Re: multi-device btrfs with single data mode and disk failure

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-09-20 13:54, Chris Murphy wrote:
> On Tue, Sep 20, 2016 at 11:03 AM, Alexandre Poux <pums974@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.