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Re: Help with XFS in VMs on VMFS

To: Ben Myers <bpm@xxxxxxx>
Subject: Re: Help with XFS in VMs on VMFS
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Sat, 30 Mar 2013 14:12:36 -0500
Cc: Jan Perci <jperci@xxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130329202735.GX22182@xxxxxxx>
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Reply-to: stan@xxxxxxxxxxxxxxxxx
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On 3/29/2013 3:27 PM, Ben Myers wrote:
> Hi Jan,
> On Thu, Mar 28, 2013 at 11:30:01PM -0400, Jan Perci wrote:
>> Back to XFS, in this context, is there any benefit in tuning some
>> parameters to get better performance, or will it all just be overshadowed
>> by poor performance of the VMDKs that tuning isn't worthwhile?
> At least get your stripe unit and width correct.
> http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance

Is this really a good idea given that XFS sits atop a virtual disk which
consists of multiple concatenated 2TB sparse files sitting on the VMFS
filesystem, which, IIRC, has a 1MB sector size?  Thus can one rely on
XFS being able to properly align to the physical RAID stripe, even if
the math is done 'properly' (if that's even possible here)?

In a complex stack like this I'd recommend defaults across the board.
Misalignment hurts performance far more than proper alignment increases
it.  No alignment is agnostic, 4KB IOs only, so you neither gain nor lose.

> Beyond that I suggest you stick with the defaults unless you have a specific
> need.  e.g. heavy usage of extended attributes might prompt you to use a 
> larger
> inode size to keep them inline.


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