If you can mount the filesystem
Then you do not need to use btrfs-zero-log.
If you try to mount the filesystem, and it fails
Look in your kernel logs (dmesg), and continue reading.
If your kernel log has an oops in it
with something like this in it:
? replay_one_dir_item+0xb5/0xb5 [btrfs] ? walk_log_tree+0x9c/0x19d [btrfs] ? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs] ? btrfs_recover_log_trees+0x195/0x29c [btrfs] ? replay_one_dir_item+0xb5/0xb5 [btrfs]
then you need to upgrade your kernel, as only older kernels are known to print oopses on log errors. If it's a recent kernel, please report the problem to the mailing list or http://kernel.bugzilla.org.
replay_one_dir_item, and references to the log (e.g.
walk_log_tree). If you don't see those, or similar messages, this entry is not for you, and you need to report the problem to the mailing list or bugzilla, or ask for help on the #btrfs IRC channel.
If your kernel log has any other messages in it
BTRFS: failed to read log tree
would indicate that you may have a problem that can be fixed by btrfs-zero-log. If that message is not there, then btrfs-zero-log is unlikely to help you, and you need to seek help elsewhere: the mailing list, htpp://bugzilla.kernel.org, or the #btrfs IRC channel.
Running btrfs-zero-log on a filesystem with any other kind of mount problem will most likely not fix it, and may make recovering it harder. If in doubt, check with the developers on IRC or the mailing list first.