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Re: Hardware vs Software RAID

From: Matthew Wakeling <matthew(at)flymine(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Hardware vs Software RAID
Date: 2008-06-25 13:03:58
Message-ID: Pine.LNX.4.64.0806251355090.4085@aragorn.flymine.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Wed, 25 Jun 2008, Merlin Moncure wrote:
>> Has anyone done some benchmarks between hardware RAID vs Linux MD software
>> RAID?
>
> I have here:
> http://merlinmoncure.blogspot.com/2007/08/following-are-results-of-our-testing-of.html
>
> The upshot is I don't really see a difference in performance.

The main difference is that you can get hardware RAID with 
battery-backed-up cache, which means small writes will be much quicker 
than software RAID. Postgres does a lot of small writes under some use 
cases.

Without a BBU cache, it is sensible to put the transaction logs on a 
separate disc system to the main database, to make the transaction log 
writes fast (due to no seeking on those discs). However, with a BBU cache, 
that advantage is irrelevant, as the cache will absorb the writes.

However, not all hardware RAID will have such a battery-backed-up cache, 
and those that do tend to have a hefty price tag.

Matthew

-- 
$ rm core
Segmentation Fault (core dumped)

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Subject: Re: Hardware vs Software RAID
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