linux-ext4 - Re: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write

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Message-ID: <20121214194340.GI9453@blackbox.djwong.org>
Date:	Fri, 14 Dec 2012 11:43:40 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andy Lutomirski <luto@...capital.net>, axboe@...nel.dk,
	lucho@...kov.net, jack@...e.cz, ericvh@...il.com,
	viro@...iv.linux.org.uk, rminnich@...dia.gov, tytso@....edu,
	martin.petersen@...cle.com, neilb@...e.de,
	Zheng Liu <gnehzuil.liu@...il.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	linux-fsdevel@...r.kernel.org, adilger.kernel@...ger.ca,
	bharrosh@...asas.com, jlayton@...ba.org,
	v9fs-developer@...ts.sourceforge.net, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 4/4] block: Optionally snapshot page contents to provide
 stable pages during write

On Fri, Dec 14, 2012 at 02:33:34PM +1100, Dave Chinner wrote:
> 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...

Yup.  I wasn't intending this flag for general service, though I /am/ curious
to hear if anyone sees any substantive performance difference with snapshots.

--D
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com
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