linux-ext4 - Re: e2fsck extremly slow after: EXT4-fs.. ext4_check_descriptors: Checksum for group .. failed

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Date:	Thu, 15 Nov 2012 12:51:04 +0100
From:	"kaefert@...il.com" <kaefert@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: e2fsck extremly slow after: EXT4-fs.. ext4_check_descriptors:
 Checksum for group .. failed

Hi there!

I've found that on that filesystem, in many folders I now have found
every 8th file has as contents instead of what it should have a copy
of every other 4th file - with some aditional zeros after it.

Maybe its more clear I give an example:
A folder with 25 files:
file 5 is a copy of 1
file 13 is a copy of 9
file 21 is a copy of 17

The original contents of file 5, 13 and 21 seem to have been lost,
maybe they are in the lost+found folder. The problem doesn't always
start with the first file in a folder, and doesn't always continue to
the end of the folder.


2012/11/13 Theodore Ts'o <tytso@...nk.org>
>
> To follow up on the list since Thomas and I have had a number of
> e-mail exchanges that were off-list, and he has sent me an compressed,
> raw e2image dump of his file system which I have investigated
>
> The proximate cause of the fs corruption seems to be a few inode table
> blocks written offset by a 1024 bytes --- there were 3 pairs of inodes
> of the form (N, N+4) which had the exact same contents in the inode
> structure (same generation number, same mtime/ctime/atimes, same
> extents).  This pattern of corruption is quite odd given that the file
> system has a 4k block size.  The best bet is that the corruption
> happened at the USB device layer, since the mis-written inodes were
> offset by a 2 512 byte sectors, as opposed to by an incorrect block
> number.  Thomas tells me this particular device has had a flaky USB
> controller and this is the not the first such failure.
>
> There also seems to be a bug in e2fsck which caused it not to be able
> to repair the corrupted file system.  I have not had a chance to track
> down the bug yet.  It may have been caused by how we handle extent
> tree blocks getting cached while trying to clone the data block.
> Something which we should fix, but ultimately, the use of metadata
> checksums is going to be the best way to deal with cases of the inode
> table block getting written to the wrong place on disk, since we will
> then know which inode not to trust, and just have e2fsck zap it.
>
> Speaking of zapping, I've given Thomas instructions on how to clri
> three of the duplicated inodes using debugfs, and that allowed e2fsck
> to be able to repair his file system.  He will have suffered some data
> loss due to the corrupted inode table, but at least this way he'll be
> able to gain access to most of the files on the disk.
>
>                                    - Ted
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