Re: RAID arrays and performance

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Matthew" <matthew(at)flymine(dot)org>
Cc: "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 16:23:22
Message-ID: 479EFE9A.EE98.0025.0@wicourts.gov
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>>> On Tue, Jan 29, 2008 at 9:52 AM, in message
<877ihsvdcb(dot)fsf(at)oxford(dot)xeocode(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>
wrote:

> I got this from a back-of-the-envelope calculation which now that I'm trying
> to reproduce it seems to be wrong. Previously I thought it was n(n+1)/2 or
> about n^2/2. So at 16 I would have expected about 128 pending i/o requests
> before all the drives could be expected to be busy.

That seems right to me, based on the probabilities of any new
request hitting an already-busy drive.

> Now that I'm working it out more carefully I'm getting that the expected
> number of pending i/o requests before all drives are busy is
> n + n/2 + n/3 + ... + n/n

What's the basis for that?

-Kevin

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