On 2016-09-20 13:54, Chris Murphy wrote:
On Tue, Sep 20, 2016 at 11:03 AM, Alexandre Poux <pums...@gmail.com> wrote:

If I wanted to try to edit my partitions with an hex editor, where would
I find infos on how to do that ?
I really don't want to go this way, but if this is relatively simple, it
may be worth to try.

Simple is relative. First you'd need
https://btrfs.wiki.kernel.org/index.php/On-disk_Format to get some
understanding of where things are to edit, and then btrfs-map-logical
to convert btrfs logical addresses to physical device and sector to
know what to edit.

I'd call it distinctly non-trivial and very tedious.

It really is. I've done this before, but I had a copy of the on-disk format documentation, a couple of working filesystems, a full copy of the current kernel sources for reference, and about 8 cups of green tea (my beverage of choice for staying awake and focused). I got _really_ lucky and it was something that really was simple to fix once I found it (it amounted to about 64 bytes of changes, it took me maybe 5 minutes to actually correct the issue once I found where it was), but it took me a good couple of hours to figure out what to even look for, plus another hour just to find it, and I'm not sure I would be able to do it any faster if I had to again (unlike doing so for ext4, which is a walk in the park by comparison).

TBH the only thing I'd worry about using a hex editor to fix in BTRFS is the super-blocks or system chunks, because they're pretty easy to find, and usually not all that hard to fix. In fact, if it hadn't been for the fact that I had no backup of the data I would lose by recreating that filesystem, and I was _really_ bored that day, I probably wouldn't have even tried.
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