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When a logical volume in a first storage box is transferred to a second storage box, information of an access object such as a network communication protocol possessed by a host which communicates with the first storage box is changed to change a network route for accessing the transferred logical v

When a logical volume in a first storage box is transferred to a second storage box, information of an access object such as a network communication protocol possessed by a host which communicates with the first storage box is changed to change a network route for accessing the transferred logical volume. Logical volume transfer is notified from the first storage box to the host by using an ICMP Redirection Error message or the ARP protocol.

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What is claimed is: 1. A storage system coupled to a plurality of host computers via a first network: wherein each of said host computers is operable to issue an input/output (I/O) request of data via said first network to said storage system on a logical volume basis, with routing information of t

What is claimed is: 1. A storage system coupled to a plurality of host computers via a first network: wherein each of said host computers is operable to issue an input/output (I/O) request of data via said first network to said storage system on a logical volume basis, with routing information of the data being stored in a routing table in each of said host computers, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said host computers, and an arbitrator coupled to said storage sub-systems via a second network and to said host computers via said first network, wherein each of said plurality of host computers is provided with a network interface card (NIC) coupled to said first network, each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, and said arbitrator is provided with an NIC coupled to said first network, wherein said arbitrator includes a logical volume transfer module and a routing information transmission module, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by a first host computer of said host computers, said logical volume transfer module instructs said first storage sub-system to transfer information as to said IP address of said logical volume to said second storage sub-system, said logical volume transfer module instructs said first and second storage sub-systems to transmit an IP datagram having said IP address as a destination address to said second storage sub-system, when receiving said IP datagram, said routing information transmission module instructs said first storage sub-system to transmit an ICMP Redirect Error message to said first host computer, when receiving an IP datagram having said IP address as a destination address from said first host computer, said routing information transmission module instructs said host computers to transmit updated routing information as to said logical volume, said first host computer updates an associated routing table according to contents of said ICMP Redirect Error message, when receiving said ICMP Redirect Error message, each of the host computers except said first host computer updates an associated routing table according to said updated routing information, and said logical volume transfer module instructs said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network. 2. The storage system according to claim 1, wherein a communication protocol of said first network is TCP/IP or an iSCSI protocol. 3. The storage system according to claim 1, wherein each of said storage sub-systems includes a plurality of storage devices. 4. The storage system according to claim 1, wherein said routing table holds correspondence relationships between IP addresses and logical volumes created by using chains of a non-oriented graph theory vertices of which correspond to an IP address and a logical volume. 5. The storage system according to claim 1, wherein said second storage sub-system to which said logical volume is to be transferred is determined by taking loads exerted on said second storage sub-system with respect to those on said first storage sub-system into consideration. 6. A method for transferring data in a storage system coupled to a plurality of host computers via a first network, wherein each of said host computers is operable to issue an I/O request of data via said first network to said storage system on a logical volume basis, with routing information of the data being stored in a routing table in each of said host computers, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said host computers, and an arbitrator coupled to said storage sub-systems via a second network and to said host computers via said first network, wherein said arbitrator includes a logical volume transfer module and a routing information transmission module, wherein each of said plurality of host computers is provided with an NIC coupled to said first network, each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, and said arbitrator is provided with an NIC coupled to said first network, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by a first host computer of said host computers, said method comprises the steps of: instructing, by said logical volume transfer module, said first storage sub-system to transfer information as to said LP address of said logical volume to said second storage sub-system; instructing, by said logical volume transfer module, said first and second storage sub-systems to transmit an IP datagram having said IP address as a destination address to said second storage sub-system, when receiving said IP datagram, instructing, by said routing information transmission module, said first storage sub-system to transmit an ICMP Redirect Error message to said first host computer, when receiving an IP datagram having said IP address as a destination address from said first host computer; instructing, by said routing information transmission module, said host computers to transmit updated routing information as to said logical volume; updating, by said first host computer, an associated routing table according to contents of said ICMP Redirect Error message, when receiving said ICMP Redirect Error message; updating, by each of the host computers except said first host computer, an associated routing table according to said updated routing information; and instructing, by said logical volume transfer module, said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network. 