Patent Publication Number: US-7912814-B2

Title: Data migration in storage system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application relates to and claims priority from Japanese Patent Application No. JP2004-139306, filed on May 10, 2004, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a storage system for use in a computer system. 
     The data migration technology from a first storage system to a second storage system is described in Patent Document 1. 
     In Patent Document 1, once connected with a host computer, the second storage system responsively issues a read request to the first storage system so that data in the first storage system is copied into the second storage system. The second storage system is provided with a copy pointer for recoding the completion level of data copying to tell the progress of data migration. 
     During such data migration, an I/O request issued by the host computer is accepted by the second storage system. In an exemplary case where a read request is issued from the host computer during data migration, the second storage system refers to the copy pointer to see whether data in request is already at hand. If at hand, the second storage system forwards the data to the host computer. If not at hand, the second storage system reads the requested data from the first storage system for transfer to the host computer. 
     Here, Patent Document 1 is JP-A-2000-187608. 
     SUMMARY 
     In Patent Document 1, first, the connection between the first storage system and the host computer is cut off to establish another connection between the host computer and the second storage system. Then, data migration is performed from the first storage system to the second storage system. Once connected to the second storage system, the host computer issues an I/O request to the second storage system. 
     The concern here is that there is no disclosure in Patent Document 1 about how an access path is changed between the host computer and the corresponding storage system, especially about how to make settings to the second storage system for an access destination of the host computer. 
     At the time of data migration, if information about data access can be taken over from a migration source to a migration destination, the host computer can be allowed to make access to the migration destination under the same conditions as for the migration source. Accordingly, it is desired such taking-over is realized. 
     In view of the above, a connection is established over a network among a storage system, a computer, and a name server for managing interrelation between initiators and targets. The storage system includes first and second storage nodes. The first storage node is provided with a first logical unit to which a first target is set. The first target is the one interrelated to a first initiator set to the computer. The second storage node is provided with a second logical unit. 
     For data migration from the first logical unit to the second logical unit, the first storage node forwards data stored in the first logical unit to the second storage node, and thus received data is then stored in the second logical unit. The first storage node also forwards information about the first target to the second storage node. Using thus received information, the second storage node then makes a target setting to the second logical unit. 
     Based on an instruction coming from the name server, the computer makes detection if a target interrelated to its initiator is set to the second storage node. If detected as such, the computer issues an access request toward the second logical unit, and the second storage node receives the request. 
     At the time of data migration, not only data, information about data access can be also migrated from a migration source to a migration destination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an exemplary structure of a computer system in a first embodiment of the present invention; 
         FIG. 2  is a diagram showing an exemplary structure of a storage node; 
         FIG. 3  is a diagram showing an exemplary structure of memory provided to the storage node; 
         FIGS. 4A and 4B  are both a diagram showing an exemplary structure of a logical unit; 
         FIGS. 5A to 5D  are all a diagram showing an exemplary structure of an LU management table; 
         FIG. 6  is a diagram showing an exemplary structure of a name server; 
         FIG. 7A  is a diagram showing an exemplary name management table during data migration; 
         FIG. 7B  is a diagram showing another exemplary name management table after data migration; 
         FIG. 8  is a schematic diagram showing an exemplary process of migrating data in a logical unit from a storage node to another; 
         FIG. 9  is a flowchart of an exemplary process of, through addition of a new SN to the storage system of the first embodiment, migrating data from an LU of any existing SN to an LU of the newly-added SN; 
         FIG. 10  is a flowchart of an exemplary process of, through addition of a new SN to a network in a second embodiment of the present invention, migrating data from an LU of any existing SN to an LU of the newly-added SN; 
         FIG. 11  is a diagram showing an exemplary system structure in a third embodiment of the present invention; 
         FIG. 12  is a diagram showing an exemplary system structure in a fourth embodiment of the present invention; 
         FIG. 13  is a diagram showing an exemplary system structure in a fifth embodiment of the present invention; 
         FIG. 14  is a diagram showing an exemplary system structure in a sixth embodiment of the present invention; 
         FIG. 15A  is a diagram showing an exemplary display screen of a management console  4  having displayed thereon the system structure before data migration; 
         FIG. 15B  is a diagram showing another exemplary display screen of the management console  4  having displayed thereon the system structure after data migration; 
         FIG. 15C  is a diagram showing still another exemplary display screen of the management console  4  having displayed thereon the interrelation among an LU, a target, and an initiator before data migration; and 
         FIG. 15D  is a diagram showing still another exemplary display screen of the management console  4  having displayed thereon the interrelation among the LU, the target, and the initiator after data migration. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the below, exemplary embodiments of the present invention are described. Note that these embodiments are no more than examples, and the present invention is not restricted thereby. 
     In the accompanying drawings, component names and numbers are each provided with a lower-case alphabetic character such as a, b, or c for component distinction among those plurally provided in the same structure. If no such component distinction is required, no alphabetic character is provided to the component numbers. 
     First Embodiment 
     1. Exemplary System Structure ( FIG. 1 ) 
       FIG. 1  is a diagram showing an exemplary system structure in a first embodiment. 
     A computer system includes: a plurality of storage nodes (in the below, simply referred to as SNs)  1 , a plurality of host computers (in the below, hosts)  2 , a network  30 , a switch  3 , a management console  4 , and a name server  5 . The switch  3  is used for establishing a connection over the network  30  among a plurality of network nodes. The network node is the collective expression including the SNs  1 , the hosts  2 , the management console  4 , the name server  5 , and others, all of which are connected to the network  30 . The name server  5  is in charge of name management of the SNs  1  and the hosts  2 , and their logical connections. The management console  4  is provided for managing a storage system  1000  structured by a plurality of SNs  1 . Herein, the network  30  is a generic name for the switch  3  and a line for connecting the switch  3  with the hosts  2 , the SNs  1 , the management console  4 , the name server  5 , and others. In  FIG. 1 , the network  30  is encircled by a dashed line. 
     The SNs  1  are each provided with a controller (CTL)  10 , and a logical unit (LU) 12Xx being a logical disk unit to be accessed by the hosts  2 . Here, Xx denotes an identification of the corresponding LU, X is an integer of 0 or larger and x is a small letter of alphabet. The controller  10  exercises control over disks connected to the corresponding SN  1 , and executes access requests coming from the hosts  2 . 
     The hosts  2  are each a computer including a network controller for establishing a connection to a CPU, memory, and the network  30 . The memory includes an initiator management table  2112 , which will be described later. 
     Similarly to the hosts  2 , the management console  4  is a computer including a network controller for establishing a connection to a CPU, memory, and the network  30 . The memory stores a structure management program  4122 , an LU management table  1111 ′, an initiator management table  2112  or  1113 , and a target management table  1112 , all of which will be described later. The management console  4  includes input units such as a keyboard and a mouse, and output units such as a display. 
     2. Exemplary Structure of Storage Node (SN) ( FIG. 2 ) 
       FIG. 2  is a diagram showing an exemplary hardware structure of the SN  1 . 
     The SN  1  includes the controller (CTL)  10 , and a plurality of disks  120   y  to be connected to the CTL  10  through a Fibre Channel  1030 . The CTL  10  exercises control over input/output to/from the disks  120   y.    
     The CTL  10  includes: a CPU  100  exercising control over the SN  1 ; memory  101 ; a network controller  102  for establishing a connection to the network  30 ; an FC controller  103 ; and a bridge  104 . Specifically, the memory  101  stores control programs to be executed by the CPU  100  and control data, and serves as cache for increase the speed of disk access. The FC controller  103  is provided for controlling the Fibre Channel (FC)  1030  to be connected to the disks  120   y . The bridge  104  exercises control over data or program transfer between the CPU  100  and the memory  101 , data transfer between the network controller  102  and the memory  101 , and data transfer between the FC controller  103  and the memory  101 . 
