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Re: read slower than write on "mv"?

To: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Subject: Re: read slower than write on "mv"?
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 3 May 2010 21:52:07 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <201005031054.38817@xxxxxx>
References: <201005031054.38817@xxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon, May 03, 2010 at 10:54:28AM +0200, Michael Monnerie wrote:
> This is not XFS specific, I see it on every filesys. I do a 
> "mv . /newlocation" and see this with iostat:
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-
> sz avgqu-sz   await  svctm  %util
> xvdb              608,60    2,60 25967,20   102,40    85,31     5,37    
> 8,77   1,58  96,72
> xvdg              0,00  608,60     0,00 26532,10    87,19     7,55   
> 12,41   0,12   7,44
> 
> Reading takes 97% I/O time, writing 7%. This is on the same raidset, but 
> I see the same when copying between two different single disks also. Or 
> is it just an effect of write caching that writes look faster than 
> reads?

Yes, just an effect of write caching hiding IO latency.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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