|University of Michigan
Technical Report CSE-TR-194-94, March, 1994.
Modern Disk Drives and
Bruce L. Worthington, Gregory
R. Ganger, Yale N. Patt
Department of Electrical Engineering
and Computer Science
The University of Michigan, Ann Arbor 48109-2122
March 1, 1994
Disk subsystem performance
can be dramatically improved by dynamically ordering, or scheduling, pending
requests. Modern disk drives have several features, such as complex logical-to-physical
mappings and large prefetching caches, that can influence scheduling effectiveness.
Via strongly validated simulation, we examine the impact of these features
on various scheduling algorithms. Using both synthetic workloads and traces
captured from six different user environments, we arrive at three main
conclusions: (1) Incorporating complex mapping information into the scheduler
provides only a marginal (less than 2%) decrease in response times for
seek-reducing algorithms. (2) Algorithms which effectively utilize a prefetching
disk cache provide significant performance improvements for workloads
with read sequentiality. The cyclical scan algorithm (C-LOOK), which always
schedules requests in ascending logical order, achieves the highest performance
among seek-reducing algorithms for such workloads (five of the six examined).
(3) Algorithms that reduce overall positioning delays produce the highest
performance provided that they recognize and exploit a prefetching cache.
This postscript version of
this report has been broken into three files for on-line access: