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

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Message-id: <20090108101145.GJ13721@webber.adilger.int>
Date:	Thu, 08 Jan 2009 03:11:45 -0700
From:	Andreas Dilger <adilger@....com>
To:	Christian Ohm <chr.ohm@....net>
Cc:	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: How to recover a damaged ext4 file system?

On Jan 07, 2009  22:42 +0100, Christian Ohm wrote:
> > 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.
> 
> I'll do that soon; I got another harddisk to copy the partition, but both
> disks aren't connected right now. 

You could also and compile and run the e2fsprogs "findsuper" tool (I've
attached it here, it isn't built by default).  This will scan the specified
device and look for ext2/3/4 superblock signatures.

> > > 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.
> 
> I did one copy of the partition already (took three hours, so not something to
> do often...), and ran fsck -y on that. The result was an endless fsck loop like
> that described in
> http://www.linuxquestions.org/questions/linux-hardware-18/corrupt-ext3-partition-need-to-recover-376366/.
> Oh, and I have to try if dumpe2fs actually works, either that or debugfs failed
> when I tried to run it on the original disk (I also ran dumpe2fs on the copy
> while fsck was doing its looping, and depending on the time it did or did not
> find a file system on the device). Anyway, I hope I can experiment some more
> tomorrow.
> 
> Oh, and is there a human understandable description of the on-disk data format
> to compare with a hexdump? A (admittedly very short) search didn't turn up
> anything.
> 
> Best regards,
> Christian Ohm
> 
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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.


View attachment "findsuper.c" of type "text/plain" (8097 bytes)