linux-ext4 - Re: Add a norecovery option to ext3/4?

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Message-Id: <88E5C632-518E-487F-B39B-926407F5A70D@mac.com>
Date:	Mon, 9 Apr 2007 12:20:58 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Samuel Thibault <samuel.thibault@...-lyon.org>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Add a norecovery option to ext3/4?

On Apr 09, 2007, at 11:43:15, Phillip Susi wrote:
> Samuel Thibault wrote:
>> Hi,
>> Distribution installers usually try to probe OSes for building a  
>> suited grub menu.  Unfortunately, mounting an ext3 partition, even  
>> in read-only mode, does perform some operations on the filesystem  
>> (log recovery).  This is not a good idea since it may silently  
>> garbage data.  XFS has a norecovery option that allows to disable  
>> that, I'd say ext3/4 should have it too.
>
> When the filesystem is told to mount the disk read only, that means  
> it should not write to it.  The fact that ext3 goes ahead and does  
> anyway is a bug and should be fixed.  There is no need for a  
> norecovery option, because read only is a sufficient directive to  
> tell the filesystem not to write to the disk.
>
> As someone else pointed out, this behavior causes havoc if you  
> hibernate a system and then boot up another system which mounts the  
> disk of the hibernated system.  Under all conditions it should be  
> safe to mount a disk read only, but here it is not because the  
> journal playback trashes the disk out from under the hibernated  
> system.

Well IIRC it is possible to prevent that by switching the blockdev to  
read-only mode first:

root@...s:~# mount /dev/hda6 /mnt
kjournald starting.  Commit interval 5 seconds
EXT3 FS on hda6, internal journal
EXT3-fs: mounted filesystem with ordered data mode
root@...s:~# umount /mnt
root@...s:~# blockdev --setro /dev/hda6
root@...s:~# mount /dev/hda6 /mnt
mount: block device /dev/loop0 is write-protected, mounting read-only
kjournald starting.  Commit interval 5 seconds
EXT3-fs: mounted filesystem with ordered data mode

Cheers,
Kyle Moffett

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