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Date: Tue, 27 Mar 2012 12:03:00 -0400
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] xfs: Do background CIL flushes via a workqueue
Message-ID: <20120327160300.GA22555@infradead.org>
References: <1332841605-3538-1-git-send-email-david@fromorbit.com>
	<20120327143127.GA11434@infradead.org>
	<20120327155759.GB28707@redhat.com>
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To: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>, xfs@oss.sgi.com

On Tue, Mar 27, 2012 at 11:57:59AM -0400, Vivek Goyal wrote:
> On Tue, Mar 27, 2012 at 10:31:27AM -0400, Christoph Hellwig wrote:
> > Vivek, does CFQ still need any hints for this sort of handoff?
> > 
> 
> Christoph, I don't understand the issue enough to comment on it.
> 
> Had a quick look at the patch. Looks like some action (writing log), has
> been moved to a worker thread.  And in some cases (log force triggered
> flush, whatever it is), we seem to prefer to do it from the submitter's
> context.

Yes.  This is to workaround the old problem of cfq getting utterly
confused if cooperating I/O beeing submitted from different threads.

The case in the previous version of this patch was:

 - thread doing the fsync will write out data, and wait for it
 - then we'd force the log by kicking a workqueue and waiting for it

quite similar to the ext3/4 fsync issues that we had long discussions
about.

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