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Date:	Sun, 15 Jul 2012 22:32:01 -0600
From:	Andreas Dilger <aedilger@...il.com>
To:	Kevin Shanahan <kmshanah@...enchant.net>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Ext4 external journal UUID mismatch?

On 2012-07-15, at 20:56, Kevin Shanahan <kmshanah@...enchant.net> wrote:

> On Mon, Jul 16, 2012 at 09:10:58AM +0930, Kevin Shanahan wrote:
>> I have created some filesystems with external journals, and after
>> reboot (clean shutdown) these will not mount anymore. I see in dmesg:
>> 
>>  [42214.365496] EXT4-fs (dm-10): journal UUID does not match
>> 
>> However, the UUIDs look fine when viewed with dumpe2fs:
>> 
> Here is what e2fsck said about it.
> 
> # e2fsck -p -j /dev/ssdvg/jnl-kvmhost0 /dev/it8/nfs-kvmhost0
> /dev/it8/nfs-kvmhost0: Superblock hint for external superblock should be 0xfd0d.  FIXED.
> /dev/it8/nfs-kvmhost0: clean, 43229/983040 files, 303609/3932160 blocks
> 
> I guess that is the "Journal Device" field? Mount worked fine after
> that and the data looks okay.
> 
> Any idea how this might have happened across all three fs with
> external journals?

Because LVM changed the block device numbers after the reboot, and the device number used previously for the external journal was different. 

The lookup of the journal by UUID (instead of relying on the "device hint" in the superblock) _should_ be handled by mount, but I don't recall if we ever got a mount.ext4 to handle this or not. It would also be possible for the "fast e2fsck" check to verify the journal UUID before mounting the filesystem, but again I'm not sure if this is done yet, and I can't check right now. 

Cheers, Andreas

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