment supports the FC interface which implements the SCSI (Small Computer System Interface) as the upper protocol. Note that it is also possible to configure the port  123  so as to support another network interface such as the IP network interface which implement the SCSI as the upper protocol.  
     [0032] The disk controller  122  is provided with a processor  131 , a cache memory  132  and a control memory  133 . The processor  131  communicates with the host  11  and controls the disk unit  121 . In addition, if a plurality of disk units  121  is provided by the storage subsystem  12  to the host  11  as one logical device or a plurality of logical devices, the processor  131  also performs management of relations between the disk units  121  and the logical devices.  
     [0033] The disk controller  122  communicates with the storage management server  13  via the network interface  125 . The cache memory  132  temporally stores the data read out from the disk unit  121  and the data received from the host  11 . It is possible to operate part of the cache memory  132  as one disk device or a plurality of disk devices in such a manner that the disk unit  121  apparently need not be accessed.  
     [0034] The control memory  133  contains a program which is executed by the processor  131 . The control memory  133  also contains the information used for management of the disk unit  121  and the logical devices which are constituted from the disk unit  121 .  
     [0035] The host  11  is provided with a network interface  113 , a FC interface  112  and a program called a client program  111  in addition to the CPU and the memory mentioned earlier. The host  11  is connected with the FC switch  18  via the FC interface  112 . In addition, the host  11  is connected with the file server  19  via the network interface  113 .  
     [0036] The client program  111  is a program executed by the host  11  in order to allow application programs running on the host  11  to use logical devices created in the storage subsystem  12 F or file systems created in the storage subsystem  12   c.    
     [0037] The client program  111  may be integrated into the operating system (not shown) of the host  11 . The client program  111  is stored in the memory of the host  111 .  
     [0038] The FC switch  18  is provided with a plurality of ports  181  and a network interface  182 . Each port  181  is connected to the interface  112  of the host  11  or the port  123  of the storage subsystem  12 F. The FC switch  18  is connected to the IP network  142  via the network interface  182 .  
     [0039] In this embodiment, all hosts  11  can use all storage subsystems  12 F connected to the FC switch  18 .  
     [0040] The file server  19  in the NAS  17  is provided with a network interface  191  connected to the host  11  via the IP network  141 , an interface  192  connected with the storage subsystem  12   c  and a server program  193  in addition to the CPU and memory mentioned earlier.  
     [0041] The server program  193  is a program executed by the file server  19  in order to allocate a device in response to a request from the host  11  and provide file access service to the host  11  and others via such a network as a NFS (Network File System).  
     [0042] By executing the server program  193 , the file server  19  allows the host  11  to use a file system created in the storage subsystem  12   c  by the file server  19 . In addition, the filer server  19  constructs a file system in the storage subsystem  12   c  by executing the server program  193 .  
     [0043] Incidentally, the storage subsystem  12   c  and the NAS  17  in this embodiment are treated as one storage subsystem by each host  11 . However, the file server  19  and the storage subsystem  12   c  may be accommodated in separate cabinets.  
     [0044] Although the interface  192  in this embodiment is an interface supporting the SCSI protocol, it may be any interface insofar as the communication protocol with the storage devices is supported.  
     [0045] The storage management server  13  is provided with a network interface  133  to connect each device of the computer system via the IP network and a device manager  131  in addition to the CPU and memory mentioned earlier. The storage manager  131  is a program executed by the CPU of the storage management server  13  in order to manage the storage subsystem  12  connected to the network.  
     [0046] As described so far, this embodiment includes different kinds of storage devices, that is, the storage subsystem  12 F having the FC interface and the NAS  17  connected with the host  11  via the IP network.  
     [0047]FIG. 8 shows an example of a logical device management table held in the storage subsystem  12  which is connected to the host  11  and the file server  19  (hereinafter denoted generically as “upper host”). The storage subsystem  12  registers logical devices to the logical device management table for management of logical devices provided to the upper host by the storage subsystem  12 .  
     [0048] The logical device management table has a logical device number entry  81 , a size entry  82 , a configuration entry  83 , a status entry  84 , a bus entry  85 , a target ID entry  86 , a LUN entry  87  and a connected host name entry  88  for each logical device owned by the storage subsystem  12 .  
