  In a virtualized infrastructure where physical resources are shared, a single
physical server failure will terminate several virtual servers and crippling
the virtual infrastructures which contained those virtual servers. In the worst
case, more failures may cascade from overloading the remaining servers. To
guarantee some level of reliability, each virtual infrastructure, at
instantiation, should be augmented with backup virtual nodes and links that
have sufficient capacities. This ensures that, when physical failures occur,
sufficient computing resources are available and the virtual network topology
is preserved. However, in doing so, the utilization of the physical
infrastructure may be greatly reduced. This can be circumvented if backup
resources are pooled and shared across multiple virtual infrastructures, and
intelligently embedded in the physical infrastructure. These techniques can
reduce the physical footprint of virtual backups while guaranteeing
reliability.
