To: <david@lang.hm>
Cc: "Decibel!" <decibel@decibel.org>,  "Henrik" <henke@mac.se>,
	"Jeff" <threshar@threshar.is-a-geek.com>,
	"Luke Lonergan" <LLonergan@greenplum.com>,
	<pgsql-performance@postgresql.org>
Subject: Re: Filesystem benchmarking for pg 8.3.3 server
In-Reply-To: <alpine.DEB.1.10.0808161714280.12859@asgard.lang.hm>
	(david@lang.hm's message of "Sat\,
	16 Aug 2008 17\:15\:41 -0700 \(PDT\)")
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	<alpine.DEB.1.10.0808161714280.12859@asgard.lang.hm>
From: Gregory Stark <stark@enterprisedb.com>
Organization: EnterpriseDB
Date: Sun, 17 Aug 2008 11:55:36 +0100
Message-ID: <874p5jykpj.fsf@oxford.xeocode.com>
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<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!