Doubly distorted mirrors

Abstract

Traditional mirrored disk systems provide high reliability by multiplexing disks. Performance is improved with parallel reads and shorter read seeks. However, writes must be performed by both disks, limiting performance. Doubly distorted mirrors increase the number of physical writes per logical write from 2 to 3, but performs logical writes more efficiently. This reduces the cost of a random logical write to 1/3 of the cost of a read. Moreover, much of the write cost can be absorbed in the rotational latency of the reads, performing under certain conditions all the writes for free. Doubly distorted mirrors achieves a 135% performance improvement over traditional mirrors in the TP1 benchmark. Although these techniques require a disk cache for writes, the cache need not be safe nor is recovery time impacted very much.