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.
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