Importance

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Abstract

A method and apparatus that maintains in stable storage an identification of which copies in a mirrored disk system are currently up to date without requiring the use of a logging mechanism. The invention may include a plurality of mirrored disk drives and a processing unit for controlling the writing and reading of data units to and from mirrored disk drives, wherein the processing unit establishes and maintains cohort sets for each mirrored disk drive. The cohort set represents the status of copies of the data units stored on the mirrored disk drives and contains membership information for redundant copies of data units stored in the plurality of mirrored disk drives. The membership information comprises a data structure of 2n status bits for identifying which disks contain current copies of the data units, where n is the number of disks in the system, and wherein the first n bits are used to indicate an intention to update the membership information for the copies and the second n bits are used indicate a current state of the membership information. The data structure may further include log(n) additional bits for identifying each mirrored disk.

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First Claim