debugfs: ex /a
Level Entries       Logical      Physical Length Flags
 0/ 7   1/  1     0 -     0    12              1
 1/ 7   1/  1     0 -     0    13              1
 2/ 7   1/  1     0 -     0    14              1
 3/ 7   1/  1     0 -     0    15              1
 4/ 7   1/  1     0 -     0    16              1
 5/ 7   1/  1     0 -     0    17              1
 6/ 7   1/  1     0 -     0     9              1
 7/ 7   1/  1     0 -     0    10 -    10      1 
Pass 1: Checking inodes, blocks, and sizes
Inode 12 extent tree could be more shallow (7; could be <= 4)
Fix? 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 1