Re: Disk Scheduling
Re: Disk Scheduling
- Subject: Re: Disk Scheduling
- From: Shawn Erickson <email@hidden>
- Date: Tue, 25 Mar 2003 14:16:13 -0800
On Tuesday, March 25, 2003, at 12:06 PM, Joey Echeverria wrote:
I'm talking about the low level scheduling of writing logical blocks
out to disk. For example, FIFO, SSTF, LOOK, C-LOOK, etc. The reason
that I'm interested is that we are looking into power efficiency in
operating systems. One thing we wanted to investigate is if there is
any effect to the energy usage when using different algorithms. One
idea that we had was to be able to queue requests for longer in an
operating system buffer and only write to disk when the queue becomes
a certain size. We were thinking that under such a scheme, you could
keep the disk stopped for longer periods of time to save the energy
from keeping it spinning.
To my knowledge Mac OS X does no buffering of writes to disk, it simply
uses a write through caching scheme (file system structure updates may
be cached for a short bit I think). I also believe it does no
scheduling or coalescing of IO requests, just simply FIFO (or whatever
the bus/disk driver is implemented to do). It leaves the scheduling up
to the disk / array controller.
Mac OS X does host a broad set of applications that have a wide set of
IO profiles that makes implementing an in OS disk scheduling system
problematic. Such a scheme can also compete or in the least be
redundant to logic housed in the disk / array controllers logic.
I would agree that caching could allow one to reduce the amount of disk
spin up for sparse in time writes. I think write caching would have far
more of a potential impact on power consumption then choosing a
particular block scheduling algorithm (especially given current drives
having rather intelligent embedded head schedulers).
To do things correctly... the write caching logic would have to be
adaptive so it can learn the current behavior of the IO profile to
correctly defer writes to maximize disk down time and that weighted
against the power draw related to spinning things back up (as I am sure
you know, it take less power to keep a disk spinning then spinning it
back up to speed).
-Shawn
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