[Discuss] Recovering a corrupted usb hard drive with XFS

[worley at alum.mit.edu (Dale R. Worley)]

> From: "Dale R. Worley" <worley at alum.mit.edu>

> In principal, it can be done by:  (1) using the right utility to shrink
> the filesystem enough to leave space for the GPT, (2) copying the
> filesystem "further" into the disk by that amount (which mostly has to
> be done *in reverse* to not lose data, so it can't be done directly by
> dd), (3) creating the GPT, (4) inserting a partition entry that
> describes the existing filesystem.

The ideal way to accomplish (2) would be for "dd" to have an "in
reverse" option, but it doesn't seem to.  OTOH, assuming that your
filesystem is an exact multiple of 1 MiB in length, it's simple enough to
write a loop that copies 1 MiB at a time in reverse, and then test it on
some sample files.

Unfortunately for (1) there seems to be no way to shrink an XFS
filesystem, so if you haven't accidentally got at least 1 MiB or so free
after the filesystem, there's little that you can do.  See e.g.
https://web.archive.org/web/20090105174540/http://www.xfs.org/index.php/XFS_FAQ#Q:_Is_there_a_way_to_make_a_XFS_filesystem_larger_or_smaller.3F

I did run into something called "convertfs" which supposedly changes a
filesystem of one format into a filesystem of another format *in
place*.  If so, you could convert the filesystem to e.g. ext4, shrink
it, then convert it back to XFS.  I'm sure it's deep magic, the only
question is whether it actually works.  At the very least, I'm sure it
requires a lot of free space in the filesystem.
https://web.archive.org/web/20160101214233/http://oss.sgi.com/archives/xfs/2002-01/msg02379.html
https://web.archive.org/web/20240507233349/https://tzukanov.narod.ru/convertfs
https://directory.fsf.org/wiki/ConvertFS
http://tzukanov.narod.ru/convertfs/convertfs-13jan2005.tar.gz

Dale