The documentation lacks clarity about depth to which recursive
defragmentation go, and was pointed out by a user.
The problem here is that the subvolume behaves the same as mount point
regarding path traversal. The nftw stops on mount boundary (FTW_MOUNT).
Add test that verifies this behaviour. Defrag has to be updated to allow
descending to subvolumes (and not mountpoints).
Issue: #185
Signed-off-by: David Sterba <dsterba@suse.com>