     3. Exemplary Structure of Memory ( FIG. 3 ) 
       FIG. 3  is a diagram showing an exemplary structure of the memory  101  provided in the SN  1 . 
     The memory  101  is structured by a cache region  110 , a control data region  111 , and a control program region  112 . 
     To increase the speed of disk access from the hosts, the cache region  110  serves as a disk cache (in the below, simply referred to as cache) for temporarily storing data of the disks  120   y  or copies thereof. 
     The control data region  111  is provided for storing various tables and others for reference by the CPU  100  at the time of execution of the control programs. The various tables include a system structure management table  1110 , an LU management table  1111 , a target management table  1112 , and an initiator management table  1113 . Specifically, the system structure management table  1110  stores structure information about the storage system  1000  that is structured by a plurality of SNs  1 . The LU management table  1111  stores structure information about the LU 12Xx in the SN  1 . The target management table  1112  stores a target name (in the below, simply referred to as target) being a logical address provided to the LU 12Xx. The initiator management table  1113  stores an initiator name (in the below, simply refereed to as initiator) being a logical address of an access sources from which the LU 12Xx is accessed. 
     Note here that the target name or initiator name is exemplified by an iSCSI name in any system using the iSCSI protocol, a WWN (World Wide Name) in any FC systems, and others. The target name is not restrictive thereto as long as being a globally unique identifier assigned to an access destination and showing no change after created until deleted. This is applicable also to the initiator name. Herein, the target address or the initiator address may be used as information for identifying the access destination or the access source. The target address is exemplified by but not restricted to a Destination ID in any system using the FC protocol, and the initiator address is exemplified by but not restricted to a Source ID and others in any system using the FC protocol. The target name and the target address are both information used for identification of address destination, and the initiator name and the initiator address are both information used for identification of address source. Thus, the target address can be an alternative option for the target name, and the initiator address for the initiator name. In consideration thereof, the target name and the target address are hereinafter collectively referred to as “target name”, and this is true to the initiator. 
     The control program region  112  is provided for storing the control programs to be executed by the CPU  100 . The control program region  112  stores various programs as follows. That is, an operating system program  1120  serves as a basic program to execute the control programs in the environment; a TCP/IP program  1121  for data transmission and reception over the network  30  using the TCP/IP protocol; an iSCSI control program  1122  for connecting between the hosts  2  and the SNs  1  using the iSCSI protocol; and a target control program  1123  for controlling a target process at the time of access reception from the host  2  being the initiator to the LU 12Xx being the target of the iSCSI. Herein, the target process includes command reception from the host  2 , command interpretation after reception, and others. The various programs further include: a RAID control program  1124  for controlling RAID (Redundant Arrays of Inexpensive Disks) structured by a plurality of disks  120   y  of the SN  1 ; a cache control program  1125  for management control of the disk cache formed in the cache region  110 ; a disk control program  1126  for executing a disk control process such as command generation with respect to a single disk  120   y ; an FC control program  1127  for transmission and reception of command and data with the disk  120   y  via the FC through control over the FC controller  103 ; an LU control program  1128  for structuring the LU 12Xx being a logical volume through formation of RAID from the disks  120   y ; a migration program  1129  for executing a migration process for migrating data of the LU 12Xx among the SNs  1 ; an initiator control program  1130  for controlling the SN  1  to operate as initiator of iSCSI at the time of migration process to forward data of the LU 12Xx to any other SN  1 ; and a communications program  1131  for carrying out communications for name management with the name server  5  based on the iSCSI protocol specifications. 
     In the present embodiment, the network  30  is exemplified as an IP network for connection between the hosts  2  and the SNs  1 , the network protocol as the TCP/IP protocol, and the data protocol between the hosts  2  and the SNs  1  as the iSCSI protocol being a block I/O interface. The present invention is not surely restrictive thereto. 
     4. Exemplary Structure of LU ( FIGS. 4A and 4B ) 
       FIGS. 4A and 4B  are both a diagram showing an exemplary structure of the LU 12Xx. 
     The SN  1  in the present embodiment is presumably provided with three disks of  1200 ,  1201 , and  1202 . Surely, the number of disks  120   y  provided to the SN  1  is not restrictive thereto, and any number will do as long as at least one or larger. 
       FIG. 4A  is a diagram showing an exemplary structure of a RAID group (in the below, referred also to as RG). 
     The three disks of  1200 ,  1201 , and  1202  structure a RAID group 12 of RAID 5 type, and the stripe size thereof is S block. Herein, the block means a logical block defined by the SCSI protocol specifications, and a disk sector or 512 bytes is often defined as a logical block. The block size is not restrictive, and surely any other value will do. In the RAID group 12, data is divided on the basis of S block for placement among other disks adjacent to one another. A stripe string includes three storage regions locating in each different disk. One of such storage regions stores parity data as a result of exclusive OR calculation from data in other two storage regions. That is,
 
 P 0= D 0+ D 1 (where + denotes exclusive OR)  Equation 1
 
     The RAID group (RG) 12 structured as such includes two logical units LU 0  and LU 1 .  FIG. 4B  is a diagram showing an exemplary structure of a logical unit. The LU 0  ( 120 ) is a logical unit having the capacity of k block, and the LU 1  ( 121 ) is a logical unit having the capacity of n block. In the RAID group, the logical block address (in the below, referred to as RG LBA) for the LU 0  is in a range from 0 to k-1, and in a range from k to (k+n-1) for the LU 1 . Once LUs are structured, the LUs are each accessed from the hosts  2  using an LBA local to the corresponding LU (Local LBA) so that each LU can behave as if being an independent disk. That is, the Local LBA for the LU 0  ( 120 ) has the address starting from 0 to (k-1) being equal to the total capacity −1, and separately therefrom, the Local LBA for the LU 1  ( 121 ) has the address starting from 0 to (n-1). 
     5. Exemplary Structure of LU Management Table ( FIGS. 5A to 5D ) 
       FIGS. 5A to 5D  are all a diagram showing an exemplary structure of the LU management table  1111  stored in the memory  101  of the SN  1 . In the table, LU denotes an LU number, and RG denotes identification information of a RAID group having LUs structured therein. Further, Start RG LBA denotes an RG LBA located at the LU head in the RG, LEN denotes the LU capacity (unit of which is block), Initiator denotes an initiator name of any initiator allowed to access the corresponding LU, e.g., initiator set to the host, and Target denotes a target name assigned to the corresponding LU. 
       FIG. 5A  shows an exemplary LU management table  1111   a  of the SNa ( 1   a ). The LU 0   a  is located in the RG 0   a , and having the Start RG LBA of 0, the capacity of k, the initiator allowed to access thereto is the host (Host a)  2   a  with the initiator name of Init-a 0 , and the target name of Targ-a 0 . Similarly, the LU 1   a  is located in the RG 0   a , and having the Start RG LBA of k, the capacity of n, the initiator allowed to access thereto is the host (Host b)  2   b  with the initiator name of Init-b 0 , and the target name of Targ-a 1 . 
     Herein, although the LU and the target have a one-to-one relationship, there may be a case where a plurality of initiators are allowed to access a target. Once the LU management table is added with an initiator name into the column of Initiator, the target control program  1123  responsively allows access only to the LU 12Xx corresponding to the initiator whose initiator name is thus entered. When a plurality of initiators are allowed to access any one specific LU 12Xx, the column of Initiator in the LU management table  1111  is provided with a plurality of entries for registration of a plurality of initiator names. If there is no access limitation for the LU 12Xx, i.e., if every initiator is allowed to access the LU 12Xx, no name is entered into the column of Initiator corresponding to the LU 12Xx (enter NULL). The details of interrelation between the initiator name and the target name are left for later description. 
