Scenario (A)
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(1) Create fedora-33 "dirty" disk image (simulating a long-running VM)
on a remote SSH server.  See "create.sh" script.

(2) The remote server is located over a 100 Mbps ethernet link.  Both
the client and server have plenty of local CPU and are otherwise idle
during the test.

(3) scp test:

scp remote:fedora-33.img local.img

(4) qcow2 snapshot + sparsify + qemu-img convert.  The time taken to
run all of the following operations in series.  See "testA4.sh".

(5) As above but without sparsify.  See "testA5.sh".

(6) As above but using nbdkit + nbdcopy.  See "testA6.sh".


Results of scenario (A)
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(3) (scp) 48m40
    ls -lsh local.img :

(4) (sparsify + qemu-img convert) 2m21

(5) (qemu-img convert)  59m39
    ls -lsh local.img :
    4.1G -rw-r--r--. 1 rjones rjones 32G Feb 10 11:11 local.img

(6) (nbdkit + nbdcopy) 3m49 [probably lock contention in cow filter]
    $ ls -lsh local.img
    1.2G -rw-r--r--. 1 rjones rjones 32G Feb 10 11:16 local.img



Scenario (B)
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(1) Create local OVA file from disk image in scenario (A) using:

tar -cf test.ova -C /path/to/remote fedora-33.img
# -S option is not usually used with OVA files

(2) Time how long it takes to copy the file:

time cp test.ova test2.ova

(3) Time how long it takes to unpack this using tar -xf:

time tar -xf test.ova fedora-33.img

(4) Time how long it takes to copy out the sparsified image.
See "testB4.sh".


Results of scenario (B)
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(2) (cp) 40s

(3) (tar xf) 43s

(4) (sparsify + qemu-img convert) 7.6s