Thwarting the Power-Hungry Disk
Fred Douglis
Matsushita Information Technology Laboratory
P. Krishnan
Brown University
Brian Marsh
Matsushita Information Technology Laboratory
Abstract
Minimizing power consumption is important for mobile computers, and
disks consume a significant portion of system-wide power. There is a
large difference in power consumption between a disk that is spinning
and one that is not, so systems try to keep the disk spinning only
when it must. The system must trade off between the power that can be
saved by spinning the disk down quickly after each access and the
impact on response time from spinning it up again too often. We use
trace-driven simulation to examine these trade-offs, and compare a
number of different algorithms for controlling disk spin-down. We
simu- late disk accesses from a mobile computer (a Macintosh Powerbook
Duo 230) and also from a desktop workstation (a Hewlett-Packard
9000/845 personal workstation running HP-UX), running on two disks
used on mobile computers, the Hewlett-Packard Kittyhawk C3014A and the
Quantum GoDrive 120. We show that the ``perfect'' off-line
algorithm---one that consumes minimum power without increasing
response time relative to a disk that never spins down---can reduce
disk power consumption by 35--50%, compared to the fixed threshold
suggested by manufacturers. An on-line algorithm with a threshold of
10 seconds, running on the Powerbook trace and GoDrive disk, reduces
energy consumption by about 40% compared to the the 5-minute threshold
recommended by manufacturers of comparable disks; however, over a
4-hour trace period it results in 140 additional delays due to disk
spin-ups.
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