From: "Darrick J. Wong" Subject: Re: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write Date: Thu, 13 Dec 2012 18:10:49 -0800 Message-ID: <20121214021048.GF9453@blackbox.djwong.org> References: <20121213080740.23360.16346.stgit@blackbox.djwong.org> <20121213080811.23360.98131.stgit@blackbox.djwong.org> <50CA8556.7030905@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: 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, david@fromorbit.com, Zheng Liu , 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: Andy Lutomirski Return-path: Content-Disposition: inline In-Reply-To: <50CA8556.7030905@mit.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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