Idle Read After Write - IRAW

Despite a low occurrence rate, silent data corruption represents a growing concern for storage systems designers. Throughout the storage hierarchy, from the file system down to the disk drives, various solutions exist to avoid, detect, and correct silent data corruption. Undetected errors during the completion of WRITEs may cause silent data corruption. A portion of the WRITE errors may be detected and corrected successfully by verifying the data written on the disk with the data in the disk cache. Write verification traditionally is scheduled immediately after a WRITE completion (Read After Write RAW) which is unattractive, because it degrades user performance. To reduce the performance penalty associated with RAW, we propose to retain the written content in the disk cache and verify it once the disk drive becomes idle. Although attractive, this approach (called IRAW Idle Read After Write) contends for resources, i.e., cache and idle time, with user traffic and other background activities. In this paper, we present a trace-driven evaluation of IRAW and show its feasibility. Our analysis indicates that idleness is present in disk drives and can be utilized for WRITE verification with minimal effect on user performance. IRAW benefits significantly if some amount of cache, i.e., 1 or 2 MB, is dedicated to retain the unverified WRITEs. If the cache is shared with the user requests then a cache retention policy that places both READs and WRITEs upon completion at the most recently used cache segment, yields best IRAW performance without effecting user READs cache hit ratio and overall user performance.