e2freefrag

Check how many free blocks are present as contiguous and aligned free space

$ e2freefrag [/dev/sdXN]

$ e2freefrag -c [chunk_size_in_kb] [/dev/sdXN]

e2freefrag [-n] [-v] device [blocks...]

-n
    Dry-run mode: scan and report without freeing blocks.

-v
    Verbose mode: print details on each block processed.

device
    Required: path to the block device (e.g., /dev/sda1).

blocks...
    Optional: space-separated list of block numbers to free; scans all if omitted.

The e2freefrag command is a low-level utility from the e2fsprogs package used to free fragmented blocks in ext2, ext3, and ext4 filesystems. These fragments are blocks marked as allocated in the block bitmap but not referenced by any inode, indirect block, or other metadata structures. Such orphans can accumulate after filesystem crashes, aggressive e2fsck repairs (e.g., with -y), or manual edits.

It scans specified block numbers (or the entire filesystem if none provided) to verify if they are truly unused, then clears their allocation bits to reclaim space. This reduces internal fragmentation, potentially improving performance and allowing better space utilization for new allocations.

e2freefrag operates directly on the raw block device, bypassing the kernel's filesystem layer, making it powerful but risky. It is essential for recovery scenarios where free space appears insufficient despite ample unused blocks. Always run on unmounted filesystems with root privileges, and consider a dry run first. Pair it with tools like debugfs to inspect blocks beforehand.

Run as root on unmounted filesystems only; direct bitmap modification risks corruption if blocks are in-use. Verify blocks with debugfs or dumpe2fs first. No journaling protection on ext3/4.

After e2fsck -y /dev/sda1, run e2freefrag -n -v /dev/sda1 to preview, then without -n.

Use debugfs -R "blockdump N" /dev/sda1 (N=block#) or hexdump to confirm fragments before freeing.

Developed by Theodore Ts'o as part of e2fsprogs (introduced ~1.19 in 2000) for ext2; extended for ext3/4 support in later versions to address fragmentation post-fsck repairs.

debugfs(8), e2fsck(8), dumpe2fs(8), tune2fs(8)