Re: RAID arrays and performance

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Gregory Stark" <gsstark(at)mit(dot)edu>
Cc: "Matthew" <matthew(at)flymine(dot)org>, "Mark Mielke" <mark(at)mark(dot)mielke(dot)cc>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: RAID arrays and performance
Date: 2008-01-29 17:13:54
Message-ID: 479F0A72.EE98.0025.0@wicourts.gov
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>>> On Tue, Jan 29, 2008 at 10:45 AM, in message
<873asginrz(dot)fsf(at)stark(dot)xeocode(dot)com>, Gregory Stark <gsstark(at)mit(dot)edu> wrote:

> 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
> 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
> an
> idle drive.

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?

-Kevin

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