Data allocation in a distributed storage system

Abstract

A method for data distribution, including distributing logical addresses among an initial set of devices so as provide balanced access, and transferring the data to the devices in accordance with the logical addresses. If a device is added to the initial set, forming an extended set, the logical addresses are redistributed among the extended set so as to cause some logical addresses to be transferred from the devices in the initial set to the additional device. There is substantially no transfer of the logical addresses among the initial set. If a surplus device is removed from the initial set, forming a depleted set, the logical addresses of the surplus device are redistributed among the depleted set. There is substantially no transfer of the logical addresses among the depleted set. In both cases the balanced access is maintained.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is divisional of U.S. patent application Ser. No. 10 / 620,080 filed Jul. 15, 2003, the content of which is incorporated herein by reference.FIELD OF THE INVENTION [0002] The present invention relates generally to data storage, and specifically to data storage in distributed data storage entities. BACKGROUND OF THE INVENTION [0003] A distributed data storage system typically comprises cache memories that are coupled to a number of disks wherein the data is permanently stored. The disks may be in the same general location, or be in completely different locations. Similarly, the caches may be localized or distributed. The storage system is normally used by one or more hosts external to the system. [0004] Using more than one cache and more than one disk leads to a number of very practical advantages, such as protection against complete system failure if one of the caches or one of the disks malfunctions. Redundancy may be inc...

AI Technical Summary

Benefits of technology

[0014] In preferred embodiments of the present invention, a data distribution system comprises a plurality of data storage devices wherein data blocks may be stored. The data blocks are stored at logical addresses that are assigned to the data storage devices according to a procedure which allocates the addresses among the devices in a manner that reduces the overhead incurred when a device is added to or removed from the system, and so as to provide a balanced access to the devices. The procedure typically distributes the addresses evenly among the devices, regardless of the number of devices in the system. If a storage device is added to or removed from the system, the procedure reallocates the logical addresses between the new numbers of devices so that the balanced access is maintained. If a device has been added, the procedure only transfers addresses to the added storage device. If a device has been removed, the procedure only transfers addresses from the removed storage device. In both cases, the only transfers of data that occur are of data blocks stored at the transferred addresses. The procedure thus minimizes data transfer and associated management overhead when the number of storage devices is changed, or when the device configuration is changed, while maintaining the balanced access.

[0054] a depleted set of storage devices, formed by subtracting a surplus storage device from the initial set, the logical addresses being redistributed among the storage devices in the depleted set so as to cause logical addresses of the surplus device to be transferred to the depleted set, while maintaining the balanced access and without requiring a substantial transfer of the logical addresses among the storage devices in the depleted set.

Problems solved by technology

Consequently, device failure has a minimal effect on the performance of the distribution system.

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