linux-ext4 - Next3 on-disk format change request

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Date:	Tue, 22 Jun 2010 13:30:54 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Next3 on-disk format change request

Hi Ted,

As you may have read, Next3 release 1.0.11 automatically migrates to
new on-disk format.
(see http://next3.sf.net)

Also, I would like to ask you again to promote Next3/Ext4 merge as a
topic on LSF,
if you think it is an appropriate topic.

My on-disk format change request is to re-assign a ro_compat feature
for IS_SNAPSHOT (currently in s_flags).

The reason for this request is the following scenario, which was
posted on next3-users list:

"If I use dd to restore a full snapshot image to another volume, can I
mount it as Next3?"

The answer to this question is that read-only mount is ok, but for
read-write mount,
fsck needs to be run first to fix the free blocks counters.

The restored image is identified with the IS_SNAPSHOT flag.
If it was a ro_compat feature, it would prevent users from mistakingly
mounting it read-write before the fix.
Also, 'tune2fs -O ^is_snapshot' could be used to fix the counters,
without running full fsck.

Please assign me a constant for the feature if you agree,
Thanks,
Amir.
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