The CMS and ATLAS online clusters consist of more than 3000 computers each. They have been exclusively used for the data acquisition that led to the Higgs particle discovery, handling 100Gbytes/s data flows and archiving 20Tbytes of data per day.
An openstack cloud layer has been deployed on the newest part of the clusters (totalling 1300 hypervisors and more than 13000 cores in CMS alone) as a minimal overlay so as to leave the primary role of the computers untouched while allowing an opportunistic usage of the cluster.
This presentation will show how to share resources with a minimal impact on the existing infrastructure. We will present the architectural choices made to deploy an unusual, as opposed to dedicated, "overlaid cloud infrastructure". These architectural choices ensured a minimal impact on the running cluster configuration while giving a maximal segregation of the overlaid virtual computer infrastructure. The use of openvswitch to avoid changes on the network infrastructure and encapsulate the virtual machines traffic will be illustrated, as well as the networking configuration adopted due to the nature of our private network. The design and performance of the openstack cloud controlling layer will be presented. We will also show the integration carried out to allow the cluster to be used in an opportunistic way while giving full control to the CMS online run control.