From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Nigel Cunningham <ncunningham@linuxmail.org>
Subject: Re: [PATCH] Freeze bdevs when freezing processes.
Date: Wed, 25 Oct 2006 21:05:56 +0200
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Cc: Pavel Machek <pavel@ucw.cz>, David Chinner <dgc@sgi.com>,
       Andrew Morton <akpm@osdl.org>, LKML <linux-kernel@vger.kernel.org>,
       xfs@oss.sgi.com
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On Wednesday, 25 October 2006 15:23, Nigel Cunningham wrote:
> Hi.
> 
> On Wed, 2006-10-25 at 14:32 +0200, Rafael J. Wysocki wrote:
> > On Wednesday, 25 October 2006 10:47, Pavel Machek wrote:
> > > On Wed 2006-10-25 18:38:30, David Chinner wrote:
> > > > On Wed, Oct 25, 2006 at 10:10:01AM +0200, Pavel Machek wrote:
> > > > > > Hence the only way to correctly rebuild the XFS state on resume is
> > > > > > to quiesce the filesystem on suspend and thaw it on resume so as to
> > > > > > trigger log recovery.
> > > > > 
> > > > > No, during suspend/resume, memory image is saved, and no state is
> > > > > lost. We would not even have to do sys_sync(), and suspend/resume
> > > > > would still work properly.
> > > > 
> > > > It seems to me that you ensure the filesystem is synced to disk and
> > > > then at some point later you record the memory state of the
> > > > filesystem, but these happen at different times. That leaves a
> > > > window for things to get out of sync again, right?
> > > 
> > > I DO NOT HAVE TO ENSURE FILESYSTEM IS SYNCED. That sys_sync() is
> > > optional.
> > > 
> > > Recording of memory state is atomic, and as long as noone writes to
> > > the disk after atomic snapshot, memory image matches what is on disk.
> > 
> > Well, my impression is that this is exactly what happens here: Something
> > in the XFS code causes metadata to be written to disk _after_ the atomic
> > snapshot.
> > 
> > That's why I asked if the dirty XFS metadata were flushed by a kernel thread.
> 
> When I first added bdev freezing it was because there was an XFS timer
> doing writes.

Yes, I noticed you said that, but I'd like someone from the XFS team to either
confirm or deny it.

Greetings,
Rafael


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