     [0049] To the logical device number entry  81 , information for identifying a logical device provided to the upper host by the storage subsystem  12  is registered. Practically, a unique number assigned to the logical device is registered. To the size entry  82 , the capacity of the logical device identified by the logical device number  81  is registered.  
     [0050] To the configuration entry  83 , information about the configuration of the logical device is registered. For example, if a RAID consisting of a plurality of disk units  121  is assigned to the logical device, information indicating the RAID type is registered. If a part of the cache memory is assigned to the logical device, information indicating the logical device consists of a “single disk unit” is set to the configuration entry  83 .  
     [0051] To the status entry  84 , information indicating the status of the logical device is set. The status of each logical device is either “online”, “offline”, “unmounted” or “disabled”. “Online” means that a disk unit  12  associated with the logical device is operating normally in the storage subsystem  12  and the upper host can use the logical device. “Offline” means that the logical device is defined in the storage subsystem  12  but it cannot be used by the upper host. Practically, the logical device is not allocated to any upper host. “Unmounted” means that the logical device is not defined in the storage subsystem  12  and cannot be used by the upper host. “Disabled” means that the logical device is defined in the storage subsystem  12  but the corresponding disk unit is failed and therefore the upper host cannot use the logical device.  
     [0052] Assuming that logical devices are created on the disk unit  121  before the product is shipped from the factory for the sake of simplicity in this embodiment, the status entry  84  for each logical device created in the storage subsystem  12  has the initial value “offline”. Incidentally, the user who uses the storage subsystem  12  can freely create the logical device so that the disk unit  121  in the storage subsystem  12  can be used more effectively.  
     [0053] For example, if the user issues a storage area allocation request to a storage subsystem  12  which is delivered with all logical devices in the “unmounted” status, the storage subsystem  12  defines one or more disk units  121  as a logical device which satisfies the requirements. Practically, on the basis of the information about the unused areas of the disk units  121  and the performance and other characteristics management information about each disk unit  121 , the storage subsystem  12  selects one or more disk units  121  which satisfy the user&#39;s requirements. Then the storage subsystem  12  registers the logical device, which is assigned storage areas of the selected disk units  121 , to the logical device management table.  
     [0054] The bus entry  85  indicates which of the ports  123  is logically connected to the logical device. Each port  123  is assigned a unique number in the storage subsystem  12 . To the bus entry, the number of the port  123  connected to the logical bus is registered.  
     [0055] To the target ID entry  86  and the LUN entry  87 , information or identifiers used by the host  11  to identify the logical device are registered. Here, information indicating SCSI-ID and LUN, identifiers used in the SCSI protocol are registered to the respective entries. It is also possible to register identifiers used in another protocol as the case may be.  
     [0056] The connected host name entry  88  is used only by the storage subsystem  12 F having the FC interface. The connected host name entry  88  specifies which host  11  is permitted to use the logical device, that is, to read data from or write data to the logical device. Practically, information indicating a host name is registered to identify one of the hosts  11 .  
     [0057] Any value, such as a world wide name (WWN) which is generally given to the I/F  112  of each host  11 , can be used as a host name if the value can designates a specific host  11  or the interconnection interface of a specific host  11 . Note that the connected host name entry  88  may be omitted from the logical device management table held by the storage subsystem  12   c.    
     [0058] Further, the storage subsystem  12  retains management information such as logical-to-physical associative information indicating associations between logical storage areas and physical storage areas, or between logical devices and disk units  121 . If the storage subsystem  12  is a RAID device, the logical-to-physical associative information includes the identifier and capacity of the disk unit  121  constituting the logical device. If a plurality of logical devices is defined in one RAID, the information includes the starting location of each logical device in the RAID. If a plurality of RAIDs constitutes one logical device, the information includes the order of the RAIDs.  
     [0059] The file server  19  has the same logical device configuration management table as that of the storage subsystem  12   c  in order to manage the allocation of logical devices provided by the storage subsystem  12   c.    
     [0060] In this embodiment, the storage management server  13  gathers information about the configuration of every storage subsystem in the computer system and retains the information as a file access storage configuration management table or the like.  
     [0061]FIG. 2 shows a configuration of a file-accessed storage&#39;s configuration management table retained by the storage management server  13 . Stored in the file-accessed storage&#39;s configuration management table is information used by the storage management server  13  for the management of the storage subsystem  12   c  owned by the NAS  17 .  
