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Re: deleting 2TB lots of files with delaylog: sync helps?

To: xfs@xxxxxxxxxxx
Subject: Re: deleting 2TB lots of files with delaylog: sync helps?
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 02:22:31 +0200
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
In-reply-to: <20100901000631.GO705@dastard>
Organization: it-management http://it-management.at
References: <201009010130.41500@xxxxxx> <20100901000631.GO705@dastard>
User-agent: KMail/1.12.4 (Linux/2.6.35.4-zmi; KDE/4.3.5; x86_64; ; )
On Mittwoch, 1. September 2010 Dave Chinner wrote:
> You're probably getting RMW cycles on inode writeback. I've been
> noticing this lately with my benchmarking - the VM is being _very
> aggressive_ reclaiming page cache pages vs inode caches and as a
> result the inode buffers used for IO are being reclaimed between the
> time it takes to create the inodes and when they are written back.
> Hence you get lots of reads occurring during inode writeback.
> 
> By issuing a sync, you clear out all the inode writeback and all the
> RMW cycles go away. As a result, there is more disk throughput
> availble for the unlink processes.  There is a good chance this is
> the case as the number of reads after the sync drop by an order of
> magnitude...

Nice explanation.
 
> > Now it can be that the sync just causes more writes and stalls
> > reads so overall it's slower, but I'm wondering why none of the
> > devices says "100% util", which should be the case on deletes? Or
> > is this again the "mistake" of the utilization calculation that
> > writes do not really show up there?
> 
> You're probably CPU bound, not IO bound.

This is a hexa-core AMD Phenom(tm) II X6 1090T Processor with up to 
3.2GHz per core, so that shouldn't be - or is there only one core used? 
I think I read somewhere that each AG should get a core or so...
 
Thanks for your explanation.

-- 
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Michael Monnerie, Ing. BSc

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