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Re: 12 disks raid setup

From: Shane Ambler <pgsql(at)Sheeky(dot)Biz>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: 12 disks raid setup
Date: 2008-03-01 00:44:09
Message-ID: 47C8A6D9.1050502@Sheeky.Biz (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Greg Smith wrote:
> On Fri, 29 Feb 2008, Franck Routier wrote:
>> my Raid controller is an Adaptec 31205 SAS/RAID controller. The battery
>> was an option, but I didn't know it at purchase time. So I have no
>> battery, but the whole system is on an UPS.
> The UPS is of no help here.  The problem is that PostgreSQL forces the 
> disk controller to commit WAL writes to disk after every transaction.  
> If you have a controller with a battery-backed cache, you can use that 
> cache to buffer those writes and dramatically increase write 
> performance.  The USP doesn't give you the same write guarantees.  Let's 
> say someone trips over the server power cord (simplest example of a 
> whole class of failures).  With the BBC controller, the cached writes 
> will get committed when you plug the server back in.  If all you've got 
> is a UPS, writes that didn't make it to disk before the outage are 
> lost.  That means you can't buffer those writes without risking database 
> corruption.
> The general guideline here is that if you don't have a battery-backed 
> cache on your controller, based on disk rotation speed you'll be limited 
> to around 100 (7200 RPM) to 200 (15K RPM) commits/second per single 
> client, with each commit facing around a 2-4ms delay.  That rises to 
> perhaps 500/s total with lots of clients.  BBC configurations can easily 
> clear 3000/s total and individual commits don't have that couple of ms 
> delay.

It may be the way you have worded this but it makes it sound like the 
cache and the battery backup are as one (or that the cache doesn't work 
unless you have the battery) The cache may be optional (or plug-in) in 
some cards, even of varied size. The battery is normally optional. You 
can normally add/remove the battery without changing the cache options.

If the raid card has the cache without the battery you would get the 
performance figures you mentioned, you just wouldn't have the 
reliability of finishing writes after a power off situation.

correct me if I am wrong here.


Shane Ambler
pgSQL (at) Sheeky (dot) Biz

Get Sheeky @ http://Sheeky.Biz

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