Does btrfs-zero-log work only on a single partition or can it handle BTRFS filesystems spanned across several devices?
 LVM functionality is only partially replaced by BTRFS
One thing about LVM Volumes in contrast to btrfs sub volumes is quota's. Every LVM VOlume has a hard quota and if it runs out of space other volumes are not running out of disk space. This is one functionality that btrfs has not (yet) implemeted.
For this reasson I stil use LVM, but with btrfs filesystems in the volumes and not with ext4 (anymore).
 SHow free blocks in btrfs
The other day I had a btrfs FS that was running out of disk space. But df -h showed that there was still 20% free (subvolumes a presumably not calculated since they are seperately mounted?). And du -s works the other way around (this also calculated subvolumes and sums, so it counts the unaltered blocks in snapshot sunbvolumes extra). Is there a way to find out how much free blocks there are available in a btrfs filesystem?