Freeblock scheduling outside of disk firmware

Abstract

Abstract Freeblock scheduling replaces a disk drive's rotationallatency delays with useful background media transfers, potentially allowing background disk I/O to occur withno impact on foreground service times. To do so, a freeblock scheduler must be able to very accurately predictthe service time components of any given disk request-- the necessary accuracy was not previously consid-ered achievable outside of disk firmware. This paper describes the design and implementation of a working ex-ternal freeblock scheduler running either as a user-level application atop Linux or inside the FreeBSD kernel.This freeblock scheduler can give 15 % of a disk's potential bandwidth (over 3.1MB/s) to a background diskscanning task with almost no impact (less than 2%) on the foreground request response times. This can increasedisk bandwidth utilization by over 6 \\Theta. 1 Introduction Freeblock scheduling is an exciting new approach to uti-lizing more of a disk's potential media bandwidth. It consists of anticipating rotational latency delays and fill-ing them with media transfers for background tasks. Via simulation, our prior work [14] indicated that 20-50%of a never-idle disk's bandwidth could be provided to background applications with no effect on foreground re-sponse times. This free bandwidth was shown to enable free segment cleaning in a busy log-structured file sys-tem (LFS), or free disk scans (e.g., for data mining or disk media scrubbing) in an active transaction process-ing system.