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Re: With 4 disks should I go for RAID 5 or RAID 10

From: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
To: Fernando Hevia <fhevia(at)ip-tel(dot)com(dot)ar>
Cc: 'pgsql-performance' <pgsql-performance(at)postgresql(dot)org>
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
Date: 2007-12-26 16:23:28
Message-ID: 47728000.4000709@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-performance
Fernando Hevia wrote:
>
> Database will be about 30 GB in size initially and growing 10 GB per 
> year. Data is inserted overnight in two big tables and during the day 
> mostly read-only queries are run. Parallelism is rare.
>
> I have read about different raid levels with Postgres but the advice 
> found seems to apply on 8+ disks systems. With only four disks and 
> performance in mind should I build a RAID 10 or RAID 5 array? Raid 0 
> is overruled since redundancy is needed.
>
> I am going to use software Raid with Linux (Ubuntu Server 6.06).
>

In my experience, software RAID 5 is horrible. Write performance can 
decrease below the speed of one disk on its own, and read performance 
will not be significantly more than RAID 1+0 as the number of stripes 
has only increased from 2 to 3, and if reading while writing, you will 
not get 3X as RAID 5 write requires at least two disks to be involved. I 
believe hardware RAID 5 is also horrible, but since the hardware hides 
it from the application, a hardware RAID 5 user might not care.

Software RAID 1+0 works fine on Linux with 4 disks. This is the setup I 
use for my personal server.

Cheers,
mark

-- 
Mark Mielke <mark(at)mielke(dot)cc>

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