Looking at free extent sizes on ext2, ext3 and ext4 filesystems

Today I stumbled on the wonders of e2freefrag, a tool for reporting the free space fragmentation on ext[2-4] filesystems. It scans the block bitmap data and reports the amount of free blocks a present in terms of free contiguous blocks and also aligned free space.

$ sudo e2freefrag  /dev/sda1
Device: /dev/sda1
Blocksize: 1024 bytes
Total blocks: 489951
Free blocks: 242944 (49.6%)

Min. free extent: 1 KB
Max. free extent: 7676 KB
Avg. free extent: 2729 KB

HISTOGRAM OF FREE EXTENT SIZES:
Extent Size Range :  Free extents   Free Blocks  Percent
    1K...    2K-  :             4             4    0.00%
    2K...    4K-  :            30            90    0.04%
    4K...    8K-  :             5            26    0.01%
  128K...  256K-  :             2           301    0.12%
  256K...  512K-  :             1           340    0.14%
  512K... 1024K-  :             6          4540    1.87%
    1M...    2M-  :             5          8141    3.35%
    2M...    4M-  :             5         14751    6.07%
    4M...    8M-  :            31        214751   88.40%


From this one can get an idea of the level of free space fragmentation in your filesystem.