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Date: Tue, 06 Mar 2012 09:17:06 +0100
From: Yeb Havinga <yebhavinga@gmail.com>
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To: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
CC: pgsql-performance@postgresql.org
Subject: Re: SSD and RAID
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On 2012-03-05 23:37, Mark Kirkwood wrote:
> Which brings up the question of should it be a pair in RAID 1 or just 
> a singe drive? Traditionally this would have been a no brainer "Of 
> course you want RAID 1 or RAID 10"! However our experience with SSD 
> failure modes points to firmware bugs as primary source of trouble - 
> and these are likely to impact both drives (nearly) simultaneously in 
> a RAID 1 configuration. Also the other major issue to watch - flash 
> write limit exhaustion - is also likely to hit at the same time for a 
> pair of drives in RAID 1.
>
> What are other folks who are using SSDs doing?

This is exactly the reason why in a set of new hardware I'm currently 
evaluating two different brands of manufacturers for the spindles 
(behind bbwc for wal, os, archives etc) and ssds (on mb sata ports). For 
the ssd's we've chosen the Intel 710 and OCZ Vertex 2 PRO, however that 
last one was EOL and OCZ offered to replace it by the Deneva 2 
(http://www.oczenterprise.com/downloads/solutions/ocz-deneva2-r-mlc-2.5in_Product_Brief.pdf). 
Still waiting for a test Deneva though.

One thing to note is that linux software raid with md doesn't support 
discard, which might shorten the drive's expected lifetime. To get some 
numbers I tested the raid 1 of ssd's setup for mediawear under a 
PostgreSQL load earlier, see 
http://archives.postgresql.org/pgsql-general/2011-11/msg00141.php

<Greg Smith imitation mode on>I would recommended that for every ssd 
considered for production use, test the ssd with diskchecker.pl on a 
filesystem that's mounted the same as you would with your data (e.g. 
with xfs or ext4 with nobarrier), and also do a mediawear test like the 
one described in the linked pgsql-general threar above, especially if 
you're chosing to run on non-enterprise marketed ssds.</>

regards,
Yeb

PS: we applied the same philosophy (different brands) also to 
motherboards, io controllers and memory, but after testing, we liked one 
IO controllers software so much more than the other so we chose to have 
only one. Also stream memory performance of one motherboard showed a 
significant performance regression in the higher thread counts that we 
decided to go for the other brand for all servers.

-- 
Yeb Havinga
http://www.mgrid.net/
Mastering Medical Data