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Message-ID: <20121214021048.GF9453@blackbox.djwong.org>
Date: Thu, 13 Dec 2012 18:10:49 -0800
From: "Darrick J. Wong" <darrick.wong@...cle.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: 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, david@...morbit.com,
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 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.
> If this becomes widely used, would it be better to snapshot on
> wait_for_stable_page instead of on io submission?
That really depends on how long you can afford to wait and how much free
memory you have. :) It's all a big tradeoff between write latency and
consumption of memory pages and bandwidth, and one that I doubt I'm qualified
to make for everyone.
> FWIW, I'm about to pound pretty hard on this whole patchset on a box
> that doesn't need stable pages. I'll let you know how it goes.
Yay!
--D
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