[vox-tech] How to erase unused disk space?

[Rod Roark]

I have Ubuntu installed in a VMware virtual machine, which is just
a collection of disk files, the largest of which is the disk image.

I want to make a tarball of these files of minimal size.  This seems
to require erasing the contents of the unused areas of the virtual
ext3 filesystem, so they will compress optimally.  Is there some
standard utility that will do this?

Note this is not a "data security erase" problem where the unused
sectors are to be repeatedly overwritten with different bit patterns;
I just want to clear them once to binary zeros.

TIA,

-- Rod