Fscking Precautions: Snapshots and the undo file

If you have a badly corrupted filesystem, e.g. because you had back blocks on the hard drive, you have want to take some precautions to make sure fsck doesn't destroy your files. This goes especially for large raids.

Something you can always do is test the fsck operation on a dmsetup snapshot. For this to work you must boot from a different partition and it must have a sparse file enabled filesystem.

# The path to your snapshot storage file.

Terabytes should be enough for your partition, otherwise increase this number
truncate -s
000G $COW 
#setup a loop for dmsetup snapshots to the COW file.
loop="$(losetup -f)"
losetup $loop $COW
#setup the snapshot device
echo 0 `blockdev --getsz $INPUT` snapshot $INPUT $loop p 8 | dmsetup create top
# let you know where the snapshot device is.
echo loop: $loop top: /dev/mapper/top
dmsetup status

After this you should be able to fsck /dev/mapper/top. You can see how much space the COW file actually occupies with du -h $COW. You may also want to get the newest fsck version (e.g. with a newer fsck-static package). If you end up with many multiply-claimed blocks, this e2fsck version may help: http://git.hpdd.intel.com/tools/e2fsprogs.git/ (checkout a -wc branch).

Good luck, you might need it!

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