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From: Gregory Stark <stark@enterprisedb.com>
To: david@lang.hm
Cc: Decibel! <decibel@decibel.org>
Cc: Henrik <henke@mac.se>
Cc: Jeff <threshar@threshar.is-a-geek.com>
Cc: Luke Lonergan <LLonergan@greenplum.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Filesystem benchmarking for pg 8.3.3 server
Date: Sun, 17 Aug 2008 11:55:36 +0100
Message-ID: <874p5jykpj.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <alpine.DEB.1.10.0808161714280.12859@asgard.lang.hm>
References: <014F2941B0A1EA47BD61D21526B806E903509704@MI8NYCMAIL08.Mi8.com>
	<F54DECFD-927B-4F64-B913-E8D903892435@mac.se>
	<alpine.DEB.1.10.0808081917080.4561@asgard.lang.hm>
	<AC012133-6932-46DD-A723-B2DCF98E5380@mac.se>
	<2DBD3E73-D9E5-4DFA-832F-3FD53480B284@torgo.978.org>
	<8795DFE4-BFE1-49A5-8341-A6823DF2BE4E@decibel.org>
	<32CD744F-2C1A-4BE2-B4BA-90CFD3A86DF7@mac.se>
	<04470CDD-146A-4725-BBC5-22E3271B958A@decibel.org>
	<alpine.DEB.1.10.0808161714280.12859@asgard.lang.hm>

<david@lang.hm> writes:

>> If you are completely over-writing an entire stripe, there's no reason to
>> read the existing data; you would just calculate the parity information from
>> the new data. Any good controller should take that approach.
>
> in theory yes, in practice the OS writes usually aren't that large and aligned,
> and as a result most raid controllers (and software) don't have the
> special-case code to deal with it.

I'm pretty sure all half-decent controllers and software do actually. This is
one major reason that large (hopefully battery backed) caches help RAID-5
disproportionately. The larger the cache the more likely it'll be able to wait
until the entire raid stripe is replaced avoid having to read in the old
parity.


-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's 24x7 Postgres support!



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