     The management console  4  also includes in the memory the LU management table  1111 ′, which is a combination result of the LU management table  1111  each included in the SNs  1  connected to the network  30 . Compared with the LU management table  1111 , the LU management table  1111 ′ is additionally provided with identification information for the corresponding SN  1  as shown in  FIG. 15C . 
     6. Exemplary Structure of Name Server ( FIG. 6 ) 
       FIG. 6  is a diagram showing an exemplary structure of the name server  5 . The name server  5  is provided with: a CPU  500  in charge of control entirely over the name server  5 ; memory  501  for storing control programs to be executed by the CPU  500  and control data; a network controller  502  for connecting to the network  30 ; and a bridge  504  exercising control over data or program transfer between the CPU  500  and the memory  501 , and data transfer between the network controller  502  and the memory  501 . 
     The memory  501  has a control data region  511 , and a control program region  512 . 
     The control data region  511  is provided for storing various tables and others for reference by the CPU  500  when executing the control programs. The control data region  511  stores a name management table  5111  including initiator and target names for iSCSI, and the connection relation between the initiator and the target. 
     The control program region  512  is provided for storing the control programs to be executed by the CPU  500 . The control program region  512  stores various programs as follows. That is, an operating system program  5120  serving as a basic program to execute the control programs in the environment; a TCP/IP program  5121  for data transmission and reception over the network  30  using the TCP/IP protocol; a name management program  5122  in charge of name management of the iSCSI nodes (i.e., hosts  2  and storage nodes SNs  1 ) to be connected over the network  30 , and controlling the interrelation between the initiators and iSCSI nodes; and a communications program  5123  for carrying out communications for name management of initiators (e.g., hosts  2 ) and targets (e.g., SNs  1 ) based on the iSCSI protocol specifications. 
     In the present embodiment, the name server  5  is exemplified by an iSNS (iSCSI Name Server) of the iSCSI protocol specifications. This is not surely restrictive, and to realize the present embodiment, any other name server specifications can be used to construct a name server. 
     7. Exemplary Structure of Name Management Table ( FIGS. 7A and 7B ) 
       FIGS. 7A and 7B  are both a diagram showing an exemplary name management table  5111  stored in the memory  501  of the name server  5 . The name management table  5111  includes the initiator management table ( 2112  or  1113 ) and the target management table  1112 . 
     In the initiator management table  2112  of  FIGS. 7A and 7B , Initiator denotes an initiator name under the management of an entry of the table, Entity denotes an identifier specifying to which device the initiator belongs, Portal denotes a portal including the initiator, and PortalGr denotes a portal group including the portal. 
     In the target management table  1112  of  FIGS. 7A and 7B , Target denotes a target name under the management of an entry of the table, Initiator denotes an initiator name allowed to access the target, Entity denotes an identifier specifying to which device the target belongs, Portal denotes a portal including the target, and PortalGr denotes a portal group including the portal. 
     Note that the initiator management table in the name management table  5111  is the same as the initiator management table stored in the memory of the device having the initiator. Similarly, the target management table in the name management table  5111  is the same as the target management table stored in the memory of the device having the target. Further, the management console  4  includes, in the memory, the initiator management table and the target management table being the same as those in the name server  5 . 
     For example, initiator management tables  2112   a  and  2112   b  of  FIG. 7A  are both an initiator management table for an initiator of the host a ( 2   a ) or the host b ( 2   b ). The Host a ( 2   a ) includes in the memory the initiator management table  2112   a  similar to the one shown in  FIG. 7A , and the Host b ( 2   b ) includes in the memory the initiator management table  2112   b  similar to the one shown in  FIG. 7A . Similarly, the initiator management table  1113  of  FIG. 7A  is an initiator management table for an initiator located in the SNa ( 1   a ), and the SNa ( 1   a ) includes in the memory  101  the initiator management table  1113  similar to the one shown in  FIG. 7A . Further, target management tables  1112   a  and  1112   b  of  FIG. 7A  are both a target management table for a target of the SNa ( 1   a ) or the SNb ( 1   b ) . The SNa ( 1   a ) includes in the memory  101  the target management table  1112  similar as the target management table  1112   a , and the SNb ( 1   b ) includes in the memory  101  a target management table  1112  similar to the target management table  1112   b.    
     As is known from the above, the name server  5  uses the name management table  5111  to collectively manage the initiator management tables of the initiators connected to the network  30 , and the target management tables of the targets connected to the network  30 . 
     Refer back to  FIG. 7A , which exemplarily shows three pairs of initiator and target. 
     A first pair includes an initiator Init-a 0  and a target Targ-a 0 . The initiator Init-a 0  is located in a portal Ia 0  of the Host a ( 2   a ), and belonging to a portal group IPGa 0 . The target Targ-a 0  is located in a portal Ta 0  of the SNa ( 1   a ), and belonging to a portal group TPGa 0  to allow the initiator Init-a 0  to access thereto. 
     A second pair includes an initiator Init-b 0  and a target Targ-a 1 . The initiator Init-b 0  is located in a portal Ib 0  of the Host b ( 2   b ), and belonging to a portal group IPGb 0 . The target Targ-a 1  is located in a portal Tal of the SNa ( 1   a ), and belonging to a portal group IPGa 1  to allow the initiator Init-a 0  to access thereto. 
     A third pair includes an initiator Init-SNa 1  and a target Targ-b 0 . The initiator Init-SNa 1  is located in a portal ISNa 1  of the SNa ( 1   a ), and belonging to a portal group IPGSNa 1 . The target Targ-b 0  is located in a portal Tb 0  of the SNb ( 1   b ), and belonging to a portal group IPGb 0 . 
     Herein, the portal denotes a logical portal located in the Host  2  or the network controller of the SN  1 , and structured by a pair of an IP address of a physical port and a TCP port number. The portal can be plurally provided if anyone specific physical port is provided with a plurality of TCP ports. The portal group includes a plurality of portals as an aggregate to be used as a single communications path. In the below, no mention is made to the portal group except for the group name. 
     The pairs of initiator and target are made between any initiators and targets connected to the network  30 , and managed by the name management table  5111 . 
     8. Exemplary SN Add-In and LU Migration Process 
     Described now is a process of achieving the load balance among the SNs  1  through addition of a new storage node  1  to the storage system  1000 , and through data migration from the LU 12Xx of any existing storage node  1  to the newly-provided SN  1 . 
       FIG. 8  is a schematic diagram showing, through addition of a new SN  1  to the storage system  1000 , an exemplary process of data migration from the LU 12Xx of any existing SN  1  to the newly-added SN  1 . Note that  FIG. 8  shows the state halfway through the construction process of the system of  FIG. 1 . 
     Assuming here is that, as the first stage, the storage system  1000  does not include the SNb ( 1   b ) but only the SNa ( 1   a ), and includes the Host a ( 2   a ) and Host b ( 2   b ). 
     The Host a ( 2   a ) is making access to an LU 0   a  ( 120   a ) of the SNa ( 1   a ), and the Host b ( 2   b ) is making access to an LU 1   a  ( 121   a ) of the SNa ( 1   a ). 
     The Host a ( 2   a ) includes an initiator, which is entered to, as the initiator name of Init-a 0 , both the initiator management table  2112   a  of the Host a ( 2   a ) and the name management table  5111  of the name server  5 . Similarly, the Host b ( 2   b ) includes an initiator, which is entered to, as the initiator name of Init-b 0 , both the initiator management table  2112   b  of the Host b ( 2   b ) and the name management table  5111  of the name server  5 . 
