linux-kernel - Re: [xfs-masters] Re: freeze vs freezer

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Message-Id: <200806300122.48204.rjw@sisk.pl>
Date:	Mon, 30 Jun 2008 01:22:47 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Dave Chinner <david@...morbit.com>
Cc:	xfs-masters@....sgi.com, Elias Oltmanns <eo@...ensachen.de>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Kyle Moffett <mrmacman_g4@....com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	David Chinner <dgc@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [xfs-masters] Re: freeze vs freezer

On Monday, 30 of June 2008, Dave Chinner wrote:
> On Thu, Jun 26, 2008 at 05:09:10PM +0200, Pavel Machek wrote:
> > > > Is this the same thing the per-device IO-queue-freeze patches for
> > > >HDAPS also
> > > > need to do?  If so, you may want to talk to Elias Oltmanns
> > > > <eo@...ensachen.de> about it.  Added to CC.
> > > 
> > > Thanks for the heads up Henrique. Even though these issues seem to be
> > > related up to a certain degree, there probably are some important
> > > differences. When suspending a system, the emphasis is on leaving the
> > > system in a consistent state (think of journalled file systems), whereas
> > > disk shock protection is mainly concerned with stopping I/O as soon as
> > > possible. As yet, I cannot possibly say to what extend these two
> > > concepts can be reconciled in the sense of sharing some common code.
> > 
> > Actually, I believe requirements are same.
> > 
> > 'don't do i/o in dangerous period'.
> > 
> > swsusp will just do sync() before entering dangerous period. That
> > provides consistent-enough state...
> 
> As I've said many times before - if the requirement is "don't do
> I/O" then you have to freeze the filesystem. In no way does 'sync'
> prevent filesystems from doing I/O.....

Well, it seems we can handle this on the block layer level, by temporarily
replacing the elevator with something that will selectively prevent fs I/O
from reaching the layers below it.

I talked with Jens about it on a very general level, but it seems doable at
first sight.

Thanks,
Rafael
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