Message-ID: <4C4B1F84.5030306@2ndquadrant.com>
Date: Sat, 24 Jul 2010 13:14:44 -0400
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Yeb Havinga <yebhavinga@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Testing Sandforce SSD
References: <4C4A9452.9070100@gmail.com>
In-Reply-To: <4C4A9452.9070100@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Yeb Havinga wrote:
> Probably like many other's I've wondered why no SSD manufacturer puts 
> a small BBU on a SSD drive. Triggered by Greg Smith's mail 
> http://archives.postgresql.org/pgsql-performance/2010-02/msg00291.php 
> here, and also anandtech's review at 
> http://www.anandtech.com/show/2899/1 (see page 6 for pictures of the 
> capacitor) I ordered a SandForce drive and this week it finally arrived.

Note that not all of the Sandforce drives include a capacitor; I hope 
you got one that does!  I wasn't aware any of the SF drives with a 
capacitor on them were even shipping yet, all of the ones I'd seen were 
the chipset that doesn't include one still.  Haven't checked in a few 
weeks though.

> * How to test for power failure?

I've had good results using one of the early programs used to 
investigate this class of problems:  
http://brad.livejournal.com/2116715.html?page=2

You really need a second "witness" server to do this sort of thing 
reliably, which that provides.

> * What filesystem to use on the SSD? To minimize writes and maximize 
> chance for seeing errors I'd choose ext2 here. 

I don't consider there to be any reason to deploy any part of a 
PostgreSQL database on ext2.  The potential for downtime if the fsck 
doesn't happen automatically far outweighs the minimal performance 
advantage you'll actually see in real applications.  All of the 
benchmarks showing large gains for ext2 over ext3 I have seen been 
synthetic, not real database performance; the internal ones I've run 
using things like pgbench do not show a significant improvement.  (Yes, 
I'm already working on finding time to publicly release those findings)

Put it on ext3, toggle on noatime, and move on to testing.  The overhead 
of the metadata writes is the least of the problems when doing 
write-heavy stuff on Linux.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us