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

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Message-Id: <200611152056.48218.rjw@sisk.pl>
Date:	Wed, 15 Nov 2006 20:56:46 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	David Chinner <dgc@....com>, 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,

On Wednesday, 15 November 2006 19:50, Pavel Machek wrote:
> 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.

There's one more thing, actually.  If the on-disk data and metadata are
changed _after_ the sync we do and _before_ we create the snapshot image,
and the subsequent  resume fails, there may be problems with recovering
the filesystem.  That is, if I correctly understand what David has told us so
far.

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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