Re: disk arms (was RE: Tips for user ASP)

["Charly Jones" <charly301@xxxxxxxxxxx>]

>From: "Dan Bale" <dbale@samsa.com>
>
> >I agree Al.  In fact I think the next performance bottleneck _has been_
> >DASD arm contention for several years now.  Larry Bolhuis and I have
> >beat our drums, and beat our drums, and beat our drums, but everyone
> >has said: no, no, no, probably not a problem, because of faster disks,
> >faster disk controllers, and write cache, and more write cache, and you
> >should believe in the new technology...
>
>I am no performance guru.  I ask this purely for education, since I am a
>little bit puzzled by the angst I'm reading here.
>
>With the recent announcements and discussion about the 4gb drives going
>away
>and, now, the smallest drives you can buy are 8gb, and both the 4gb & 8gb
>drives have the same number of arms and, so, therefore, the arms must cover
>twice the amount of DASD.
>
>However, the other part of the announcement & discussion was that the price
>of the 8gb drives dropped to the level of the discontinued 4gb drives.  If
>so, what has been lost?  The managers concerned about disk arm performance
>now have to, generally speaking (and perhaps easier said than done), ensure
>that DASD utilization on the 8gb drives never exceeds half of what they
>would allow the 4gb drives to exceed.  Their costs have not been increased,
>as far as I can tell.  They are paying the same amount for the number of
>arms, which is the critical factor, more so than the drive capacity.
>

I am not a manager.  But I have met a few.  And very few of the ones I've
met would be willing (or able) to actually follow through on the
consequences of what I believe you have proposed.  If you (and your
managers, to say nothing of the CFO) were willing to cut in half the
"storage threshold" value and forever and always not allow your systems to
go above 45% disk used (assuming that the threshold was 90% before), then I
would almost agree with you.  There are _some_ performance reasons to prefer
a 90% full 4 gigabyte drive over a 45% full 8 gigabyte drive.  You can think
of it as the performance "sweet spot" on the disk.  Especially if you have
RAID enabled.  But I don't really think very many folks are using RAID, are
they?

And if your disk vendor ever encouraged you to buy 18 gigabyte drives, you
would lower the threshold again, wouldn't you?  Another 50% or so?

The angst is real.  The pain is experienced by almost all...

--  Charly


"Nothing would please me more than being able to hire ten programmers and
deluge the hobby market with good software."  -  Bill Gates in 1976

"We are still waiting..."  -  Alan Cox in 2002

"Linux is only free if your time is worthless."


Charly Jones
253 265-6244
Gig Harbor
Washington USA



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