Date: Thu, 02 Apr 2009 19:31:11 +1300
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Raid 10 chunksize
In-reply-to: <alpine.GSO.2.01.0903261309220.7258@westnet.com>
To: Greg Smith <gsmith@gregsmith.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Message-id: <49D45BAF.7050709@paradise.net.nz>
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Greg Smith wrote:
>
>> Yeah - with 64K chunksize I'm seeing a result more congruent with 
>> yours (866 or so for 24 clients)
>
> That's good to hear.  If adjusting that helped so much, you might 
> consider aligning the filesystem partitions to the chunk size too; the 
> partition header usually screws that up on Linux.  See these two 
> references for ideas:  
> http://www.vmware.com/resources/techresources/608 
> http://spiralbound.net/2008/06/09/creating-linux-partitions-for-clariion
>

Well I went away and did this (actually organized for for the system 
folks to...). Retesting showed no appreciable difference (if anything 
slower). Then I got to thinking:

For a partition created on a (hardware) raided device, sure - alignment 
is very important, however in my case we are using software (md) raid - 
which creates devices out of individual partitions (which are on 
individual SAS disks) e.g:

md3 : active raid10 sda4[0] sdd4[3] sdc4[2] sdb4[1]
     177389056 blocks 256K chunks 2 near-copies [4/4] [UUUU]

I'm thinking that alignment issues do not apply here, as md will 
allocate chunks starting at the beginning of wherever sda4 (etc) begins 
- so the absolute starting position of sda4 is irrelevant. Or am I 
missing something?

Thanks again

Mark