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Re: Opinions on Raid

From: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
To: Joe Uhl <joeuhl(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Opinions on Raid
Date: 2007-02-27 17:56:07
Message-ID: 1172598967.20651.72.camel@state.g2switchworks.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 2007-02-27 at 07:12, Joe Uhl wrote:
> We have been running Postgres on a 2U server with 2 disks configured in
> raid 1 for the os and logs and 4 disks configured in raid 10 for the
> data.  I have since been told raid 5 would have been a better option
> given our usage of Dell equipment and the way they handle raid 10.

Some controllers do no layer RAID effectively.  Generally speaking, the
cheaper the controller, the worse it's gonna perform.

Also, some controllers are optimized more for RAID 5 than RAID 1 or 0.

Which controller does your Dell have, btw?

>   I
> have just a few general questions about raid with respect to Postgres:
> 
> [1] What is the performance penalty of software raid over hardware raid?
>  Is it truly significant?  We will be working with 100s of GB to 1-2 TB
> of data eventually.

For a mostly read system, the performance is generally pretty good. 
Older linux kernels ran layered RAID pretty slowly.  I.e. RAID 1+0 was
no faster than RAID 1.  The best performance software RAID I found in
older linux kernels (2.2, 2.4) was plain old RAID-1.  RAID-5 was good at
reading, but slow at writing.


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