     The LU 0   a  ( 120   a ) of the SNa ( 1   a ) is added as the target name of Targ-a 0  to the target management table  1112  of the SNa ( 1   a ) and the name management table  5111  of the name server  5 . Also added to the target management table  1112  and the name management table  5111  is Init-a 0  as the initiator allowed to access the target Targ-a 0 . Similarly, the LU 1   a  ( 121   a ) of the SNa ( 1   a ) is added as the target name of Targ-a 1  to the target management table  1112  of the SNa ( 1   a ) and the name management table  5111  of the name server  5 . Also added to the target management table  1112  and the name management table  5111  is Init-b 0  as the initiator allowed to access the target of Targ-a 1 . 
     As such, two pairs of Init-a 0  and Targ-a 0 , and Init-b 0  and Targ-a 1  are made.  FIG. 7A  shows the name management table  5111  under such pair making. The target management table  1112  and the name management table  5111  are added with initiators in accordance with the iSCSI protocol specifications. Assuming here is that the Host a ( 1   a ) is already operating under the state accessible to the LU 0   a  ( 120   a ), and the Host b ( 1   b ) under the state accessible to the LU 1   a  ( 121   a ). That is, as shown in  FIG. 5A , the LU management table  1111  in the memory  101  of the SNa ( 1   a ) includes Targ-a 0  as the target name of the LU 0   a  ( 120   a ), and Init-a 0  as the initiator in the Host a ( 1   a ) that is allowed to access the Lu 0   a  ( 120   a ). Similarly, the LU management table  1111  includes Targ-a 1  as the target name of the LU 1   a  ( 121   a ), and Init-b 0  as the initiator in the Host b ( 1   b ) allowed to access the Lu 1   a ( 121   a ) 
     By referring to  FIGS. 8 and 9 , described next is a process of data migration from the LU 1   a  ( 121   a ) to the SNb ( 1   b ) newly added to the storage system  1000  due to overloaded SNa ( 1   a ), for example.  FIG. 9  is a flowchart of an exemplary process of, through addition of a new SN  1  to the storage system  1000 , migrating data from an LU 12Xx of any existing SN  1  to an LU 12Xx of the newly-added SN  1 . 
     9. Add-In of Storage Node SNb (Step  9001  of  FIG. 9 ) 
     First, the SNb ( 1   b ) is connected to the switch  3  to add the SNb ( 1   b ) to the storage system  1000  (step  9001  of  FIG. 9 ). The SNb ( 1   b ) is assumed to have a storage region enough for storage of data in the LU 1   a  ( 121   a ) of the SNa ( 1   a ). 
     10. Study of Migration Source LU (Step  9002  of  FIG. 9 ) 
     The CPU of the management console  4  goes through the structure management program  4122  to acquire information about the LU 1   a  ( 121   a ), which is the destination LU (step  9002 ). In the below, when a process is executed by the CPU going through any corresponding program, simply referred to as “the program goes through the process”. 
     To be specific, the structure management program  4122  asks the SNa ( 1   a ) for structure information of the LU 1   a  ( 121   a ). In response to such a request, the LU control program  1128  of the SNa ( 1   a ) refers to the LU management table  1111  to forward the applicable structure information of the LU 1   a  ( 121   a ) to the management console  4 . The structure information includes information in the LU management table  1111  of the SNa ( 1   a ), and information about the RG structure (RAID structure) including the LU 1   a  ( 121   a ) structured therein. The structure management program  4122  enters, into the LU management table  1111 ′ stored in its own memory, the information received from the SNa ( 1   a ) together with the identification information of the SNa ( 1   a ). Then, based on thus received information, the LU 1   a  ( 121   a ) is identified as being the LU having the capacity of n block in the RAID group of RAID5 structure. 
     Herein, the structure management program  4122  may skip step  9002  if the management console  4  already has information about the SNs  1  in the storage system  1000 , i.e., information in the LU management table  1111 , and the RAID structure of the respective LUs, and if the management console  4  is exercising control over the structure information using its own LU management table  1111 ′. 
     11. Construction of Migration Destination LU and Target Registration (Step  9003  of  FIG. 9 ) 
     Next, the structure management program  4122  of the management console  4  instructs the SNb ( 1   b ) to construct an LU 0   b  ( 120   b ) having the same capacity as the LU 1   a  ( 121   a ) being the migration source to any appropriate RAID group of the newly added SNb ( 1   b ). Here, the RAID group considered appropriate may be the one having the same RAID structure as the LU 1   a  ( 121   a ). 
     The structure management program  4122  also instructs the SNb ( 1   b ) to set thus newly constructed LU 0   b  ( 120   b ) as a target to the portal Tb 0  of the physical port and the portal number designated by the SNb ( 1   b ), and the Portal group TPGb 0 . 
     When the SNb ( 1   b ) receives such an instruction, the LU control program  1128  constructs the LU 0   b  ( 120   b ) so that a target having the target name of Targ-b 0  is created to the portal Tb 0  and the portal group TPGb 0 . Then, as shown in  FIG. 5B , the LU management table  1111   b  is added with Targ-b 0  for target name, LU 0   b  for LU, RG 0   b  for RG, 0 for Start RG LBA, and n for LEN. 
     The communications program  1131  of the SNb ( 1   b ) forwards a request to the name server  5  to enter any new target thereto. Upon reception of such a request, the name server  5  registers the target management table  1112   b  of  FIG. 7A  to the name management table  5111  as information about the new target. At this point, the target management table  1112   b  is storing Targ-b 0  for target name, SNb for Entity, Tb 0  for Portal, and TPGb 0  for PortalGroup, and the column of Initiator is vacant, which will be filled in step  9005  that is described later. 
     The target control program  1123  of the SNb ( 1   b ) enters, also to the target management table  1112  in its own memory  101 , the same contents as stored in the target management table  1112   b  in the name management table  5111  of the name server  5 , i.e., Targ-b 0  for target name, SNb for Entity, Tb 0  for Portal, and TPGb 0  for PortalGroup (step  9003  of  FIG. 9 ). 
     In the above manner, by the SNb ( 1   b ), the LU 0   b  ( 120   b ) is constructed, and the target Targ-b 0  is registered. The construction information about the LU 0   b  ( 120   b ) and the contents of the target management table  1112  of the target Targ-b 0  are forwarded from the SNb ( 1   b ) to the structure management program  4122  of the management console  4 . In this manner, the information is also registered into the LU management table  1111 ′ and the target management table  1112  of the management console  4 . Here, the structure information about the LU 0   b  ( 120   b ) includes the RAID structure of the RAID group of the LU 0   b  ( 120   b ), and the information of the LU 0   b  ( 120   b ) entered to the LU management table of the SNb ( 1   b ). 
     12. Construction of Initiator to Migration Source SN (Step  9004  of  FIG. 9 ) 
     Next, the structure management program  4122  of the management console  4  instructs the SNa ( 1   a ) being the migration source for initiator construction to the portal ISNa 1  having the designated physical portal and port number, and the portal group IPGSNa 1 . 
     When the SNa ( 1   a ) receives such an instruction, the initiator control program  1130  responsively creates an initiator having the initiator name of init-SNa 1  to the portal ISNa 1 , and the portal group IPGSNa 1 . Then, the communications program  1131  asks the name server  5  to enter the resulting initiator thereto. 
