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

[Alexandre Poux <pums974@gmail.com>]

Le 20/09/2016 à 20:56, Austin S. Hemmelgarn a écrit :
> 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.
OK I will forget this.
Thank you