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Re: Stalls during writeback for mmaped I/O on XFS in 3.0

To: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxxx>
Subject: Re: Stalls during writeback for mmaped I/O on XFS in 3.0
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 15 Sep 2011 10:55:57 -0400
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20110915144755.GB2235@xxxxxxxxxxxxxxxxxxxxxxxxx>
References: <20110915144755.GB2235@xxxxxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Sep 15, 2011 at 09:47:55AM -0500, Shawn Bohrer wrote:
> I've got a workload that is latency sensitive that writes data to a
> memory mapped file on XFS.  With the 3.0 kernel I'm seeing stalls of
> up to 100ms that occur during writeback that we did not see with older
> kernels.  I've traced the stalls and it looks like they are blocking
> on wait_on_page_writeback() introduced in
> d76ee18a8551e33ad7dbd55cac38bc7b094f3abb "fs: block_page_mkwrite
> should wait for writeback to finish"
> 
> Reading the commit description doesn't really explain to me why this
> change was needed.

It it there to avoid pages beeing modified while they are under
writeback, which defeats various checksumming like DIF/DIX, the iscsi
CRCs, or even just the RAID parity calculations.  All of these either
failed before, or had to work around it by copying all data was
written.

If you don't use any of these you can remove the call and things
will work like they did before.

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