When a failover occurs in an HA cluster, the software components have to be restarted on the standby node. Assuming the storage has been replicated to the remote location using synchronous or asynchronous mode5, the standby node can continue to handle the application safely.
For cloud computing, it is usually necessary to keep the session stateful during and after the migration of a VM. When stateful session is necessary during the movement of the system, the distance between the two physical hosts is driven by the maximum latency supported, the synchronous replications for data mirroring, and the services associated with the storage replication. This means a maximum distance of 100 km between two hosts should not be exceeded when using an Active/Active storage mode requiring synchronous data replication engines. It is important to note that this distance drops to 40-50 km when using the default shared storage mode.
The elasticity of cloud computing is also driven by the requirement of the active sessions to be maintained with no interruption of service, therefore live migration services in real time are limited to metro distances due to the synchronous mirroring (zero RPO). Beyond metro distances6and using current storage tools, the whole cloud computing solution becomes a DRservice. Therefore service functions such as Site Recover Manager (SRM®) are very efficient and built for that purpose, but in a stateless session mode.
Cisco addresses DCI requirements with a complete set of network and storage architecture solutions. The Cisco solution is based on the four components that are critical to providing transport elasticity, flexibility, transparency, and resiliency between two or more sites: