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Re: [PATCH] xfs: improve sync behaviour in face of aggressive dirtying

To: xfs@xxxxxxxxxxx
Subject: Re: [PATCH] xfs: improve sync behaviour in face of aggressive dirtying
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 21 Jun 2011 09:21:46 +0200
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>
In-reply-to: <20110621003343.GJ32466@dastard>
Organization: it-management http://it-management.at
References: <20110617131401.GC2141@xxxxxxxxxxxxx> <20110620081802.GA27111@xxxxxxxxxxxxx> <20110621003343.GJ32466@dastard>
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On Dienstag, 21. Juni 2011 Dave Chinner wrote:
> > The minor one is that we always flush all work items and not just
> > those on the filesystem to be flushed.  This might become an issue
> > for lager systems, or when we apply a similar scheme to fsync,
> > which has the same underlying issue.
> For sync, I don't think it matters if we flush a few extra IO
> completions on a busy system.

Couldn't that be bad on a system with mixed fast/slow storage (say 15k 
SAS and 7.2k SATA), where on the busy fast SAS lots of syncs occur and 
lead to extra I/O on the SATA disks? Especially if there are 16 SAS 
disks in an array with RAID-0 against 4 SATA disks in RAID-6, to say the 
worst. If the SATAs are already heavy used (say >=50%), those extra 
writes could bring them to their knees.

I'm not sure how often syncs occur though, maybe that's why Dave says it 
shouldn't matter? AFAIK, databases generate heavy syncs though.

mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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