Such an utility is zerofree.
From its description:
Zerofree finds the unallocated, non-zeroed blocks in an ext2 or ext3 file-system and fills them with zeroes. This is useful if the device on which this file-system resides is a disk image. In this case, depending on the type of disk image, a secondary utility may be able to reduce the size of the disk image after zerofree has been run. Zerofree requires the file-system to be unmounted or mounted read-only.
The usual way to achieve the same result (zeroing the unused blocks) is to run "dd" do create a file full of zeroes that takes up the entire free space on the drive, and then delete this file. This has many disadvantages, which zerofree alleviates:
- it is slow
- it makes the disk image (temporarily) grow to its maximal extent
- it (temporarily) uses all free space on the disk, so other concurrent write actions may fail.
Zerofree has been written to be run from GNU/Linux systems installed as guest OSes inside a virtual machine. If this is not your case, you almost certainly don't need this package.
UPDATE #1
The description of the .deb package contains the following paragraph now which would imply this will work fine with ext4 too.
Description: zero free blocks from ext2, ext3 and ext4 file-systems Zerofree finds the unallocated blocks with non-zero value content in an ext2, ext3 or ext4 file-system and fills them with zeroes...
Other uses
Another application this utility is to compress disk images that are a backup of a real disk. A typical example of this is the dump of the SD card in a BeagleBone or a Raspberry Pi. Once empty spaces have been zeroed, backup images can be compressed more efficiently.