7. The method according to claim 6, wherein a communication protocol of said first network is TCP/IP or an iSCSI protocol. 8. The method according to claim 6, wherein each of said storage sub-systems includes a plurality of storage devices. 9. The method according to claim 6, wherein said routing table holds correspondence relationships between IP addresses and logical volumes created by using chains of a non-oriented graph theory vertices of which correspond to an IP address and a logical volume. 10. The method according to claim 6, wherein said second storage sub-system to which said logical volume is to be transferred is determined by taking loads exerted on said second storage sub-system with respect to those on said first storage sub-system into consideration. 11. A storage system coupled to a first host computer coupled to a first network, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said first host computer, said storage sub-systems being coupled to each other via a second network, and an arbitrator coupled to said storage sub-systems via said second network, wherein said first host computer is provided with an NIC coupled to said first network, and each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, wherein said first host computer has an Address Resolution Protocol (ARP) cache holding correspondence relationships between IP addresses of logical volumes and Media Access Control (MAC) addresses of said NICs, wherein said arbitrator includes a logical volume transfer module, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by an I/O request issued from said first host computer, said logical volume transfer module instructs said first storage sub-system to disconnect I/O connections established for the IP address of the logical volume, said logical volume transfer module instructs said first storage sub-system to transfer information as to said IP address of said logical volume to said second storage sub-system, said logical volume transfer module instructs said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network, said logical volume transfer module instructs said second storage sub-system to broadcast, on said first network, an ARP packet indicating that the IP address of the logical volume now corresponds to an MAC address of the NIC of said second storage sub-system, said first host computer extracts a correspondence relationship between the IP address and the MAC address from the ARP packet, when receiving the ARP packet on said first network, to update the correspondence relationships in an associated ARP cache, and said logical volume transfer module makes I/O connections be reestablished between said first host and said second storage sub-system. 12. The storage system according to claim 11, wherein each of said storage sub-systems includes a plurality of storage devices. 13. The storage system according to claim 11, wherein said second storage sub-system to which said logical volume is to be transferred is determined by taking loads exerted on said second storage sub-system with respect to those on said first storage sub-system into consideration. 14. The system according to claim 11, wherein, after completion of transfer of data of a logical volume belonging to a group from said first storage sub-system to said second storage-subsystem, data of all logical volumes included in said group is transferred from said second storage sub-system to said first host computer. 15. The system according to claim 14, wherein the data of all logical volumes included in said group is transferred from said second storage sub-system to said first host computer without passing said first storage sub-system. 16. The system according to claim 11, wherein said first network and said second network are a same network. 17. A method for transferring data in a storage system coupled to a first host computer coupled to a first network, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said first host computer, said storage sub-systems being coupled to each other via a second network, and an arbitrator coupled to said storage sub-systems via said second network, wherein said first host computer is provided with an NIC coupled to said first network, and each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, wherein said first host computer has an ARP cache holding correspondence relationships between IP addresses of logical volumes and MAC addresses of said NICs, wherein said arbitrator includes a logical volume transfer module, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by an I/O request issued from said first host computer, said method comprises the steps of: instructing, by said logical volume transfer module, said first storage sub-system to disconnect I/O connections established for the IP address of the logical volume; instructing, by said logical volume transfer module, said first storage sub-system to transfer information as to said IP address of said logical volume to said second storage sub-system; instructing, by said logical volume transfer module, said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network; instructing, by said logical volume transfer module, said second storage sub-system to broadcast, on said first network, an ARP packet indicating that the IP address of the logical volume now corresponds to an MAC address of the NIC of said second storage sub-system; extracting, by said first host computer, a correspondence relationship between the IP address and the MAC address from the ARP packet, when receiving the ARP packet on said first network, to update the correspondence relationships in an associated ARP cache; and making, by said logical volume transfer module, I/O connections be reestablished between said first host and said second storage sub-system. 