I got something new about Condor VM Universe here : http://cca11.org/files/2011/04/p09.pdf
In section 2.2, it says:
In order to improve the performance of VM jobs at start
and eviction, the Condor developers added support for
pre-staging VM images on execute hosts in Condor version 7.5.3.
By pre-staging the VM on local disk, job startup and eviction no
longer saturates the network connection. The bulk
queueing of VM jobs is also faster since the scheduler does
not have to copy the same image out to each execute node.
How to configure this new pre-staging feature in Condor?