SlackBuildsOrg/system/zerofree
..
README
slack-desc
zerofree.8
zerofree.info
zerofree.SlackBuild

Zerofree is a utility which scans the free blocks in an ext2/3/4
filesystem and fills any non-zero blocks with zeroes.

This is useful if you have an ext2/3/4 virtual disk image
(e.g. VirtualBox' .vdi) that you wish to compact.

See project homepage for information and usage.