2007 USENIX Annual Technical Conference
Pp. 261–274 of the Proceedings
DiskSeen: Exploiting Disk Layout and Access History to Enhance I/O Prefetch
Xiaoning Ding, Ohio State University; Song Jiang, Wayne State University; Feng Chen, Ohio State University; Kei Davis, Los Alamos National Laboratory; Xiaodong Zhang, Ohio State University
Abstract
Current disk prefetch policies in major operating systems track
access patterns at the level of the file abstraction. While this is
useful for exploiting application-level access patterns,
file-level prefetching cannot realize the full
performance improvements achievable by prefetching. There are two
reasons for this. First, certain prefetch opportunities can only be
detected by knowing the data layout on disk, such as the contiguous
layout of file meta-data or data from multiple files. Second,
non-sequential access of disk data (requiring disk head movement) is
much slower than sequential access, and the penalty for
mis-prefetching a `random' block, relative to that of a sequential
block, is correspondingly more costly.
To overcome the inherent limitations of prefetching at the logical
file level, we propose to perform prefetching directly at the level
of disk layout, and in a portable way. Our technique, called
DiskSeen, is intended to be supplementary to, and to work
synergistically with, file-level prefetch policies, if present.
DiskSeen tracks the locations and access times of disk blocks, and
based on analysis of their temporal and spatial relationships, seeks
to improve the sequentiality of disk accesses and overall
prefetching performance.
Our implementation of the DiskSeen scheme in the Linux 2.6 kernel
shows that it can significantly improve the effectiveness of
prefetching, reducing execution times by 20%-53% for
micro-benchmarks and real applications such as grep,
CVS, and TPC-H.
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