From the zerofree man page:
zerofree finds the unallocated, blocks with non-zero value content in an ext2, ext3 or ext4 filesystem (e.g. /dev/hda1) and fills them with zeroes (or another octet of your choice).
Filling unused areas with zeroes 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.
This can help further reduce the size of the compressed image.
I don't really know if zerofree should happen as part of the new SHRINK command, but it is a possibility considering that SHRINK is similar in intent and already has a loop device setup on the image.
From the zerofree man page:
This can help further reduce the size of the compressed image.
I don't really know if zerofree should happen as part of the new SHRINK command, but it is a possibility considering that SHRINK is similar in intent and already has a loop device setup on the image.