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Date: Tue, 29 Jan 2008 11:13:54 -0600
From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
To: "Gregory Stark" <gsstark@mit.edu>
Cc: "Matthew" <matthew@flymine.org>, "Mark Mielke" <mark@mark.mielke.cc>,
	<pgsql-performance@postgresql.org>
Subject: Re: RAID arrays and performance
References: <Pine.LNX.4.58.0712041213490.3731@aragorn.flymine.org>
	<87lk8aiqf3.fsf@oxford.xeocode.com>
	<Pine.LNX.4.58.0712041247540.3731@aragorn.flymine.org>
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	<4755648C.90905@mark.mielke.cc>
	<Pine.LNX.4.58.0712041454110.3731@aragorn.flymine.org>
	<87bq96ii3b.fsf@oxford.xeocode.com>
	<Pine.LNX.4.58.0712041603490.3731@aragorn.flymine.org>
	<4755810E.4000707@mark.mielke.cc>
	<Pine.LNX.4.58.0712041637260.3731@aragorn.flymine.org>
	<87zlwq6yz9.fsf@oxford.xeocode.com>
	<Pine.LNX.4.64.0801291417330.4642@aragorn.flymine.org>
	<87bq74vfys.fsf@oxford.xeocode.com>
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	<Pine.LNX.4.64.0801291505180.4642@aragorn.flymine.org>
	<877ihsvdcb.fsf@oxford.xeocode.com> <479EFE9A.EE98.0025.0@wicourts.gov>
	<873asginrz.fsf@stark.xeocode.com>
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>>> On Tue, Jan 29, 2008 at 10:45 AM, in message
<873asginrz.fsf@stark.xeocode.com>, Gregory Stark <gsstark@mit.edu> =
wrote:=20
=20
> Well consider when you've reached n-1 drives; the expected number of =
requests
> before you hit the 1 idle drive remaining out of n would be n requests. =
When
> you're at n-2 the expected number of requests before you hit either of =
the=20
> two
> idle drives would be n/2. And so on. The last term of n/n would be the =
first
> i/o when all the drives are idle and you obviously only need one i/o to =
hit=20
> an
> idle drive.
=20
You're right.  Perhaps the reason more requests continue to improve
performance is that a smart controller will move across the tracks
and satisfy the pending requests in the most efficient order?
=20
-Kevin
=20