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Date:	Tue, 3 Apr 2012 14:26:54 -0700
From:	Ted Ts'o <tytso@....edu>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Andreas Dilger <aedilger@...il.com>,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: backup of the last group'descriptor when it is the 1st group
 of a meta_bg

On Tue, Apr 03, 2012 at 01:28:14PM -0600, Andreas Dilger wrote:
> It would probably not even cause the backup group descriptor to be
> lost in the worst case (new mke2fs/e2fsck/resize2fs creates gd backup,
> old e2fsck "deletes" gd backup block, use filesystem for a long time,
> corrupt primary group descriptors, try to recover using new e2fsck).

Well, it can only be repaired if that block hasn't been allocated and
assigned to a file.  If it has, then you can't easily repair it and
you have to resign yourself to not having a backup of the bgd.  And
that means more complexity since e2fsck would have to deal with the
possibility that the last block might contain a backup bgd, or might
be allocated to a file.

And even if there is a "harmless" corruption, it will still
potentially alarm users who happen to format an ext4 file system with
this this change implemented, and then they boot a rescue CD which is
using an older e2fsprogs.

Ultimately I suspect the best approach might be to simply try to
reconstruct the last bgd by attempting to find the inode table in case
the last meta_bg bgd is destroyed.  Since this only comes up for file
systems with a single block group in a meta_bg, it's a relatively easy
thing to do.....

						- Ted
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