Storage server and storage system

1. A storage server comprising:

first tiered storage devices that store data items, the first tiered storage devices comprising at least a first storage device associated with a first tier level, and a second storage device associated with a second tier level and having an access speed slower than that of the first storage device;

a network interface that communicates with each of a client and another storage server through a network, wherein said another storage server comprises second tiered storage devices; and

a processor configured to control the first tiered storage devices and the network interface,

wherein the processor is configured to:

read, when a read request is received from the client through the network interface, a data item designated by the read request from the first tiered storage devices, and transmit the read data item to the client through the network interface;

relocate data items among the first tiered storage devices, the relocated data items comprising a first data item of first access frequency that is moved from the second storage device to the first storage device and a second data item of second access frequency lower than the first access frequency that is moved from the first storage device to the second storage device; and

transmit first information indicative of placement of the relocated data items in the first tiered storage devices as a hint for data relocation among the second tiered storage devices, to said another storage server through the network interface,

wherein the first information comprises information indicative of a tier level at which each of the relocated data items is placed.

2. The storage server of claim 1 , wherein

when the first data item is placed in the first storage device and the second data item is placed in the second storage device by the relocation of the data items, the first information comprises information indicating that the first data item is placed at the first tier level and information indicating that the second data item is placed at the second tier level.

3. The storage server of claim 2 , wherein

when the first access frequency is higher than a threshold value, the first data item is locked in the first storage device and the first information further comprises information indicating that the first data item is locked at the first tier level.

4. The storage server of claim 1 , further comprising a memory cache having an access speed higher than that of the first storage device,

wherein the processor is further configured to:

store in the memory cache the data item read from the first tiered storage devices;

determine, when a subsequent read request is received from the client, whether a data item designated by the subsequent read request is present in the memory cache; and

when the data item designated by the subsequent read request is present in the memory cache:

read the data item designated by the subsequent read request from the memory cache; and

transmit the read data item to the client through the network interface, and

wherein the first information further comprises information indicative of the data item stored in the memory cache.

5. The storage server of claim 1 , wherein

the processor is further configured to, when a read request from a client is redirected from the storage server to said another storage server, transmit the first information to said another storage server through the network interface.

6. The storage server of claim 1 , wherein

the processor is further configured to, when a scheduled date and time to transmit the first information comes, transmit the first information to said another storage server through the network interface.

7. The storage server of claim 1 , wherein

the processor is further configured to, if a failure occurs in a part of the storage server, transmit the first information to said another storage server through the network interface.

8. The storage server of claim 1 , wherein

the processor is further configured to, if software is added to the storage server, transmit the first information to said another storage server through the network interface.

9. A storage system comprising:

a first storage server comprising:

a first network interface; and

first tiered storage devices that store data items, the first tiered storage devices comprising at least a first storage device associated with a first tier level, and a second storage device associated with a second tier level and having an access speed slower than that of the first storage device; and

a second storage server comprising:

a second network interface; and

second tiered storage devices that store data items, the second tiered storage devices comprising at least a third storage device associated with the first tier level, and a fourth storage device associated with the second tier level and having an access speed slower than that of the third storage device,

wherein the first storage server is configured to:

read, when a first read request is received from a client through the first network interface, a data item designated by the first read request from the first tiered storage devices, and transmit the read data item to the client through the first network interface;

relocate data items among the first tiered storage devices, the relocated data items comprising a first data item of first access frequency that is moved from the second storage device to the first storage device and a second data item of second access frequency lower than the first access frequency that is moved from the first storage device to the second storage device; and

transmit first information indicative of placement of the relocated data items in the first tiered storage devices to the second storage server through the first network interface,

wherein the first information comprises information indicative of a tier level at which each of the relocated data items is placed,

wherein the second storage server is configured to:

receive the first information from the first storage server through the second network interface; and

relocate data items among the second tiered storage devices based on the first information, the relocated data items comprising a third data item and a fourth item, wherein the third item corresponds to the first data item and is moved from the fourth storage device to the third storage device and the fourth data item corresponds to the second data item and is moved from the third storage device to the fourth storage device.

10. The storage system of claim 9 , wherein

the first information comprises information indicating that the first data item is placed at the first tier level and information indicating that the second data item is placed at the second tier level.

11. The storage system of claim 10 , wherein

when the first access frequency is higher than a threshold value, the first data item is locked in the first storage device and the first information further comprises information indicating that the first data item is locked at the first tier level, and

the second storage server is further configured to lock the third data item corresponding to the first data item in the third storage device based on the first information.

12. The storage system of claim 9 , wherein

the second storage server is further configured to read, when a second read request is received from a client through the second network interface after relocating the data items among the second tiered storage devices based on the first information, a data item designated by the second read request from the second tiered storage devices and transmit the read data item to the client through the second network interface.

13. The storage system of claim 9 , wherein

the first storage server is further configured to, when a read request from a client is redirected from the first storage server to the second storage server, transmit the first information to the second storage server through the first network interface.

14. The storage system of claim 9 , wherein

the first storage server is further configured to, when a scheduled date and time to transmit the first information comes, transmit the first information to the second storage server through the first network interface.

15. The storage system of claim 9 , wherein

the first storage server is further configured to, if a failure occurs in a part of the storage server, transmit the first information to the second storage server through the first network interface.

16. The storage system of claim 9 , wherein

the first storage server is further configured to, if software is added to the first storage server, transmit the first information to the second storage server through the first network interface.