linux-ext4 - Re: Q wrt LVM snapshot of ext4 w/ external journal

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Message-ID: <4BC4DCE1.3050600@nuclearwinter.com>
Date:	Tue, 13 Apr 2010 16:06:41 -0500
From:	Larkin Lowrey <llowrey@...learwinter.com>
To:	tytso@....edu
CC:	linux-ext4@...r.kernel.org
Subject: Re: Q wrt LVM snapshot of ext4 w/ external journal

On 4/13/2010 2:22 PM, tytso@....edu wrote:
> It's safe to mount the LVM2 snapshot only if you use the mount option
> noload.  This will prevent it from using the journal, which is good,
> since the journal is in use by the original file system.  :-)
>
>   
I was able to verify that mounting with noload worked as advertised.
Thank you. The exact message was "mounted filesystem without journal".
> Hopefully, mounting the snapshot without using noload _should_ fail,
> since the journal is already in use, but I'm not sure we have that
> check in place, so I don't recommend trying it on a production file
> system.
I also verified that mounting w/o noload causes the mount operation to
fail. The message was "failed to claim external journal device".

When I unmounted the original fs so that the journal device was not in
use and then tried to mount the snapshot w/o noload, the mounted
snapshot was able to claim the journal. Of course I would never do that
on purpose but I wanted to see what would happen. I believe this falls
under the "don't do that or bad things will happen" category.

Thank you very much for your response to my query.

--Larkin
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