From: Thierry Vignaud <tvignaud@mandriva.com>
Subject: Re: fsck.ext4: Group descriptors look bad... trying backup blocks...
Date: Tue, 21 Apr 2009 17:14:40 +0200
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To: Jeremy Sanders <jss@ast.cam.ac.uk>
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Jeremy Sanders <jss@ast.cam.ac.uk> writes:

> However, the system seems to mostly work, so I recreated the ext4 device, 
> I've just run my backup script again and fsck'd the device. It seems the 
> problem is reproducible with the new kernel:
> 
> [root@xback2 ~]# fsck /dev/md0
> fsck 1.41.4 (27-Jan-2009)
> e2fsck 1.41.4 (27-Jan-2009)
> fsck.ext4: Group descriptors look bad... trying backup blocks...
> Group descriptor 0 checksum is invalid.  Fix<y>?
> 
> Looks like there's a real problem in ext4 causing this under certain 
> circumstances (unless an obscure hardware error is somehow giving the same 
> problem).
> 
> To cause this, all I did was rsync a set of directories to the disk. No hard 
> link trees were created.

For the record, I reproduced this bug with 2.6.30-rc2-git6 on a new
1.5Tb disk. Formated as ext4, using relatime, copied 20Gb.
On reboot, I got such errors.
The hd was partitionned (all ext4) as:
/ (5Gb)  |  /usr (20Gb)  |  /pub (1.5Tb)

The smaller system fses didn't saw those errors.