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From: Gregory Stark <stark@enterprisedb.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance\@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Need help with 8.4 Performance Testing
Date: Wed, 10 Dec 2008 00:54:37 +0000
Message-ID: <87y6yohmsy.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <alpine.DEB.1.10.0812091732130.4666@aragorn.flymine.org>
References: <200812071138.01215.josh@agliodbs.com>
	<dcc563d10812072140q1cb257cbs8140b390759cb1a5@mail.gmail.com>
	<Pine.GSO.4.64.0812081442410.5242@westnet.com>
	<dcc563d10812081231y6c71601am677ba828ff0b515e@mail.gmail.com>
	<Pine.GSO.4.64.0812081731510.26019@westnet.com>
	<b42b73150812081723qdf7f203tcba3c3660023890f@mail.gmail.com>
	<Pine.GSO.4.64.0812082225470.13669@westnet.com>
	<493E6DBE.2040605@verizon.net>
	<BDFBB77C9E07BE4A984DAAE981D19F961ACA17D9FC@EXVMBX018-1.exch018.msoutlookonline.net>
	<dcc563d10812090921l90fd2eci8e1d163258d49ce0@mail.gmail.com>
	<alpine.DEB.1.10.0812091732130.4666@aragorn.flymine.org>


Matthew Wakeling <matthew@flymine.org> writes:

> On Tue, 9 Dec 2008, Scott Marlowe wrote:
>> I wonder how many hard drives it would take to be CPU bound on random
>> access patterns?  About 40 to 60?  And probably 15k / SAS drives to
>> boot.  Cause that's what we're looking at in the next few years where
>> I work.
>
> There's a problem with that thinking. That is, in order to exercise many
> spindles, you will need to have just as many (if not more) concurrent requests.
> And if you have many concurrent requests, then you can spread them over
> multiple CPUs. So it's more a case of "How many hard drives PER CPU". It also
> becomes a matter of whether Postgres can scale that well.

Well:

$ units
2445 units, 71 prefixes, 33 nonlinear units
You have: 8192 byte/5ms
You want: MB/s
	* 1.6384
	/ 0.61035156

At 1.6MB/s per drive if find Postgres is cpu-bound doing sequential scans at
1GB/s you'll need about 640 drives to keep one cpu satisfied doing random I/O
-- assuming you have perfect read-ahead and the read-ahead itself doesn't add
cpu overhead. Both of which are false of course, but at least in theory that's
what it'll take.


-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning



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