A Framework for Building Unobtrusive Disk Maintenance Applications

This paper describes a programming framework for clean construction of disk maintenance applications. They can use it to expose the disk activity to be done, and then process completed requests as they are reported. The system ensures that these applications make steady forward progress without competing for disk access with a system's primary applications. It opportunistically completes maintenance requests by using disk idle time and freeblock scheduling. In this paper, three disk maintenance applications (backup, write-back cache destaging, and disk layout reorganization) are adapted to the system support and evaluated on a FreeBSD implementation. All are shown to successfully execute in busy systems with minimal (e.g., less than 2%) impact on foreground disk performance. In fact, by modifying FreeBSD's cache to write dirty blocks for free, the average read cache miss response time is decreased by 12-30%. For non-volatile caches, the reduction is almost 50%.