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Subject: Re: Random write result differences between RAID device and XFS
References: <56AB44AF.8020807@purplehaze.ch> <20160129222553.GK20456@dastard>
From: Christian Affolter <c.affolter@purplehaze.ch>
Message-ID: <56AC93EC.80202@purplehaze.ch>
Date: Sat, 30 Jan 2016 11:43:56 +0100
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To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com

Hi Dave,

On 29.01.2016 23:25, Dave Chinner wrote:
> On Fri, Jan 29, 2016 at 11:53:35AM +0100, Christian Affolter wrote:
>> Hi everyone,
>>
>> I'm trying to understand the differences of some bandwidth and IOPs test
>> results I see while running a random-write full-stripe-width aligned fio
>> test (using libaio with direct IO) on a hardware RAID 6 raw device
>> versus on the same device with the XFS file system on top of it.
>>
>> On the raw device I get:
>> write: io=24828MB, bw=423132KB/s, iops=137, runt= 60085msec
>>
>> With XFS on top of it:
>> write: io=14658MB, bw=249407KB/s, iops=81, runt= 60182msec
> 
> Now repeat with a file that is contiguously allocated before you
> start. And also perhaps with the "swalloc" mount option.

Wow, thanks! After specifying --fallocate=none (instead of the default
fallocate=posix), bandwidth and iops increases and are even higher than
on the raw device:

write: io=30720MB, bw=599232KB/s, iops=195, runt= 52496msec

I'm eager to learn what's going on behind the scenes, can you give a
short explanation?

Btw. mounting the volume with "swalloc" didn't make any change.


Thanks a lot!
Chris

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