zerofree

zero free blocks on ext2/3/4 filesystems

TLDR

Zero the free blocks of an unmounted filesystem

$ sudo zerofree /dev/[sda1]

Show progress while clearing

$ sudo zerofree -v /dev/[sda1]

Dry run: count non-zero free blocks without writing

$ sudo zerofree -n /dev/[sda1]

Remount the root filesystem read-only first, then clear

$ sudo mount -o remount,ro / && sudo zerofree /dev/[sda1]

zerofree scans an ext2, ext3, or ext4 filesystem and overwrites every unallocated block that is not already zero with zeros (or a chosen fill byte). Data in allocated files is left untouched.Zeroing free space is mainly useful before imaging or compressing a disk. A virtual machine image or sparse file shrinks dramatically once its free blocks are zero, and the unused space can then be reclaimed by the host or by sparse-aware tools.The target filesystem must be unmounted or mounted read-only, since zerofree reads and writes the block device directly and cannot run safely while the kernel is also modifying it.

PARAMETERS

-n

Dry run: report how many free blocks are non-zero without writing anything.

-v

Verbose: print progress as a percentage while clearing.

-f fillval

Fill free blocks with the byte value fillval (0-255) instead of zero.

INSTALL

sudo apt install zerofree

sudo dnf install zerofree

sudo apk add zerofree

sudo zypper install zerofree

nix profile install nixpkgs#zerofree

CAVEATS

The filesystem must not be mounted read-write. Run zerofree from a live/rescue environment, or remount the device read-only first (the root filesystem usually needs single-user or recovery mode). Only ext2/3/4 are supported. Zeroing is not a secure erase; it only makes free space compress well. Use shred or blkdiscard to destroy previously written data.

HISTORY

zerofree was written by Ron Yorston to help shrink User-Mode Linux and virtual machine disk images. It is packaged as zerofree on most distributions.