Richard Lewis Mattson

Maintaining good performance in disk arrays during failure via uniform parity group distribution

Disk arrays are increasingly being used in distributed computing systems, as the vehicle for providing reliable and high performance data storage. When a disk in a RAID-5 fails, data in that failed disk can still be made available through parity reconstruction by reading from the other disks. However, this poses an increased burden on the surviving disks, and if consideration is not given to this failure consequence, then the performance of the system may degrade to an unacceptable level. This paper describes techniques that will enable the disk array to maintain good performance in the event of a disk failure. After a failed disk has been repaired, its content must be reconstructed from all the associated parity groups. In RAID-5, this must be a single thread sequential process. With the techniques introduced in this paper, it is shown how this sequential process can now be broken down into multiple parallel processes distributed throughout the array, thus shortening the reconstruction time. While the techniques introduced in this paper are applied to disk arrays, they may potentially have general applications in other areas of distributed computing.<<ETX>>

Uniform parity group distribution in disk arrays with multiple failures

Several new disk arrays have recently been proposed in which the parity groupings are uniformly distributed throughout the array so that the extra workload created by a disk failure can be evenly shared by all the surviving disks, resulting in the best possible degraded mode performance. Many arrays now also put in multiple spare disks so that expensive service calls can be deferred. Furthermore, in a new sparing scheme called distributed sparing, the spare spaces are actually distributed throughout the array. This means after a rebuild the new array will be logically different from the original array. The authors present an algorithm for constructing and maintaining arrays with distributed sparing so that repeated uniform parity group distribution is achieved with each successive failure. >

A version management method for distributed information

Information networks in which workstations access central data collections primarily to extract information are a growing application segment. When a workstation is likely to use an item more than once it may be economical to retain a copy locally. We describe how to propagate updates for an arbitrary relationship between the source database and cached items when the network connection is intermittent, unreliable, and/or slow.