debugfs: ex /a
Level Entries       Logical      Physical Length Flags
 0/ 1   1/  1     0 -     0     9              1
 1/ 1   1/  1     0 -     0    10 -    10      1 
Pass 1: Checking inodes, blocks, and sizes
Inode 12 extent tree (at level 1) could be shorter.  Optimize? yes

Pass 1E: Optimizing extent trees
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information

test_filesys: ***** FILE SYSTEM WAS MODIFIED *****
test_filesys: 12/128 files (0.0% non-contiguous), 19/512 blocks
Exit status is 0