LHC experiments are now in Run-II data taking and approaching new challenges in the operation
of the computing facilities in future Runs. Despite having demonstrated to be able to sustain
operations at scale during Run-I, it has become evident that the computing infrastructure for RunII
already is dimensioned to cope at most with the average amount of data recorded, and not
for peak usage. The latter are frequent and may create large backlogs and have a direct impact
on data reconstruction completion times, hence to data availability for physics analysis. Among
others, the CMS experiment is exploring (since the first Long Shutdown period after Run-I) the
access and utilisation of Cloud resources provided by external partners or commercial providers.
In this work we present proof of concepts of the elastic extension of a CMS Tier-3 site in Bologna
(Italy), on an external OpenStack infrastructure. We start from presenting the experience on a first
work on the “Cloud Bursting” of a CMS Grid site using a novel LSF configuration to dynamically
register new worker nodes. Then, we move to an even more recent work on a “Cloud Site as-aService”
prototype, based on a more direct access/integration of OpenStack resources into the
CMS workload management system. Results with real CMS workflows and future plans are also
presented and discussed.