Track-aligned Extents: Matching Access Patterns to Disk Drive Characteristics
Conference on File and Storage Technologies (FAST) January 28-30, 2002. Monterey, CA. Supercedes Carnegie Mellon University SCS Technical Report CMU-CS-01-119.
Jiri Schindler, John Linwood Griffin, Christopher R. Lumb, Gregory R. Ganger
Dept. Electrical and Computer Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
Track-aligned extents (traxtents) utilize disk-specific knowledge to match access patterns to the strengths of modern disks. By allocating and accessing related data on disk track boundaries, a system can avoid most rotational latency and track crossing overheads. Avoiding these overheads can increase disk access efficiency by up to 50% for mid-sized requests (100-500 KB). This paper describes traxtents, algorithms for detecting track boundaries, and some uses of traxtents in file systems and video servers. For large-file workloads, a version of FreeBSDs FFS implementation that exploits traxtents reduces application run times by up to 20% compared to the original version. A video server using traxtent-based requests can support 56% more concurrent streams at the same startup latency and buffer space. For LFS, 44% lower overall write cost for track-sized segments can be achieved.