     Upon reception of such a request, the name server  5  registers to the name management table  5111  an initiator management table  1113 SNa 1  of  FIG. 7A  as information about thus newly-constructed initiator. The initiator management table  1113 SNa 1  already has init-SNa 1  for initiator name, SNa for Entity, ISNa 1  for Portal, and IPGSNa 1  for PortalGroup. 
     Here, the initiator control program  1130  of the SNa ( 1   a ) enters, also to the initiator management table  1113  in its own memory  101 , the same contents as stored in the initiator management table  1113 SNa 1  in the name management table  5111  of the name server  5 , i.e., init-SNa 1  for initiator name, SNa for Entity, ISNa 1  for Portal, and IPGNa 1  for PortalGroup. 
     In the above manner, the SNa ( 1   a ) is through with initiator construction, and the contents of the initiator management table  1113  of the initiator init-SNa 1  are forwarded from the SNa ( 1   a ) to the structure management program  4122  of the management console  4  so as to be entered to the initiator management table  1113  of the management console  4 . 
     13. Initiator Registration of Migration Source SN to Target of Migration Destination SN (Step  9005  of  FIG. 9 ) 
     Next, the structure management program  4122  of the management console  4  issues an instruction towards the SNb ( 1   b ) to provide the initiator init-SNa 1  of the SNa ( 1   a ) with an access permission for the target Targ-b 0 . 
     After the SNb ( 1   b ) receives such an instruction, as shown in  FIG. 5B , the LU control program  1128  enters an initiator of Init-SNa 1  to the LU management table  111   b  as an initiator for access permission to the target Targ-b 0 , i.e., the LU 0   b . Further, the target control program  1123  of the SNb ( 1   b ) enters the initiator of Init-SNa 1  to the target management table  1112  of the target Targ-b 0  as an initiator for access permission to the target Targ-b 0 . 
     Then, the SNb ( 1   b ) asks the name server  5  to enter an initiator of Init-SNa 1  to the target management table  1112   b  as an initiator allowed to access the target Targ-b 0 . Here, the target management table  1112   b  is the one registered into the name management table  5111  in step  9003 . In this manner, on the name management table  5111  of the name server  5 , the relation between the initiator Init-SNa 1  and the target Targ-b 0  (LU 0   b ) is established. 
     As such, the initiator of the migration source SN is successfully entered to the target of the migration destination SN. 
     Here, also to the LU management table  1111 ′ in the memory and the target management table  1112  of the target Targ-b 0 , the structure management program  4122  of the management console  4  enters Init-SNa 1  as an initiator allowed to access the target Targ-b 0 . 
     14. Execution of Discovery (Step  9006  of  FIG. 9 ) 
     Through registration of a new pair of initiator and target to the name management table  5111  of the name server  5  in step  9005 , the initiator-target relation under the management of the name server  5  shows some change. To deal with such a change, the name management program  5122  of the name server  5  issues a State Change Notification (SCN) to the corresponding initiators, i.e., devices such as the hosts  2  and SNs  1  each including an initiator. The initiators received such an SCN go through a process referred to as discovery. During discovery, the initiators each make an inquiry to the name server  5  whether any change has occurred to the targets accessible thereby, i.e., whether the accessible target(s) have been added or deleted. Upon reception of such an inquiry, the name server  5  responsively makes a search of the name management table  5111  based on the initiator name included in the inquiry. After the search, a response is made about the target management information about any target(s) accessible by the inquiring initiator, i.e., information having been registered in the target management table. 
     In step  9006 , as for the initiators located in the hosts  2 , no change is observed for the targets accessible by the corresponding initiator. Thus, even if the host  2  goes through discovery, no target change is discovered, and nothing happens. 
     On the other hand, after the SNa ( 1   a ) receives the SCN, the initiator control program  1130  asks the iSCSI control program  1122  to go through discovery. As a result, the iSCSI control program  1122  is notified, by the name server  5 , of a new target Targ-b 0  corresponding to the initiator Init-SNa 1  of the SNa ( 1   a ). 
     In response thereto, the initiator control program  1130  of the SNa ( 1   a ) instructs the TCP/IP program  1121  to establish any new TCP connection between the TCP port of the SNa ( 1   a ) and the TCP port of the SNb ( 1   b ). 
     Then, the initiator control program  1130  instructs the iSCSI control program  1122  to go through an iSCSI log-in process to establish a new iSCSI session between the portal ISNa 1  and the portal Tb 0  of the SNb ( 1   b ). In this manner, a communications path using iSCSI is established between the SNa ( 1   a ) and the SNb ( 1   b ). 
     Next, the initiator control program  1130  of the SNa ( 1   a ) issues an iSCSI Inquiry command to the target Targ-b 0  of the SNb ( 1   b ) to detect an LU 0   b . This allows the SNa ( 1   a ) to access the LU 0   b  ( 120   b ) of the SNb ( 1   b ). 
     15. Execution of LU Migration (Step  9007  of  FIG. 9 ) 
     The structure management program  4122  of the management console  4  issues an instruction toward the SNa ( 1   a ) to migrate data in the LU 1   a  ( 121   a ) to the LU 0   b  ( 120   b ) of the SNb ( 1   b ). 
     Upon reception of such an instruction, the SNa activates the migration program  1129 . Using the TCP session established in step  9006 , the migration program  1129  communicates with the migration program  1129  of the SNb ( 1   b ) under any specific protocol to check the state of LU 0   b  ( 120   b ), and whether the LU 1   a  ( 121   a ) and the LU 0   b  ( 120   b ) are in the same size or not, for example. Then, the SNb ( 1   b ) is notified that migration is now started. 
     Then, the migration program  1129  of the SNa ( 1   a ) issues a command to the target control program  1123 . In response thereto, the target control program  1123  reads, to the cache  110 , data of the LU 1   a  ( 121   a ) by any appropriate size. The migration program  1129  issues another command to the initiator control program  1130 . In response, the initiator control program  1130  issues an iSCSI writing command to the LU 0   b  ( 120   b ) of the SNb ( 1   b ) to write the data read to the cache  110 . After receiving the writing command and the data, the SNb ( 1   b ) stores the data into the cache  110 , and then writes the data thus stored in the cache  110  to the LU 0   b  ( 120   b ). By repeating such a procedure, the data in the LU 1   a  ( 121   a ) is completely copied into the LU 0   b  ( 120   b ) (( 1 ) of  FIG. 8 ). 
     Note here that during such a copying process, the initiator init-b 0  of the Host b ( 2   b ) keeps accessing the LU 1   a  ( 121   a ) of the SNa ( 1   a ), i.e., target Targ-a 1 . 
     During the copying process, if the SNa ( 1   a ) receives from the Host b ( 2   b ) the writing command and the writing data to the LU 1   a  ( 121   a ), the migration program  1129  of the SNa ( 1   a ) writes the writing data to the LU 1   a  ( 121   a ), and also forwards the writing data to the LU 0   b  ( 120   b ) of the SNb ( 1   b ). Then, the SNa ( 1   a ) reports the Host b ( 2   b ) that the writing process is through, i.e., periodical data writing to the LU 0   b  ( 120   b ). 
     As an alternative manner, storage regions storing different data between the migration source LU 1   a  ( 121   a ) and the migration destination LU 0   b  ( 120   b ) may be managed by the SNa ( 1   a ) using a differential bit map. To be specific, the SNa ( 1   a ) makes a registration of a differential bit for any storage region on the differential bit map. Here, the storage region is the one not yet through with data copying from the LU 1   a  ( 121   a ) to the LU 0   b  ( 120   b ), and the one through with copying but thereafter showing no data coincidence between the LU 1   a  ( 121   a ) and the LU 0   b  ( 120   b ) due to data update in the LU 1   a  ( 121   a ). This update is caused by reception of writing data addressed to the LU 1   a  ( 121   a ) from the Host b ( 2   b ). Based on the differential bit map, the SNa ( 1   a ) may write the data stored in the LU 1   a  ( 121   a ) to the LU 0   b  ( 120   b ) after the data copying process is through only for the storage region having been registered with the differential bit. In this manner, the writing data received from the Host b ( 2   b ) during the copying process can be copied to the LU 0   b  ( 120   b ) being the migration destination. 