     [0062] For each NAS  17 , the file-accessed storage&#39;s configuration management table has a host name entry  201 , an IP address list entry  202 , a performance/reliability level entry  203 , a total capacity entry  204 , an unused capacity entry  205  and a device maximum size entry  206 .  
     [0063] To the host name entry  201 , information indicating the host name of the file server  19  is registered. Generally, an arbitrary host name is given to the file server  19  by the user. On the IP network, the relation between the IP address and the host name given to the file server  19  is managed by a DNS (Domain Name Server).  
     [0064] To the IP address list  202 , one or more IP addresses owned by the network interface  191  of the file server  19  are registered. The registered IP addresses are used as destination addresses for communication when the host  11  accesses the file server  19  (NAS  17 ). If the file server  19  is connected to the IP network  141  via a plurality of network interfaces  191 , the IP address entry  202  indicates the IP addresses assigned to each of the plurality of network interfaces  191 .  
     [0065] To the other entries, information about the attributes of the NAS  17  comprising the file server  19  and the storage subsystem  12   c  is registered by the storage management server  13 .  
     [0066] The performance/reliability level entry  203  retains evaluative values given objectively, on the basis of the computer system, to the performance and reliability of the storage subsystem  12   c.    
     [0067] The performance evaluation indexes may include the seek and rotation speeds of the disk unit  121  mounted in the storage subsystem  12   c , the storage capacity of the disk unit  121 , the RAID level configured in the storage subsystem  12   c , the communication bandwidth between the controller  122  and the disk unit  121 , the communication bandwidth of the port  123 , the number of communication lines, the storage capacity of the cache  132 , the nominal, total performance values of the storage subsystem  12   c , the performance of the processor owned by the file server  19 , the number of processors, the transmission bandwidth of the network interface  191 , and the number of interfaces.  
     [0068] The reliability evaluation indexes may include the redundancy of elements constituting the disk unit  121  and the disk controller  122  in the storage subsystem  12   c , the RAID level implemented in the storage subsystem  12   c , the number of available alternative paths and other various configurational conditions subject to product specifications.  
     [0069] Although it is assumed in this embodiment that the requirements presented by the host  11  in a request to allocate a logical device concern performance and reliability, it is also possible to include functional requirements or criteria owned by each storage subsystem. For example, such functions as to replication and backup of logical devices, provided by the file server  19  and the storage subsystem  12 , can be taken into consideration.  
     [0070] The total capacity entry  204  registers information indicating the total of available storage areas in the storage subsystem  12   c . This total available capacity is determined by the storage capacity of each disk unit  121  owned by the storage subsystem  12   c , the number of such disk units  121  and the RAID level implemented in the storage subsystem  12   c . In this embodiment, since it is assumed that available logical devices are predetermined, the total capacity of the available logical devices is registered to this entry.  
     [0071] The unused capacity entry  205  registers information indicating which logical devices owned by the storage subsystem  12   c  are not yet allocated to the host  11 . In this embodiment, since the logical devices in the “unmounted” state cannot be used by the host  11 , the total storage capacity of the logical devices in the “offline” state is registered to this entry.  
     [0072] The device maximum size entry  206  registers information indicating the storage capacity of the largest logical device the storage subsystem  12   c  can create therein. Of the logical devices in the “offline” state, the size of the largest one is registered to this entry.  
     [0073] This embodiment where all logical devices are already defined may be modified in such a manner that the storage subsystem  12   c  can define a logical device in response to a request from the host. In such an embodiment, information indicating the total capacity of the unused storage areas of the disk units  121  may be registered to the unused capacity entry  205 . Further, in such an environment, the capacity of the largest logical device which can be created by using the unused areas of the disk units  121  within the limits of control by the storage subsystem  12  may be registered to the device maximum size entry  206 . The limits of control mean, for example, the maximum allowable number of disks in a disk array per logical device and the maximum allowable size of control information relative to a capacity per logical device.  
     [0074]FIG. 3 is a block-accessed storage&#39;s configuration management table owned by the storage management server  13 .  
     [0075] To the block-accessed storage&#39;s configuration management table, information used by the storage management server  13  to manage the storage subsystem  12 F is registered.  
