Re: btrfs recovery

[Michael Born <Michael.Born@aei.mpg.de>]

Am 30.01.2017 um 22:20 schrieb Chris Murphy:
> On Mon, Jan 30, 2017 at 2:07 PM, Michael Born <Michael.Born@aei.mpg.de> wrote:
>> The files I'm interested in (fstab, NetworkManager.conf, ...) didn't
>> change for months. Why would they change in the moment I copy their
>> blocks with dd?
> 
> They didn't change. The file system changed. While dd is reading, it
> might be minutes between capturing different parts of the file system,
> and each superblock is in different locations on the disk,
> guaranteeing that if the dd takes more than 30 seconds, your dd image
> has different generation super blocks. Btrfs notices this at mount
> time and will refuse to mount because the file system is inconsistent.
> 
> It is certainly possible to fix this, but it's likely to be really,
> really tedious. The existing tools don't take this use case into
> account.
> 
> Maybe btfs-find-root can come up with some suggestions and you can use
> btrfs restore -t with the bytenr from find root, to see if you can get
> this old data, ignoring the changes that don't affect the old data.
> 
> What you do with this is btrfs-find-root and see what it comes up
> with. And work with the most recent (highest) generation going
> backward, plugging in the bytenr into btrfs restore with -t option.
> You'll also want to use the dry run to see if you're getting what you
> want. It's best to use the exact path if you know it, this takes much
> less time for it to search all files in a given tree. If you don't
> know the exact path, but you know part of a file name, then you'll
> need to use the regex option; or just let it dump everything it can
> from the image and go dumpster diving...

I really want to try the "btrfs-find-root / btrfs restore -t" method.
But, btrfs-find-root gives me just the 3 lines output and then nothing
for 16 hours.
I think, I saw a similar report that the tool just doesn't report back
in the mailing list archive (btrfs-find-root duration? Markus Binsteiner
Sat, 10 Dec 2016 16:12:25 -0800)

./btrfs-find-root /dev/loop0
Couldn't read tree root
Superblock thinks the generation is 550114
Superblock thinks the level is 1

Hans, also thank you for the explanation even though I'm not sure, I
understand.
I would be happy with older parts of the tree which then have lower
numbers than the 550112.

Michael