Truncate complete disk image created with dd

https://superuser.com/questions/610819/how-to-resize-img-file-created-with-dd First make sure the free space is actually empty, and doesn't contain leftovers of deleted files. The easiest way to achieve this is to create a huge file on the disk, containing only null bytes, then delete it.

# losetup --find --partscan foo.img
# lsblk
NAME      MAJ:MIN RM    SIZE RO TYPE MOUNTPOINT
loop0       7:0    0   4096M  0 loop 
├─loop0p1 259:0    0   2048M  0 loop 
└─loop0p2 259:1    0   2048M  0 loop 
# for part in /dev/loop0p*; do
    mount $part /mnt
    dd if=/dev/zero of=/mnt/filler conv=fsync bs=1M
    rm /mnt/filler
    umount /mnt
  done
dd: error writing ‘/mnt/filler’: No space left on device
dd: error writing ‘/mnt/filler’: No space left on device
# losetup --detach /dev/loop0

Then compress it with a tool like gzip or xz. Even at lowest compression levels, a long series of zeros will compress well:

# ls -s
4096M foo.img
# gzip foo.img
# ls -s
11M foo.img.gz

Note that you must uncompress the image when writing it back to disk. This will uncompress it 'live':

# cat foo.img.gz | gunzip | dd of=/dev/sda

Note that the output device (sda) must be of sufficient size to fit the original image, otherwise data will be lost or corrupted.

hlev/shrink.md