     [0076] The block-accessed storage&#39;s configuration management table has a storage name entry  301 , a port name list entry  302 , a performance/reliability level entry  203 , a total capacity entry  204 , an unused capacity entry  205  and a device maximum size entry  206 .  
     [0077] To the storage name entry  301 , information about an identifier for identifying one of the storage subsystems  12 F is registered. For example, the identifier may be the fiber channel&#39;s platform WWN or a combination of the vendor&#39;s identifier and product number owned by the storage subsystem  12 F.  
     [0078] To the port name list entry  302 , information indicating the WWN assigned to the port  123  owned by the storage subsystem  12 F is registered. As mentioned above, the port name (WWN) is used as an identifier by which the host  11  can identify the port of the storage subsystem  12 F as the destination.  
     [0079] Information registered to the performance/reliability level entry  203 , the total capacity entry  204 , the unused capacity entry  205  and the device maximum size entry  206  is the same as the information registered to those in the file-accessed storage&#39;s configuration management table.  
     [0080]FIG. 4 is a flowchart of file system creation processing by the client program  111 . This processing is performed by the host  11  through the client program  111  in response to a request to create a new file system issued from, for example, an application program running on the host  11 .  
     [0081] The new file system creation request received by the client program  111  from the user, the application program or the like is accompanied with requirements for the size and attribute of the file system to be created, the storage capacity of the storage area in which the file system is to be created, and the performance and reliability of the logical device for the file system.  
     [0082] The size of the storage area, mentioned above, means the size of a logical device described earlier. Information about the performance and reliability requirements indicates objective performance/reliability any logical device to be selected for the file system must meet on the basis of the criteria in the computer system described above.  
     [0083] The performance requirements may concern such an aspect of performance as the access speed of the logical device by assuming a low speed disk drive, a high speed disk drive, a cache-resident disk or the like.  
     [0084] The reliability requirements may include such configurational requirements for the logical device as a certain RAID level, dual path and remote mirror.  
     [0085] Dual path allows the host  11  to access the same device via a plurality of interfaces in the case where the host  11  has a plurality of interfaces. Even when a path cannot be used, the device can be accessed via the other path (alternative path).  
     [0086] The remote mirror generates a copy of a logical device owned by a storage subsystem  12  in another storage subsystem  12 . Reliability of the computer system can be raised because even when the storage subsystem  12  fails due to an earthquake, a fire or the like, the data copy held by the remote storage subsystem is available (Step  401 ).  
     [0087] The client program  111  sends a file system creation request to the storage management server  13  and waits for a response (Step  402 ).  
     [0088] From the storage management server  13 , the client program  111  receives information about the allocated storage area.  
     [0089] The received information indicates whether the allocated storage area is a logical device owned by a storage subsystem  12 F or a file system provided by the file server  19 . In addition, the received information includes information necessary to access the logical device or the file system. For example, in the former case, the information includes the logical device&#39;s WWN and LUN assigned to the port  123  of the storage subsystem  12 . In the latter case, the information includes the host name or IP address of the file server  19  and the directory name of the file system if the NFS protocol is implemented (Step  403 ).  
     [0090] Using the received information, the client program  111  judges whether the allocated storage area is a logical device owned by the storage subsystem  12 F or a file system provided by the file server (Step  404 ).  
     [0091] If the allocated storage area is a logical device of the storage subsystem  12 F, the host  11  reconfigures itself so that it can use the logical device. Practically, this reconfiguration is done as follows:  
     [0092] The host  11  is assumed to be running under what is called an open operating system. Therefore, for the host  11  to use logical devices, a device file is prepared for each logical device. Device files are prepared when device configuration recognizing processing is done by the host  11 . Thus, logical devices have no device files if they didn&#39;t exist when the last device configuration recognizing processing was done. Due to this, a device file must be prepared in the host  11  for the newly allocated logical device in this step.  
     [0093] More practically, Hewlett-Packard&#39;s operating system uses the “IOSCAN”; command to recognize new logical devices and create device files for them. As a result of the device configuration recognizing processing like this, the host  11  gets able to use the newly allocated logical device (Step  405 ).  
     [0094] Once the newly allocated logical device gets available, the host  11  creates a file system, concretely a directory, in the logical device (Step  406 ). Then, the host  11  mounts the file system, in other words, concatenates the new file system to the existing directory tree (Step  407 ). After that, information indicating the mount point is sent back by the client program  111  to the application program or the like (Step  408 ).  
