  High availability has always been one of the main problems for a data center.
Till now high availability was achieved by host per host redundancy, a highly
expensive method in terms of hardware and human costs. A new approach to the
problem can be offered by virtualization. Using virtualization, it is possible
to achieve a redundancy system for all the services running on a data center.
This new approach to high availability allows to share the running virtual
machines over the servers up and running, by exploiting the features of the
virtualization layer: start, stop and move virtual machines between physical
hosts. The system (3RC) is based on a finite state machine with hysteresis,
providing the possibility to restart each virtual machine over any physical
host, or reinstall it from scratch. A complete infrastructure has been
developed to install operating system and middleware in a few minutes. To
virtualize the main servers of a data center, a new procedure has been
developed to migrate physical to virtual hosts. The whole Grid data center
SNS-PISA is running at the moment in virtual environment under the high
availability system. As extension of the 3RC architecture, several storage
solutions have been tested to store and centralize all the virtual disks, from
NAS to SAN, to grant data safety and access from everywhere. Exploiting
virtualization and ability to automatically reinstall a host, we provide a sort
of host on-demand, where the action on a virtual machine is performed only when
a disaster occurs.