     As such, by the time when the copying process is through, the data in the LU 1   a  ( 121   a ) and the data in the LU 0   b  ( 120   b ) are to be the same (( 1 ) of  FIG. 8 ). This is the end of data copying. 
     16. Copying of Target (Step  9008  of  FIG. 9 ) 
     Once the copying process is through, the migration program  1129  of the SNa ( 1   a ) instructs the LU control program  1128  to refer to the LU management table  1111  so that the target of the LU 1   a  ( 121   a ), i.e., Targ-a 1 , and the initiator thereof, i.e., Init-b 0 , are acquired from the LU management table  1111   a  of  FIG. 5A . Then, the migration program  1129  of the SNa ( 1   a ) uses any new or existing TCP connection between the SNa ( 1   a ) and the SNb ( 1   b ), e.g., the TCP connection established in step  9006 , to transfer information about thus acquired initiators and targets of the LU 1   a  ( 121   a ). 
     Then, the migration program  1129  of the SNb ( 1   b ) issues an instruction to the LU management program  1128 . The LU management program  1128  responsively enters, to the LU management table  1111  of the LU 0   b  ( 120   b ) of  FIG. 5C , Targ-a 1  to Target, and Init-b 0  to Initiator. More in detail, the LU management program  1128  enters the target and initiator of the LU 1   a  received from the SNa ( 1   a ) to the LU management table of the LU 0   b  ( 120   b ) to change the target and initiator of the LU 0   b  ( 120   b ) to those of the LU 1   a  ( 121   a ). In this manner, the data and the access information, i.e., target and initiator, of the LU 1   a  ( 121   a ) of the SNa ( 1   a ) are taken over by the LU 0   b  ( 120   b ) of the SNb ( 1   b ), and this is the end of LU migration. 
     After completion of LU migration as such, a completion notice is forwarded by the SNb ( 1   b ) to the SNa ( 1   a ), and by the SNa ( 1   a ) to the structure management program  4122  of the management console  4 . Upon reception of the completion notice, the management console  4  enters, also the its own LU management table  1111 ′, Targ-a 1  to the Target of the LU 0   b  ( 120   b ), and Init-b 0  to the Initiator thereof. 
     As such, the LU migration process is completed. 
     17. Deletion of Initiator being Migration Source (Step  9009  of  FIG. 9 ) 
     After receiving the completion notice of LU migration, the structure management program  4122  of the management console  4  instructs the SNa ( 1   a ) to go through initiator deletion. The SNa ( 1   a ) responsively instructs the initiator control program  1130  to cut off the connection between the initiator Init-SNa 1  and the target Targ-b 0  used for data migration, and delete the initiator Init-SNa 1 . The initiator control program  1130  instructs the iSCSI control program  1122  to cut off the session between the initiator Init-SNa 1  and the target Targ-b 0 . Also, the initiator control program  1130  deletes the initiator management table  1113  about the initiator Init-SNa 1  from the memory  101 , and instructs the name server  5  to delete the initiator management table  1113 SNa 1  about the initiator Init-SNa 1 . 
     The name server  5  instructed as such accordingly deletes the initiator management table  1113 SNa 1  having been registered in the name management table  5111 . 
     As such, the initiator Init-SNa 1  is deleted by following, in reverse, steps  9004  and  9005  of initiator registration. 
     The structure management program  4122  of the management console  4  also deletes the initiator management table  1113  of the initiator Init-SNa 1  stored in its own memory. 
     18. Deletion of Migration Source Target (Step  9010  of  FIG. 9 ) 
     The structure management program  4122  of the management console  4  instructs the SNa ( 1   a ) to cut off the session established between the target Targ-a 1  set to the LU 1   a  ( 121   a ) being the migration source and the initiator Init-b 0  located in the Host b ( 2   b ), and to delete the target Targ-a 1  set to the migration source LU 1   a  ( 121   a ). 
     The LU control program  1128  of the SNa ( 1   a ) instructed as such then responsively issues an instruction toward the iSCSI control program  1122  to cut off the session between the initiator Init-b 0  of the Host-b ( 2   b ) and the target Targ-a 1  of the SNa ( 1   a ), and the iSCSI program  1122  responsively executes the instruction. The LU control program  1128  deletes, from the LU management table  1111   a  of  FIG. 5A , any entry relating to the LU 1   a  ( 121   a ) As a result, the LU management table in the memory  101  of the SNa ( 1   a ) looks like an LU management table  1111   a  of  FIG. 5D . Further, the SNa ( 1   a ) deletes the entry of Targ-a 1  from the target management table  1112  in the memory  101 . 
     The communications program  1131  of the SNa ( 1   a ) instructs the name server  5  to delete, also from the name management table  5111 , any entry relating to the target Targ-a 1  in the target management table  1112 . The name server  5  then responsively goes through deletion as instructed (( 2 ) of  FIG. 8 ). 
     Here, the structure management program  4122  of the management console  4  deletes any entry relating to the LU 1   a  ( 121   a ) from the LU management table  1111 ′ in its own memory, and also deletes the target management table relating to the target Targ-a 1 . 
     19. Change of Migration Destination Target (Step  9011  of  FIG. 9 ) 
     The structure management program  4122  of the management console  4  then instructs the SNb ( 1   b ) to enter, to the name server  5 , the target Targ-a 1  having been set to the migration destination LU 0   b  ( 120   b ) in step  9008 . 
     The communications program  1131  of the SNb ( 1   b ) instructed as such notifies, in a similar manner to step  9003 , the name server  5  to change the target name and the initiator name in the target management table  1112   b  of the name management table  5111  into target: Targ-a 1 , and initiator: Init-b 0  (( 3 ) of  FIG. 8 ). The name management program  5122  of the name server  5  changes the name management table  5111  as notified. The resulting name management table  5111  looks like the one shown in  FIG. 7B . 
     The target control program  1123  of the SNb ( 1   b ) also applies the same change to be done by the name server  5 . That is, the target management table  1113  stored in the memory  101  of the SNb ( 1   b ) is changed similarly. Specifically, in the target management table  1113 , target is changed from Targ-b 0  to Targ-a 1 , and initiator is changed from Init-SNa 1  to Init-b 0  so as to include Target: Targ-a 1 , Initiator: Init-b 0 , Entity: SNb, Portal: Tb 0 , and PortalVr: TPGb 0 . 
     The structure management program  4122  of the management console  4  stores, into its own memory, a new target table  1113  of the target Targ-a 1 , which is including Target: Targ-a 1 , Initiator: Init-b 0 , Entity: SNb, Portal: Tb 0 , and PortalVr: TPGb 0 . 
     20. Execution of Discovery (Step  9012  of  FIG. 9 ) 
     In consideration of the initiator-target relation changed in step  9011 , the name management program  5122  of the name server  5  issues a State Change Notification (SCN) to the initiators (( 4 ) of  FIG. 8 ). In response to such an SCN, the initiators each execute discovery to inquire the name server  5  whether any change has occurred to their own accessible targets. 