     [0095] Meanwhile, if the storage area allocated by the storage management server is a file system provided by the file server  19 , steps  407  and  408  are performed in order to make the provided file system available (Steps  407  and  408 ).  
     [0096]FIG. 5 is a flowchart of file system creation processing by the storage management server  13 . Receiving a file system creation request from the host  11 , the storage management server  13  performs this processing by executing the storage manager  131 .  
     [0097] The storage management server  13  receives from the host  11  the file system creation request that includes information about what characteristics are required of the file system to be created and the logical device in which the file system is to be created (hereinafter denoted as “device requirements”(Step  501 ).  
     [0098] The storage management server  13  selects storage subsystems  12  which can be accessed by the host  11  which has issued the file system creation request (hereinafter denoted as requester host  11 ”) via the FC or the IP network. To make this selection, the storage management server  13  requires information about the configuration of the network connecting the hosts  11  and the storage subsystems  12 .  
     [0099] Practically, this selection is made by using the FC network topology prepared on the basis of configurational information gathered by the storage management server  13  from each device and the zoning information which is set to the fibre channel switch  18 . Even if the I/F  112  of a host  11  is connected to a port  123  of a storage subsystem  12 F via the fiber channel, the host  11  is judged as not accessible to the storage subsystem  12 F if access between the two ports is prohibited due to the zoning. Detail of the zoning information is not explained here because this information is retained by most storage management servers (Step  502 ).  
     [0100] Then, from the storage subsystems  12  selected in step  12 , the storage management server  13  chooses a storage subsystem  12  which meets the device requirements presented by the requester host  11 . Practically, the storage management server  13  compares the file/block-accessed storage&#39;s configuration table held therein with the device requirements contained in the file system creation request and determines a storage subsystem  12  which meets the device requirements (Steps  503  and  504 ).  
     [0101] If any of the storage subsystems  12  does not meet the device requirements, the storage management server  13  notifies the requester host  11  that no device cannot be allocated and terminates the processing (Step  514 ).  
     [0102] If a storage subsystem  12  exists which meets the device requirements, the storage management server  13  determines the type of the storage subsystem  12  to be allocated (Step  505 ).  
     [0103] If the storage subsystem  12   c  is to be allocated, the storage management server  13  sends a file system creation request to the file server  19 , including the device requirements received from the requester host  11  (Step  506 ). Then, from the file server  19 , the storage management server  13  receives information about the file system to be allocated (Step  507 ). The storage management server  13  also sends information about the file system to the requester host (Step  508 ).  
     [0104] If a storage subsystem  12 F is to be allocated, the storage management server  13  instructs the selected storage subsystem  12 F to set its logical device “online” with the requester host  11  (hereinafter, referred to as “online instruction”). This online instruction sent to the storage subsystem  12 F also includes the device requirements received from the requester host  11  (Step  509 ).  
     [0105] In addition, the storage management server  13  instructs the selected storage subsystem  12 F to reconfigure itself so that a logical device owned by the selected storage subsystem  12 F can be used by the requester host  11 . Practically, the storage management server  13  sends information indicating the WWN assigned to the interface  112  of the requester host  11  to the selected storage subsystem  12 F and instructs the storage subsystem  12 F to allow accesses through the interface  112 . (Step  510 )  
     [0106] Receiving information about the allocated logical device from the storage subsystem  12 F (Step  511 ), the storage management server  13  instructs the FC switch  18  to change its zoning so as to allow the logical device to be accessed through the interface  112  of the requester host  11 . The logical device information sent from the storage subsystem  12 F includes the WWN assigned to the port  123  of the storage subsystem  12 F and the LUN which allows the port  123  to identify the logical device (Step  512 ).  
     [0107] Then, the storage management server  13  sends the logical device information received from the storage subsystem  12 F to the requester host  11  (Step  513 ).  
     [0108]FIG. 6 is a flowchart of logical device allocation processing by a storage subsystem  12 F. This processing is executed by the storage subsystem  12 F when a logical device allocation request is received from the storage management server  13 .  
     [0109] Receiving a logical device allocation request from the storage management server  13 , the storage subsystem  12 F searches the logical device management tables for an offline logical device which meets the received device requirements (Step  601  and Step  602 ).  
