linux-kernel - Re: Back to the future.

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Date:	Fri, 04 May 2007 08:52:03 +0100
From:	David Greaves <david@...eaves.com>
To:	Kyle Moffett <mrmacman_g4@....com>
Cc:	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

Kyle Moffett wrote:
> On May 03, 2007, at 11:10:47, Pavel Machek wrote:
>> How mature is freezing filesystems -- will it work on at least ext2/3
>> and vfat?
> 
> I'm pretty sure it works on ext2/3 and xfs and possibly others, I don't
> know either way about VFAT though.  Essentially the "freeze" part
> involves telling the filesystem to sync all data, flush the journal, and
> mark the filesystem clean.  The intent under dm/LVM was to allow you to
> make snapshots without having to fsck the just-created snapshot before
> you mounted it.
> 
>> What happens if you try to boot and filesystems are frozen from
>> previous run?
> 
> If you're just doing a fresh boot then the filesystem is already clean
> due to the dm freeze and so it mounts up normally.  All you need to do
> then is have a little startup script which purges the saved image before
> you fsck or remount things read-write since either case means the image
> is no longer safe to resume.

Wouldn't it be better if freeze wrote a freeze-ID to the fs and returned it?
This would naturally be kept in the image and a UUID mismatch would be
detectable - seems safer and more flexible than 'a script'.

"This isn't the freeze you're looking for, move along"

David
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