Re: Opinions on Raid

Really appreciate all of the valuable input. The current server has the
Perc4ei controller.

The impression I am taking from the responses is that we may be okay with
software raid, especially if raid 1 and 10 are what we intend to use.

I think we can collect enough information from the archives of this list to
help make decisions for the new machine(s), was just very interested in
hearing feedback on software vs. hardware raid.

We will likely be using the 2.6.18 kernel.

Thanks for everyone's input,

Joe

-----Original Message-----
From: Scott Marlowe [mailto:smarlowe(at)g2switchworks(dot)com]
Sent: Tuesday, February 27, 2007 12:56 PM
To: Joe Uhl
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] Opinions on Raid

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.