debugfs: ex /a
Level Entries       Logical      Physical Length Flags
 0/ 1   1/  2     0 -    16     9             17
 1/ 1   1/  4     0 -     0    10 -    10      1 
 1/ 1   2/  4    11 -    11   100 -   100      1 
 1/ 1   3/  4    13 -    13   101 -   101      1 
 1/ 1   4/  4    15 -    15   102 -   102      1 
 0/ 1   2/  2    17 -    21    12              5
 1/ 1   1/  3    17 -    17   103 -   103      1 
 1/ 1   2/  3    19 -    19   104 -   104      1 
 1/ 1   3/  3    21 -    21   105 -   105      1 
Pass 1: Checking inodes, blocks, and sizes
Inode 12 extent tree (at level 1) could be narrower.  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 (8.3% non-contiguous), 26/512 blocks
Exit status is 0
