Use-case for 'diff' of images: Useful for converting a monolithic image back into a thin image on top of a common base. (useful when you have copied or cloned a guest, and you want to get back to a thin image on top of a template or base image)

NOTE: (1) the base-image is base-f17.img ; . 

#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info ../base-f17.qcow2 
image: ../base-f17.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 5.0G
cluster_size: 65536
[root@localhost difftest]# 
#----------------------------------------------------------------#
[root@localhost difftest]# rsync -avh --progress ../base-f17.qcow2 modified-f17.qcow2
[root@localhost difftest]# ls
modified-f17.qcow2
#----------------------------------------------------------------#

The modified-f17.qcow2 has 3 extra textfiles, and a directory /export  (that's the 'difference' between base-f17.img)
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info modified-f17.qcow2 
image: modified-f17.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 5.0G
cluster_size: 65536
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img create -f qcow2 -b modified-f17.qcow2 diff.qcow2
Formatting 'diff.qcow2', fmt=qcow2 size=5368709120 backing_file='modified-f17.qcow2' encryption=off cluster_size=65536 lazy_refcounts=off 
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info diff.qcow2 
image: diff.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 196K
cluster_size: 65536
backing file: modified-f17.qcow2
[root@localhost difftest]# 
#----------------------------------------------------------------#

Now, let's do the 'rebase' operation. (this is an expensive operation)
- At the end of operation, the diff.qcow2 will have its backing file changed to 'base-f17.qcow2'
- Note that: (In 'Safe mode', which is the default -- any clusters that differ between backing_file and the old backing file of filename are merged into filename before actually changing the backing file. )
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img rebase -b ../base-f17.qcow2 diff.qcow2
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info diff.qcow2 
image: diff.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 1.4M
cluster_size: 65536
backing file: ../base-f17.qcow2
[root@localhost difftest]# 
#----------------------------------------------------------------#

guestfish is able to boot into the diff, and also list the files added here (at this point, we can discard 'modified.qcow2
#----------------------------------------------------------------#
[root@localhost difftest]# guestfish --rw -i -a diff.qcow2 

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: Fedora release 17 (Beefy Miracle)
/dev/mapper/vg_none-lv_root mounted on /
/dev/sda1 mounted on /boot

><fs> ls /export
testfile1
testfile2
testfile3
><fs> 
#----------------------------------------------------------------#

Now, the overlay is quite small, as it can be noticed here:
#----------------------------------------------------------------#
[root@localhost difftest]# ls -lash diff.qcow2 
1.4M -rw-r--r--. 1 root root 1.5M Oct  4 05:58 diff.qcow2
[root@localhost difftest]# 
#----------------------------------------------------------------#

Also, you can quickly boot into the diff.qcow2 to see what's up with it.
#----------------------------------------------------------------#
[root@localhost difftest]#  qemu-kvm -enable-kvm -m 1024 diff.qcow2  -nographic
#----------------------------------------------------------------#