From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Pavel Machek <pavel@ucw.cz>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex
Date: Wed, 15 Nov 2006 21:00:33 +0100
User-Agent: KMail/1.9.1
Cc: David Chinner <dgc@sgi.com>, Alasdair G Kergon <agk@redhat.com>,
       Eric Sandeen <sandeen@redhat.com>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, dm-devel@redhat.com,
       Srinivasa DS <srinivasa@in.ibm.com>,
       Nigel Cunningham <nigel@suspend2.net>
References: <20061107183459.GG6993@agk.surrey.redhat.com> <20061115185029.GA3722@elf.ucw.cz> <200611152056.48218.rjw@sisk.pl>
In-Reply-To: <200611152056.48218.rjw@sisk.pl>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200611152100.35054.rjw@sisk.pl>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2633
Lines: 65

On Wednesday, 15 November 2006 20:56, Rafael J. Wysocki wrote:
> 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,

Well, but this is equivalent to a power failure immediately after the sync, so
there _must_ be a way to recover the filesystem from that, no?

I think I'll prepare a patch for freezing the work queues and we'll see what
to do next.

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/