zerofree

Zerofree is designed to zero out unallocated blocks in ext2, ext3, and ext4 file-systems that contain non-zero data. This tool is primarily useful when working with disk images, as it allows secondary utilities to shrink the image size after zerofree has cleared the unused space. The file-system must be unmounted or mounted read-only for zerofree to operate.

Unlike the traditional method of using 'dd' to create a zero-filled file that consumes all free space before deletion, zerofree avoids several issues: it doesn't temporarily expand the disk image to maximum size, doesn't exhaust free space (preventing concurrent writes), and is significantly faster. It was specifically written for GNU/Linux guest OSes in virtual machines, though it can also securely erase sensitive data better than a simple 'rm'.

Zerofree requires minimal dependencies and has a small installed size of 30 KB.

Zerofree scans the specified ext2, ext3, or ext4 filesystem for unallocated blocks containing non-zero data and overwrites them with zeroes (or a specified fill value). It operates directly on the filesystem structure without requiring the filesystem to be mounted writable, targeting only unused blocks to reclaim space in disk images for subsequent compression by virtualization tools.