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SubjectRe: disk head scheduling
Date
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Rik van Riel wrote:
> On Fri, 19 Mar 1999, Mark H. Wood wrote:
> > On Fri, 19 Mar 1999, Ingo Molnar wrote:
> > > On Thu, 18 Mar 1999, Yasushi Saito wrote:
> > > > What I tried to implement was two-way elevator seeking (SCAN). In my
> > >
> > > the bigger problem is that dumber devices will just execute non-forwards
> > > ordered requests. Most modern harddisks will either cache a full track, or
> >
> > Well of course a 2-way elevator should sort by *ascending* sector within
> > descending track. I take it this is difficult?
>
> Modern devices never give you the physical layout of the disk,
> so yes, it is extremely difficult, if not impossible...

Guys, we're all concerned about "seek times" as they are quite
significant compared to the read times. A disk capable of 5Mb per
second can do about 200k - 400k in random-reads.

About 6 years ago, I decided that "fsck" was taking too long, and I
rewrote it to use the elevator algorithm. (as it is a single task,
waiting for every IO to complete, the kernel elevator algorithm didn't
have a chance to do its work: at most one IO request in the queue at
any time).

The result was less than 5% improvement.

Intuitively the elevator algorithm is pretty important. In practise it
isn't all that important:

-- Even During heavy IO there is not often more than a few threads
doing the IO, so the list of IOs that can be rearranged is not
significant.

-- The parameters for modern disks make things less profitable. My first
harddisk had a stepper motor for head-position that did around 1200
steps per second seeking 10 tracks takes 10 times longer than one track.
And a 100 tracks takes ten times longer than that. Modern disks seek
track-to-track in a few milliseconds, and do the whole disk in under
twenty. So fetching 20 random blocks in-order is going to be just
a few milliseconds faster than just fetching in the random order.


Roger.


--
** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
*-- BitWizard writes Linux device drivers for any device you may have! --*
* Never blow in a cat's ear because if you do, usually after three or *
* four times, they will bite your lips! And they don't let go for at *
* least a minute. -- Lisa Coburn, age 9

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