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

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Date:	Thu, 13 Dec 2012 17:48:06 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
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 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?

If this becomes widely used, would it be better to snapshot on
wait_for_stable_page instead of on io submission?

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.

--Andy
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