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From: Kevin Grittner <Kevin.Grittner@wicourts.gov>
To: Gregory Stark <stark@enterprisedb.com>
To: Matthew <matthew@flymine.org>
Cc: Mark Mielke <mark@mark.mielke.cc>
Cc: pgsql-performance@postgresql.org
Subject: Re: RAID arrays and performance
Date: Tue, 29 Jan 2008 10:23:22 -0600
Message-ID: <479EFE9A.EE98.0025.0@wicourts.gov> (raw)
In-Reply-To: <877ihsvdcb.fsf@oxford.xeocode.com>
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	<877ihsvdcb.fsf@oxford.xeocode.com>

>>> On Tue, Jan 29, 2008 at  9:52 AM, in message
<877ihsvdcb.fsf@oxford.xeocode.com>, Gregory Stark <stark@enterprisedb.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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