linux-ext4 - Re: How to recover a damaged ext4 file system?

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Message-ID: <20090106193404.GA18957@mit.edu>
Date:	Tue, 6 Jan 2009 14:34:04 -0500
From:	Theodore Tso <tytso@....edu>
To:	Andreas Dilger <adilger@....com>
Cc:	Christian Ohm <chr.ohm@....net>, linux-ext4@...r.kernel.org
Subject: Re: How to recover a damaged ext4 file system?

On Tue, Jan 06, 2009 at 05:05:27AM -0700, Andreas Dilger wrote:
> 
> You should try to run e2fsck with the backup group descriptors, using
> the -B and/or -b options (at a guess -B 4096 and -b 32768).

That probably won't help, given that the fsck transcript already says
this:

> > fsck.ext4: Group descriptors look bad... trying backup blocks...

It looks like both the primary and the backup block group descriptors
are bad.  I'm not sure how this happened; normally nothing touches the
backup block superblocks at all.  Stupid question --- are you sure the
partition table is sane; that's always the first thing to check.

Can you upload someplace the output of

dumpe2fs /dev/XXX
dumpe2fs -o superblock=32768 /dev/XXX
dumpe2fs -o superblock=98304 /dev/XXX

That would be helpful to see what had happened.

> 2. Is this corruption a fault of ext4? I guess this is difficult to
> answer, but I had ext3 survive any lockups without much problems. So
> far ext4 seems not quite that robust, but perhaps another file
> system would have blown up as well in this situation. Is there any
> information I can give you to help make ext4 more robust?

I'm not sure what the hard system hang did, but it looks like it
splattered a lot of random crap all over the harddrive.  I doubt ext4
did this, and I doubt ext3 would have done any better.... we need to
know a lot more about exactly what sort damage was done to the
filesytem to say for certain, though.

					- Ted

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