The HP lhtoRAID hierarchical storage system
John Wilkes, Richard Gelding, Carl Staelin, Tim J. Sullivan · 1995
Configuring redundant disk arrays is a black art. To properly configure an array, a system administrator must understand the details of both the array and the workload it will support; incorrect understanding of eithec or changes in the workload over time, can lead to poor performance. Wepresent a solution to this problem: a two-level storage hierarchy implemented inside a single disk-array controller In theupper level of this hierarchy, two copies of active data are stored to provide full redundancy and excellent performance. In the lower level, RAID 5 parity protection is used to provide excel[ent storage cost for inactive data, at somewhat lower performance. The technology we describe in this pape< known as HP AutoRAID, automatically and transparently manages migration of data blocks between these two levels as access patterns change. The result is a fully-redundant storage system that is extremely easy to use, suitable for a wide variety of workloads, largely insensitive to dynamic workload changes, and that performs much better than disk arrays with comparable numbers of spindles and much larger amounts offront-end ~ ~ cache. Because the implementation of the I-W AUtORAID technology is almost entirely in embedded software, the additional hardware cost for these benejits is very small. We describe the HP AUtORAID technology in detail, and provide performance data for an embodiment of it in a prototype storage array, together with the results of simulation studies used to choose algorithms used in the array. 1