linux-kernel - Re: [PATCH 2.6.19 5/5] fs: freeze

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Date:	Wed, 15 Nov 2006 19:50:29 +0100
From:	Pavel Machek <pavel@....cz>
To:	David Chinner <dgc@....com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alasdair G Kergon <agk@...hat.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, Srinivasa DS <srinivasa@...ibm.com>,
	Nigel Cunningham <nigel@...pend2.net>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

Hi!

> > > This means, however, that we can leave the patch as is (well, with the minor
> > > fix I have already posted), for now, because it doesn't make things worse a
> > > bit, but:
> > > (a) it prevents xfs from being corrupted and
> > 
> > I'd really prefer it to be fixed by 'freezeable workqueues'.
> 
> I'd prefer that you just freeze the filesystem and let the
> filesystem do things correctly.

Well, I'd prefer filesystems not to know about suspend, and current
"freeze the filesystem" does not really nest properly.

> > Can you
> > point me into sources -- which xfs workqueues are problematic?
> 
> AFAIK, its the I/O completion workqueues that are causing problems.
> (fs/xfs/linux-2.6/xfs_buf.c) However, thinking about it, I'm not
> sure that the work queues being left unfrozen is the real problem.
> 
> i.e. after a sync there's still I/O outstanding (e.g. metadata in
> the log but not on disk), and because the kernel threads are frozen
> some time after the sync, we could have issued this delayed write
> metadata to disk after the sync. With XFS, we can have a of queue of

That's okay, snapshot is atomic. As long as data are safely in the
journal, we should be okay.

> However, even if you stop the workqueue processing, you're still
> going to have to wait for all I/O completion to occur before
> snapshotting memory because having any I/O complete changes memory
> state.  Hence I fail to see how freezing the workqueues really helps
> at all here....

It is okay to change memory state, just on disk state may not change
after atomic snapshot.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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