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Re: A huge latency in ext4 and xfs because of stable page write

To: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Subject: Re: A huge latency in ext4 and xfs because of stable page write
From: Zheng Liu <gnehzuil.liu@xxxxxxxxx>
Date: Thu, 13 Dec 2012 10:30:26 +0800
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, tytso@xxxxxxx, hch@xxxxxxxxxxxxx
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Hi Darrick,

On Wed, Dec 12, 2012 at 11:47:10AM -0800, Darrick J. Wong wrote:
> On Wed, Dec 12, 2012 at 01:13:32PM +0800, Zheng Liu wrote:
> > On Wed, Dec 12, 2012 at 03:49:49PM +1100, Dave Chinner wrote:
> > [cut...]
> > > > > > Hence, I wonder whether or not we could revert stable page write 
> > > > > > temporarily.
> > > > > > After it is improved, we could add it back again.
> > > > > 
> > > > > The plan is to turn it off for filesystems/devices that don't
> > > > > require it. That list of devices will grow in future, so you
> > > > > probably should plan to handle latencies in the application
> > > > > properly...
> > > > 
> > > > I wonder whether we can provide a sysctl to turn on/off stable page
> > > > write.  At least we need to give sysadmin an opportunity to control it.
> > > 
> > > That's already been considered and discarded because turning off
> > > stable pages on devices that require it will cause validation or
> > > data corruption problems. The discussion was for these patches (and
> > > I think a followup series as well):
> > > 
> > > http://www.spinics.net/lists/linux-fsdevel/msg59421.html
> > 
> > Thanks for pointing out.  So now it seems that only I can do is to
> > present a proposal to revert stable page write temporarily because it
> > causes a huge latency for some applications.
> 
> Hrm... just to be clear, is your complaint that you have one of these
> checksum-happy disks and overwrites are slow on it, or that you have a regular
> SATA disk that doesn't require stable pages and you don't want to take the
> speed hit?
> 
> If it's the second, then let's just push the bdi flag thing upstream.  Given
> your comments a couple of days ago, I'm pretty sure it's the second, but I
> figured I ought to clarify the record. :)

Yes, it's the second.  In our product system we have a large number of
SATA disks that don't require stable pages.  So that would be great if
we have a method to turn on/off stable page.

> 
> I haven't posted new patches because I've been busy writing a fix for ext3.
> It seems to be working, so I'll clean it up and send out a new series.

Cool!  We are looking forward the new patch series. :-)

Regards,
                                                - Zheng

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