linux-kernel - Re: Back to the future.

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Message-Id: <E92E80B1-2D3F-4942-A17B-AED903F535C6@mac.com>
Date:	Thu, 3 May 2007 12:53:48 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Pavel Machek <pavel@....cz>
Cc:	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.

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.

If the kernel is later modified to purge all filesystem data (dcache/ 
pagecache) during snapshot and effectively remount and reopen all the  
files by path during restore then you could remove that requirement.   
You'd just need to make sure that the restore-from-disk scripts did  
an fsck or journal-restore before reloading the old kernel data.

Cheers,
Kyle Moffett
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