btrfs-zero-log will remove the log tree if log tree is corrupt, which will allow you to mount the filesystem again.
The common case where this happens has been fixed a long time ago, so it is unlikely that you will see this particular problem.
One can determine whether btrfs-zero-log is needed according to the kernel backtrace:
? 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] ? btree_read_extent_buffer_pages+0x76/0xbc [btrfs] ? open_ctree+0xff6/0x132c [btrfs]
If the errors are like above, then btrfs-zero-log should be used to clear the log and the filesystem may be mounted normally again.
If you use btrfs as the root filesystem, you may want to include btrfs-zero-log into initramdisk if the log problems hits you often.
btrfs-zero-log will return 0 if no error happened. Other exit code means some problems happened.