     [0110] If no logical device does not exist which meets the device requirements, the storage subsystem  12 F notifies the storage management server  13  that it cannot allocate a logical device and terminates this allocation processing (Step  605 ).  
     [0111] If a logical device exists which meets the device requirements, the storage subsystem  12 F changes the status entry  8  in the corresponding logical device management table to “online”and defines the LUN to the port  123  reachable from the interface  12  of the requester host  11 . In addition, the storage subsystem  12 F registers the received requester host&#39;s WWN to the connected host name entry  88  of the logical device management table so as to allow the requester host  11  to access the allocated logical device via the interface  112  of the requester host (Step  603 ). Then, logical device information about the allocated logical device is sent to the storage management device (Step  604 ).  
     [0112]FIG. 7 is a flowchart of file system creation processing the file server  19  performs by executing the server program  193 . This processing is executed when a file system creation request is received from the storage management server  13 .  
     [0113] Receiving a file system creation request and device requirements from the storage management server  13 , the file server searches the logical device management tables for an offline logical device which meet the received device requirements (Steps  701  and  702 ). If no logical device does not exist which meets the device requirements, the file server  19  notifies the storage management server  13  that it cannot create a file system and terminates this allocation processing (Step  708 ).  
     [0114] If a logical device exists which meets the device requirements, the file server  19  instructs the storage subsystem  12   c  to set the logical device “online” (Step  703 ).  
     [0115] Receiving logical device information about the logical device, which is set online as above, from the storage subsystem  12   c  (Step  704 ), the file server  19  reconfigures itself so as to make the logical device of concern available from the file server  19  (Step  705 ). Then, the file server  19  creates a file system in the logical device (Step  706 ) and sends information about the created file system to the storage management server  13  (Step  707 ). Explanation of Steps  705  and  706  is omitted here because these steps can be implemented in the same manner as Steps  405  and  406  in FIG. 4.  
     [0116] Implementation of the present invention is not limited to the embodiment described so far. A number of variations are possible within the scope of the present invention.  
     [0117] For example, instead of selecting a logical device of concern on the basis of the performance/reliability level information about each storage subsystem  12  included in the block-accessed storage&#39;s/file-accessed storage&#39;s configuration information held by the storage management server  13 , the computer system can be configured in such a manner that the storage management server  13  receives logical device configuration information from the storage subsystems  12  each time the configuration is changed and determines the performance/reliability level of each storage subsystem  12  on the basis of the received information.  
     [0118] While such an embodiment increases communication between the storage management server  13  and the storage subsystems  12  and therefore may impose substantial loads on the server and the network, it has such advantages that the latest information about the logical device configuration can be obtained and the storage server  13  need not install a memory to retain the configurational information about all logical devices.  
     [0119] It is also possible to configure the computer system in such a manner that the storage management server  13  selects some candidate storage subsystems  12  for allocation on the basis of the performance/reliability level and other evaluation values the storage management server  13  retains about each storage subsystem  12 , sends the file system allocation request from the host  11  to each of the selected storage subsystems  12 , each selected storage subsystem  12  selects some candidate logical devices for allocation and the storage management server  13  finally selects a logical device for allocation from those candidate logical devices.  
     [0120] In addition, while the logical device allocation method is described so far based on the assumption that the states of the storage subsystems  12  are static, it is possible to realize more appropriate logical device allocation in terms of performance, etc. by taking into consideration the dynamically changing states of the storage subsystems  12  during operation of the computer system.  
     [0121] For example, what should dynamically be evaluated may include the load on the network when a storage subsystem  12  is accessed, the occupancy rates of the port  123 , the controller  122  and the disk unit  121  in the storage subsystem  12 , and the occupancy rate of the cache in the controller  122 . Accordingly, the computer system can be configured in such a manner that the storage management server  13  retains information about the current load on each storage subsystem  12 , which is evaluated comprehensively according to criteria prescribed by the computer system&#39;s administrator, user, etc., and selects a logical device on the basis of this information. Further, each storage subsystem  12  can be configured in such a manner that selection of a logical device within the storage subsystem  12  is made by taking into consideration the internal dynamic status information the storage subsystem  12  retains.  
     [0122] According to the present invention, it is possible to use storage devices having appropriate characteristics in a computer system where different kinds of storage devices coexist.  
     [0123] While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.