Digital Equipment Corporation

Alan Rollow

David William Thiel

Steven J Malan

Robert A Ellis

Richard B Wells

Sid Snyder

A storage system having a plurality of disks arranged in a RAID array with one of said disks being a replacement disk. A method and apparatus for restoring the contents of the replacement disk without interrupting concurrent access to the RAID array. An access request by a user application to a data block in the replacement disk results in regeneration and reconstruction of the accessed data block. In addition, the systematic reconstruction of data or parity blocks not yet accessed by the user application occurs by locating status bits associated with each data and parity block of the new disk.

Method and apparatus for the on-line restoration of a disk in a RAID-4

Data storage systems using a RAID-4 or RAID-5 organization divide an application I/O request into a number of individual I/O requests, each of which is contained within the boundaries of a single strip. The data blocks of each chunk on a strip responsive to an I/O request can form a complex geometric pattern requiring complicated operations to perform the I/O request. To simplify the necessary operations, each individual I/O request to a strip is divided into at most three requests targeted to data blocks forming a rectangle and each of these rectangles are processed as a unit. If a data block within a rectangle is unavailable, then the request to that rectangle is further subdivided into at most two requests targeted to data blocks forming smaller, non-overlapping rectangles which collectively are the original rectangle. The recursive decomposition of rectangles into smaller rectangles isolates the data block with an error and permits the selection of fewer and less complicated operation algorithms to complete the I/O request.

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Alan Rollow