Information Technology Reference
In-Depth Information
Chapter 10
Exploiting Disk Layout
and Block Access History
for I/O Prefetch
Feng Chen
The Ohio State University, USA
Xiaoning Ding
The Ohio State University, USA
Song Jiang
Wayne State University, USA
abStract
As the major secondary storage device, the hard disk plays a critical role in modern computer system. In
order to improve disk performance, most operating systems conduct data prefetch policies by tracking
I/O access pattern, mostly at the level of file abstractions. Though such a solution is useful to exploit
application-level access patterns, file-level prefetching has many constraints that limit the capability
of fully exploiting disk performance. The reasons are twofold. First, certain prefetch opportunities can
only be detected by knowing the data layout on the hard disk, such as metadata blocks. Second, due to
the non-uniform access cost on the hard disk, the penalty of mis-prefetching a random block is much
more costly than mis-prefetching a sequential block. In order to address the intrinsic limitations of file-
level prefetching, we propose to prefetch data blocks directly at the disk level in a portable way. Our
proposed scheme, called DiskSeen, is designed to supplement file-level prefetching. DiskSeen observes
the workload access pattern by tracking the locations and access times of disk blocks. Based on analy-
sis of the temporal and spatial relationships of disk data blocks, DiskSeen can significantly increase
the sequentiality of disk accesses and improve disk performance in turn. We implemented the DiskSeen
scheme in the Linux 2.6 kernel and we show that it can significantly improve the effectiveness of file-
level prefetching and reduce execution times by 20-53% for various types of applications, including
grep, CVS, and TPC-H.
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