Computer Science Department
School of Computer Science, Carnegie Mellon University


Track-aligned Extents:
Matching Access patterns to Disk Drive Characteristics

Jiri Schindler, John Linwood Griffin,
Christopher R. Lumb, Gregory R. Ganger

April 2001

Keywords: Data layout, disk characterization, disk efficiency, file systems, track-aligned access

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 use of traxtents in file systems and video servers. For large file workloads, a modified version of FreeBSD's FFS implementation reduces application run times by 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.

20 pages

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