I've encountered the issue described in the first bullet here: https://btrfs.wiki.kernel.org/index.php/Gotchas To quote: Stable kernel version 3.19.1+ can cause a deadlock at mount time workaround: boot with older kernel, or run btrfs-zero-log to clear the log (beware of the consequences) fix: scheduled for 3.19.5, or apply 9c4f61f01d269815bb7c37. also affected: 3.14.35+, 3.18.9+
*** This bug has been marked as a duplicate of bug 1207618 ***