     After the Host b ( 2   b ) receives the SCN, and after an inquiry is issued to the name server  5  through execution of discovery (( 5 ) of  FIG. 8 ), the Host b ( 2   b ) is notified from the name server  5  of management information about the target Targ-a 1  relating to the initiator Init-b 0 . Here, the management information is the one registered in the target management table  1112   b  of the target Targ-a 1 . Accordingly, this tells the Host b ( 2   b ) that the target Targ-a 1  relating to the initiator Init-b 0  has moved to the SNb ( 1   b ). 
     Thus, a TCP/IP program (not shown) of the Host b ( 2   b ) establishes a new TCP connection between the TCP port of the Host b ( 2   b ) and the TCP port of the SNb ( 1   b ). 
     Then, the iSCSI control program (not shown) of the Host b ( 2   b ) goes through an iSCSI log-in process to the SNb ( 1   b ) to establish a new iSCSI session between the portal Ib 0  of the Host b ( 2   b ) and the portal Tb 0  of the SNb ( 1   b ). As a result, a communications path using iSCSI is established between the Host b ( 2   b ) and the SNb ( 1   b ), and thus path switching is completed (( 6 ) of  FIG. 8 ). Accordingly, hereinafter, if the initiator Init-b 0  of the Host b ( 2   b ) forwards a writing command and writing data to the target Targ-a 1 , the SNb ( 1   b ) including the target Targ-a 1  receives the command and data. The writing data is thus stored in the LU 0   b  ( 120   b ) including the target Targ-a 1 . 
     In the present embodiment, when data stored in the LU 1   a  ( 121   a ) of the SNa ( 1   a ) is migrated into the LU 0   b  ( 120   b ) of the SNb ( 1   b ) being the migration destination, the LU 0   b  ( 120   b ) takes over not only the data but also access information. Here, the access information includes target names of targets set to the LU 1   a  ( 121   a ) being the migration source, and initiator names of initiators allowed to access the targets. Therefore, the Host b ( 2   b ) having gone through discovery acknowledges that the target Targ-a 1  corresponding to its initiator init-b 0  is changed in location from SNa ( 1   a ) to SNb ( 1   b ). That is, the Host b ( 2   b ) does not acknowledge that the target has been changed. This is because the target name Targ-a 1  corresponding to the initiator Init-b 0  shows no change even after data migration. Thus, in the present embodiment, as long as the target name Targ-a 1  is not changed, even if the location of the target is changed, the data stored in the LU corresponding to the target is guaranteed as not having been changed. That is, the Host  2  can access the same data as long as accessing the target having the same target name. 
     If the session is temporarily cut off In step  9010  between the initiator Init-b 0  of the Host b ( 2   b ) and the target Targ-a 1  of the SNa ( 1   a ), the session from the Host b ( 2   b ) is temporarily cut off until a session is established in step  9012  between the initiator Init-b 0  of the Host b ( 2   b ) and the target Targ-a 1  of the SNb ( 1   b ). However, the iSCSI command process generally has a retry mechanism, and thus if no command is received by the target, the Host b ( 2   b ) continuously retries for duration of 10 seconds. During this duration, if an SCN is issued, if discovery is completed, and if a new session is established between the initiator Init-b 0  of the Host b ( 2   b ) and the target Targ-a 1  of the SNb ( 1   b ), the application executed by the Host b ( 2   b ) does not acknowledge such a momentarily cut-off. Thus, without interrupting the application of the Host  2 , data migration can be performed from any specific SN  1  to another SN  1 . In such a manner, without interrupting the application of the Host  2 , the SN  1  can be additionally provided, and the load can be distributed among a plurality of SNs  1  connected to the switch  3 . 
     What is better, the programs applying control over layers lower to the operating system of the Host b ( 2   b ) such as the TCP/IP program and the iSCSI control program acknowledge that the location of the target Targ-a 1  is changed due to data migration as above. The issue here is that, the TCP/IP program and the iSCSI control program establish a TCP connection and an iSCSI session. Thus, the operating system of the Host b ( 2   b ) does not necessarily have to acknowledge the location of the target as long as the LU is acknowledged as a logical volume. In view thereof, the operating system of the Host b ( 2   b ) and the application program operating thereon do not acknowledge that data migration has been executed. That is, data migration can be favorably performed without causing the operating system of the Host  2  and the application program to notice data migration among the SNs  1 . 
     21. Method for Target Generation 
     Next, the method for target generation is described in more detail. The target name has to be a unique identifier. To retain such a uniqueness of the target name, an exemplary method is described below. 
     Assuming here is that a target name is a character string of an appropriate length. An exemplary character string is a combination of various codes and numbers, e.g., a code identifying a manufacturing company, a code identifying a specific organization in the manufacturing company, a code for identifying a storage system, a code for identifying the type of a storage node, a code of a revision of the storage node, a serial number of the storage node, and a sequential number assigned to a target in the storage node. With such a structure, even if any new target is generated in a certain storage node, the newly-generated target can be provided with a target name unique thereto only by incrementing the sequential number. 
     In the present embodiment above, when data in the LU 12Xx is migrated from a specific SN  1  to another, the LU 12Xx being the migration destination takes over the target name of the LU 12Xx being the migration source. As such, even if the target name is passed between the SNs, the target name remains unique. Thus, the target name can be continuously used by the SN  1  being the migration destination after taken over. 
     Herein, it is preferable to use nonvolatile memory such as Flash memory for the CTL  10  of the storage node  1  for storing the maximum value of the sequential number used at the time providing a target name to the target in the SN  1 . Here, the maximum value of the sequential number is the maximum value of the sequential number already in use. With such a structure, even if power failure or error occurs to the SN  1 , the Flash memory has stored the sequential number. Thus, after recovery, the SN  1  can keep generating a series of unique numbers to any new targets set in the SN  1  only by incrementing thus stored sequential number. 
     Note here that, shown in the above embodiment is the example of taking over a target name provided to any specific LU 12Xx in response to data migration from the LU 12Xx to another. Alternatively, at the time of data migration, the LU 12Xx being the migration destination may be provided with any new target name. If this is the case, to the LU 12Xx being the migration destination, a target name unique to the destination SN  1  can be set using a sequential number of the SN  1 , the serial number of the SN  1 , a revision code of the SN  1 , and others. If any new target name is set to the LU 12Xx being the destination, the LU control program  1128  of the SNb ( 1   b ) enters in step  9008  of  FIG. 9  thus newly-set target name to the LU management table  1111 . Also in step  9011 , the SNb ( 1   b ) is required to enter the newly-set target name to the name server  5 . As a result, at the time of discovery of step  9012 , the initiator Init-b 0  of the Host b ( 2   b ) detects the new target, enabling the initiator to construct a session with the target. 
     22. Setting of Target 
     In the above embodiment, shown is the example that the SN  1  generates target or initiator for registration into the name server  5 . Instead of the SNs  1  generating the target and initiator as such, the name server  5  may generate those. If this is the case, the SNs  1  issue an instruction for the name server  5  to enter the target and initiator, and in return, the name server  5  forwards the target and initiator back to the corresponding SN  1 . Then, the SN  1  makes an entry of the target and initiator received by the name server  5 . 
     23. Display Screen of Management Console ( FIG. 15 ) 
       FIG. 15  shows an exemplary display screen of the management console  4 . 
     The structure management program  4122  of the management console  4  displays on its screen the LU management table  1111 ′, the target management table  1112 , and the initiator management table  2112  or  1113 , all of which are stored in the memory of the management console  4 .  FIGS. 15C and 15D  both show such a display screen. Specifically,  FIG. 15C  shows an exemplary display screen before data migration, and  FIG. 15D  shows an exemplary display screen after data migration. 
