The Computing facility CNAF, in Bologna (Italy), is the biggest WLCG Computing Center in
Italy, and serves all WLCG Experiments plus more than 20 non-WLCG Virtual Organizations
and currently deploys more than 200 kHS06 of Computing Power and more than 20 PB of Disk
and 40 PB of tape via a GPFS SAN. The Center has started a program to evaluate the possibility
to extend its resources on external entities, either commercial or opportunistic or simply remote,
in order to be prepared for future upgrades or temporary burst in the activity from experiments.
The approach followed is meant to be completely transparent to users, with additional external
resources directly added to the CNAF LSF batch system; several variants are possible, like the
use of VPN tunnels in order to establish LSF communications between hosts, a multi-master
LSF approach, or in the longer term the use of HTCondor. Concerning the storage, the simplest
approach is to use Xrootd fallback to CNAF storage, unfortunately viable only for some experiments;
a more transparent approach involves the use of GPFS/AFM module in order to cache files
directly on the remote facilities. In this paper we focus on the technical aspects of the integration,
and assess the difficulties using different remote virtualisation technologies, as made available at
different sites. A set of benchmarks is provided in order to allow for an evaluation of the solution
for CPU and Data intensive workflows. The evaluation of Aruba as a resource provider for CNAF
is under test, with limited available resources; a ramp up to a larger scale is being discussed. On
a parallel path, this paper shows a similar attempt of extension using proprietary resources, at
ReCaS-Bari; the chosen solution is simpler in the setup, but shares many commonalities.