Message-ID: <4D026A09.3080108@tweakers.net>
Date: Fri, 10 Dec 2010 18:57:29 +0100
From: Arjen van der Meijden <acmmailing@tweakers.net>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US;
	rv:1.9.2.13) Gecko/20101207 Lightning/1.0b2 Thunderbird/3.1.7
MIME-Version: 1.0
To: pgsql-performance@postgresql.org
Subject: Re: Hardware recommendations
References: <685022.3525.qm@web111301.mail.gq1.yahoo.com>
In-Reply-To: <685022.3525.qm@web111301.mail.gq1.yahoo.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

On 10-12-2010 14:58 Andy wrote:
>> We use ZFS and use SSDs for both the log device and L2ARC.  All
>> disks and SSDs are behind a 3ware with BBU in single disk mode.
>
> Out of curiosity why do you put your log on SSD? Log is all
> sequential IOs, an area in which SSD is not any faster than HDD. So
> I'd think putting log on SSD wouldn't give you any performance
> boost.

The "common knowledge" you based that comment on, may actually not be 
very up-to-date anymore. Current consumer-grade SSD's can achieve up to 
200MB/sec when writing sequentially and they can probably do that a lot 
more consistent than a hard disk.

Have a look here: http://www.anandtech.com/show/2829/21
The sequential writes-graphs consistently put several SSD's at twice the 
performance of the VelociRaptor 300GB 10k rpm disk and that's a test 
from over a year old, current SSD's have increased in performance, 
whereas I'm not so sure there was much improvement in platter based 
disks lately?

Apart from that, I'd guess that log-devices benefit from reduced latencies.

Its actually the recommended approach from Sun to add a pair of (small 
SLC-based) ssd log devices to increase performance (especially for 
nfs-scenario's where a lot of synchonous writes occur) and they offer it 
as an option for most of their "Unified Storage" appliances.

Best regards,

Arjen