How to fix unmountable BTRFS

(from https://superuser.com/a/1248201)

First, ensure you've got valid superblocks:

btrfs rescue super-recover -v <device>

Use:

btrfs find-root <device>

to find the best tree root to use in repair:

The root node should have the highest level
The higher generation, the higher chance the fs can be recovered using that root.
Note: in your error message, wanted is the generation in the log, the found is the generation based on the tree root.

Find the cleanest output from the following commands:

btrfs check --tree-root <block> --super <sup>

Where:

<sup> is either 0, 1 or 2.
<block> is is given by btrfs find-root (note: don't multiply by block size, even though the manual says )

Then repair the filesystem as follows:

btrfs check --repair --tree-root <block> --super <sup>

Note the addition of --repair to actually change the filesystem.