zerofree - 1.1.1-1 main

Zerofree finds the unallocated blocks with non-zero value content in
an ext2, ext3 or ext4 file-system and fills them with zeroes
(zerofree can also work with another value than zero). This is mostly
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.
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The usual way to achieve the same result (zeroing the unused
blocks) is to run "dd" to 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.
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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. (One other use case would
be to erase sensitive data a little bit more securely than with a
simple "rm").