     The structure management program  4112  displays on its screen the LU management table  1111 ′, the target management table  1112 , the initiator management table  2112  or  1113 , and pointers therefor. Thus, a manager using the management consoler  4  can easily grasp the relationship between the LU and the initiator or the target from the information displayed on the display screen. 
     The structure management program  4112  also displays the system structure on its screen based on the LU management table  1111 ′, the target management table  1112 , and the initiator management table  2112  or  1113  stored in the memory of the management console  4 .  FIGS. 15A and 15B  both show such a display screen. Specifically,  FIG. 15A  shows the system structure before data migration, and  FIG. 15B  shows the system structure after data migration. 
       FIGS. 15A and 15B  both show a display screen in a case where the LU-b ( 120   b ) being the migration destination takes over the target name set to the LU 1   a  ( 121   a ) being the migration source. Once data migration is performed, the target name is taken over from the migration source LU to the migration destination LU, causing the target Targ-a 1  to be changed in location on the display screen before and after data migration. However, the combination of initiator and target remains the same, i.e., pair of init-a 0  and Targ-a 0 , and pair of init-b 0  and Targ-a 1 . As such, even if data migration is performed between the SNs  1 , no change occurs to the combination of initiator and target. Accordingly, this eases the management of initiator sand targets for the manager in the system using the management console  4 . 
     Note here that the information displayed on the display screen is updated every time the LU management table  1111 ′, the target management table  1112 , or the initiator management table  2112  or  1113  is updated. Such update is performed responding to an instruction coming from the structure management program to the SNs  1  as described by referring to  FIG. 9 , or a notification received by the structure management program from the SNs  1  about any change applied to the system structure. 
     Second Embodiment 
     Described next is a second embodiment. In the first embodiment, exemplified is the case of migrating data stored in the LU 1   a  ( 121   a ) of the SNa ( 1   a ) to the SNb ( 1   b ), which is newly added. In the second embodiment, as shown in  FIG. 10 , an SNc ( 1   c ) is additionally added to the switch  3 , and the LU 0   a  ( 120   a ) left in the SNa ( 1   a ) is migrated to thus newly-added SNc ( 1   c ). 
     The LU 0   a  ( 120   a ) with the target Targ-a 0  in the SNa ( 1   a ) is connected with the initiator Init-a 0  of the Host a ( 2   a ). Thus, in the second embodiment, the initiator-target relationship is different from that in the first embodiment, and the discovery and other processes are to be executed by the Host a ( 2   a ). The procedure, however, remains the same that the data in the LU 0   a  ( 120   a ) of the SNa ( 1   a ) is migrated to the LU 0   c  ( 120   c ) of the SNc ( 1   c ), the LU 0   c  ( 120   c ) being the migration destination takes over the target Targ-a 0  of the LU 0   a  ( 120   a ), and the access path is changed between the initiator Init-a 0  and the target Targ-a 0 . 
     After completion of such data migration, the SNa ( 1   a ) has no LU 12Xx to be accessed by the Hosts  2 . Accordingly, the SNa ( 1   a ) can be removed from the switch  3 , leading to reduction of the SN. 
     Utilizing the process as such, the SNa ( 1   a ) can be replaced to the SNc ( 1   c ) without interrupting access from the Hosts  2 . More in detail, during the process of changing the access path from the Hosts  2  by migrating the data stored in the LU 0   a  ( 120   a ) of the SNa ( 1   a ) to the newly-added SNc ( 1   c ), the Hosts  2  can be accessible to the data stored in these both LUs. Thus, even if data storage is required for a longer time than the SN lasts, i.e., if data lasts longer than the SN, due to law, for example, data remains available through exchange of any out-of-life storage node  1  instead of replacing the storage system  1000  in its entirety. 
     According to the present embodiment, data storage can be achieved over a long period of time as long as data lasts while suppressing cost increase required for system replacement without temporary data saving, and without interrupting data access. 
     Third Embodiment 
       FIG. 11  is a diagram showing another exemplary system structure. A third embodiment has differences from the first and second embodiments that the storage node  1  has two controllers of CTL 0  and CTL 1 , and LU 120   x  are so structured as to be accessible by these two controllers  10 . Moreover, the network  30  is provided with two switches of  0 ( 3 ) and  1 ( 31 ), and the Hosts  2  and the storage nodes  1  are each connected to these two switches. In the present embodiment, the wiring between the LU 120   x  and CTL 10 , the wiring between the SN  1  and the switch, and the wiring between the Hosts  2  and the switch are all doubly provided. In such a manner, the resulting storage system can be high in reliability. The method for replacing the storage node land the load distribution through LU migration is the same as that in the first and second embodiments. 
     Fourth Embodiment 
       FIG. 12  is a diagram showing another exemplary system structure. In the present embodiment, the storage system  1000  is provided with a plurality of CTL  10 , and these CTL  10  share the LU 12Xx via a disk connector  150 . Add-in and removal of the SNs in the first and second embodiments correspond to add-in and removal of the CTL  10 . As an example, a CTLc ( 10   c ) may be added as a replacement for the out-of-life CTLa ( 10   a ), and after thus newly-added CTLc ( 10   c ) takes over the LU 12Xx that was under the control of the CTLa ( 10   a ), the CTLa ( 10   a ) is removed. At this time, the procedure taken for taking over the LU management information in the LU management table  1111  of the CTLa ( 10   a ), for taking over the target in the target management table  1112  of the CTLa ( 10   a ), and for changing the access path is executed in the same manner as that in the first and second embodiments. Herein, the CTLs  10  are each connected to the corresponding LU 12Xx via the disk connector  150 , thus there is no need for data migration from the LU 12Xx. For example, to take over the LU 0  ( 120   a ) that was under the control of the CTLa ( 10   a ) to the CTLc ( 10   c ), the CTLc ( 10   c ) is allowed to access the LU 0  ( 120   a ) through the disk connector  150 . Here, exclusive control is to be exercised, and thus the same procedure in the first and second embodiments are to be executed for the CTLc ( 10   c ) to take over the LU management information about the LU 0  ( 120   a ) from the CTLa ( 10   a ), take over target information set to the LU ( 120   a ), i.e., contents of the target management table  1112  about the target, and others. The procedure can skip the data copying process. In this manner, cost efficiency and system change can be swiftly done to a greater degree. 
     Fifth Embodiment 
       FIG. 13  is a diagram showing still another exemplary system structure. In the present embodiment, the switch  3  and the management console  4  are included in the storage system  1000 . The switch  3 , the management console  4 , and the SN  1  are all components of the storage system  1000 , and the user is provided those as a set. As a preferred embodiment, these components are so structured as a unit, providing the user with better manageability. 
     Sixth Embodiment 
       FIG. 14  is a diagram showing still another exemplary system structure. In the present embodiment, the management console  4  of  FIG. 13  is not provided, and the structure management program  4122  in the management console  4  of the above embodiments is provided to the CTL ( 10 ) of the respective storage nodes. Whenever any structure change occurs, the structure management program  4122  communicates with other structure management programs  4122  to see what structure change has occurred. Further, prior to structure change, exclusive control is applied to any needed resources. Such a structure eliminates the management console  4 , leading to the storage system with better cost efficiency. 
     In the above embodiments, the access path from the host is changed after LU data migration is performed. This change may be done in the following order: 
     1. Migrate LU information (target information and initiator access permission information included) 
     2. Switching of access path from host to migration destination (migration of target name, and registration change of name server included) 
     3. LU data migration 
     If this is the case, data access during migration can be handled in the same manner as the background technology. Also in this case, the same effects as the other embodiments can be successfully achieved. Specifically, LU migration can be performed without causing the operating system and the applications of the hosts to notice, which is the characteristics of the present invention.