From: Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH 4/4] block: Optionally snapshot page contents to provide
 stable pages during write
Date: Fri, 14 Dec 2012 14:33:34 +1100
Message-ID: <20121214033334.GA9806@dastard>
References: <20121213080740.23360.16346.stgit@blackbox.djwong.org>
 <20121213080811.23360.98131.stgit@blackbox.djwong.org>
 <50CA8556.7030905@mit.edu>
 <20121214021048.GF9453@blackbox.djwong.org>
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Cc: Andy Lutomirski <luto@amacapital.net>, axboe@kernel.dk,
	lucho@ionkov.net, jack@suse.cz, ericvh@gmail.com,
	viro@zeniv.linux.org.uk, rminnich@sandia.gov, tytso@mit.edu,
	martin.petersen@oracle.com, neilb@suse.de,
	Zheng Liu <gnehzuil.liu@gmail.com>,
	linux-kernel@vger.kernel.org, hch@infradead.org,
	linux-fsdevel@vger.kernel.org, adilger.kernel@dilger.ca,
	bharrosh@panasas.com, jlayton@samba.org,
	v9fs-developer@lists.sourceforge.net, linux-ext4@vger.kernel.org
To: "Darrick J. Wong" <darrick.wong@oracle.com>
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In-Reply-To: <20121214021048.GF9453@blackbox.djwong.org>
Sender: linux-ext4-owner@vger.kernel.org

On Thu, Dec 13, 2012 at 06:10:49PM -0800, Darrick J. Wong wrote:
> On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> > On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> > > Several complaints have been received regarding long file write latencies when
> > > memory pages must be held stable during writeback.  Since it might not be
> > > acceptable to stall programs for the entire duration of a page write (which may
> > > take many milliseconds even on good hardware), enable a second strategy wherein
> > > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > > while writes continue.
> > > 
> > > This provides a band-aid to provide stable page writes on jbd without needing
> > > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > > to allow administrators to enable it there.
> > 
> > I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> > useful as a mount option everywhere, though?
> 
> ext3 requires snapshots; the rest are ok with either strategy.
> 
> *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> option.

It's copying every single IO, right? If so, then please don't
propagate any further than is necessary to fix the broken
filesystems...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com