Message-ID: <48B709A5.3090906@emolecules.com>
Date: Thu, 28 Aug 2008 13:25:09 -0700
From: Craig James <craig_james@emolecules.com>
User-Agent: Thunderbird 2.0.0.16 (Macintosh/20080707)
MIME-Version: 1.0
To: pgsql-performance@postgresql.org
Subject: Re: Best hardware/cost tradoff?
References: <g96ttt$2l3m$1@news.hub.org>
	<064701c90949$51d117d0$8f01010a@iptel.com.ar>
In-Reply-To: <064701c90949$51d117d0$8f01010a@iptel.com.ar>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

>> -----Mensaje original-----
>> De: pgsql-performance-owner@postgresql.org 
>> * I think we will go for hardware-based RAID 1 with a good 
>> battery-backed-up controller. I have read that software RAID 
>> perform surprisingly good, but for a production site where 
>> hotplug replacement of dead disks is required, is software 
>> RAID still worth it?
>> ... 
> I havent had any issues with software raid (mdadm) and hot-swaps. It keeps
> working in degraded mode and as soon as you replace the defective disk it
> can reconstruct the array on the fly. Performance will suffer while at it
> but the service keeps up.
> The battery backup makes a very strong point for a hw controller. Still, I
> have heard good things on combining a HW controller with JBODS leaving the
> RAID affair to mdadm. In your scenario though with "*lots* of random reads",
> if I had to choose between a HW controller & 2 disks or software RAID with 4
> or 6 disks, I would go for the disks. There are motherboards with 6 SATA
> ports. For the money you will save on the controller you can afford 6 disks
> in a RAID 10 setup.

This is good advice.  Hot-swapping seems cool, but how often will you actually use it? Maybe once every year?  With Software RAID, replacing a disk means shutdown, swap the hardware, and reboot, which is usually less than ten minutes, and you're back in business.  If that's the only thing that happens, you'll have 99.97% uptime on your server.

If you're on a limited budget, a software RAID 1+0 will be very cost effective and give good performance for lots of random reads.  Hardware RAID with a battery-backed cache helps with writes and hot swapping.  If your random-read performance needs outweigh these two factors, consider software RAID.

Craig