Re: backup of the last group'descriptor when it is the 1st group of a meta_bg

[Ted Ts'o <tytso@mit.edu>]

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