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Re: How to setup disk spindles for best performance

From: Christiaan Willemsen <cwillemsen(at)technocon(dot)com>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: How to setup disk spindles for best performance
Date: 2008-08-29 05:07:58
Message-ID: E3A3D7E0-04D5-48F6-B498-8A02F864316D@technocon.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Aug 29, 2008, at 4:43 AM, Greg Smith wrote:

> On Thu, 21 Aug 2008, Christiaan Willemsen wrote:
>
>> Anyway, I'm going to return the controller, because it does not  
>> scale very well with more that 4 disks in raid 10. Bandwidth is  
>> limited to 350MB/sec, and IOPS scale badly with extra disks...
>
> How did you determine that upper limit?  Usually it takes multiple  
> benchmark processes running at once in order to get more than 350MB/ 
> s out of a controller.  For example, if you look carefully at the  
> end of http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_50_spindles_testing_for_postgresql/ 
>  you can see that Joshua had to throw 8 threads at the disks in  
> order to reach maximum bandwidth.

I used IOmeter to do some tests, with 50 worker thread doing jobs. I  
can get more than 350 MB/sec, I'll have to use huge blocksizes  
(something like 8 MB). Even worse is random read and 70%read, 50%  
random tests. They don't scale at all when you add disks.  A 6 disk  
raid 5 is exactly as fast as a 12 disk raid 10 :(

>> The idea for xlog + os on 4 disk raid 10 and the rest for the data  
>> sound good
>
> I would just use a RAID1 pair for the OS, another pair for the xlog,  
> and throw all the other disks into a big 0+1 set.  There is some  
> value to separating the WAL from the OS disks, from both the  
> performance and the management perspectives.  It's nice to be able  
> to monitor the xlog write bandwidth rate under load easily for  
> example.

Yes, that's about what I had in mind.

Kind regards,

Christiaan

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