18. The method according to claim 17, wherein each of said storage sub-systems includes a plurality of storage devices. 19. The method according to claim 17, wherein said second storage sub-system to which said logical volume is to be transferred is determined by taking loads exerted on said second storage sub-system with respect to those on said first storage sub-system into consideration. 20. The method according to claim 17, wherein, after completion of transfer of data of a logical volume belonging to a group from said first storage sub-system to said second storage-subsystem, data of all logical volumes included in said group is transferred from said second storage sub-system to said first host computer. 21. The method according to claim 20, wherein the data of all logical volumes included in said group is transferred from said second storage sub-system to said first host computer without passing said first storage sub-system. 22. The method according to claim 17, wherein said first network and said second network are a same network. 23. A storage system coupled to a router coupled to a first network, said router being coupled to host computers via another network, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said router, said storage sub-systems being coupled to each other via a second network, and an arbitrator coupled to said storage sub-systems via said second network, wherein said router is provided with an NIC coupled to said first network, and each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, wherein said router has an Address Resolution Protocol (ARP) cache holding correspondence relationships between IP addresses of logical volumes and Media Access Control (MAC) addresses of said NICs, wherein said arbitrator includes a logical volume transfer module, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by an I/O request issued from said router, said logical volume transfer module instructs said first storage sub-system to disconnect I/O connections established for the IP address of the logical volume, said logical volume transfer module instructs said first storage sub-system to transfer information as to said IP address of said logical volume to said second storage sub-system, said logical volume transfer module instructs said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network, said logical volume transfer module instructs said second storage sub-system to broadcast, on said first network, an ARP packet indicating that the IP address of the logical volume now corresponds to an MAC address of the NIC of said second storage sub-system, said router extracts a correspondence relationship between the IP address and the MAC address from the ARP packet, when receiving the ARP packet on said first network, to update the correspondence relationships in an associated ARP cache, and said logical volume transfer module makes I/O connections be reestablished between said first host and said second storage sub-system. 24. A method for transferring data in a storage system coupled to a router coupled to a first network, said router being coupled to host computers via another network, wherein said storage system comprises: a plurality of storage sub-systems storing data on a logical volume basis to be accessible by said router, said storage sub-systems being coupled to each other via a second network, and an arbitrator coupled to said storage sub-systems via said second network, wherein said router is provided with an NIC coupled to said first network, and each of said plurality of storage sub-systems is provided with an NIC coupled to said first network, wherein said router has an ARP cache holding correspondence relationships between IP addresses of logical volumes and MAC addresses of said NICs, wherein said arbitrator includes a logical volume transfer module, wherein, when data of a logical volume stored in said first storage sub-system is to be transferred to said second storage sub-system, with said logical volume having an IP address and being accessed by an I/O request issued from said router, said method comprises the steps of: instructing, by said logical volume transfer module, said first storage sub-system to disconnect I/O connections established for the IP address of the logical volume; instructing, by said logical volume transfer module, said first storage sub-system to transfer information as to said IP address of said logical volume to said second storage sub-system; instructing, by said logical volume transfer module, said first and second storage sub-systems to transfer said logical volume from said first storage sub-system to said second storage sub-system via said second network; instructing, by said logical volume transfer module, said second storage sub-system to broadcast, on said first network, an ARP packet indicating that the IP address of the logical volume now corresponds to an MAC address of the NIC of said second storage sub-system; extracting, by said router, a correspondence relationship between the IP address and the MAC address from the ARP packet, when receiving the ARP packet on said first network, to update the correspondence relationships in an associated ARP cache; and making, by said logical volume transfer module, I/O connections be reestablished between said first host and said second storage sub-system.