A VMware vSphere Metro Storage Cluster (vMSC) configuration is a VMware-certified solution that combines replication with array-based clustering. These solutions are typically deployed in environments where the distance between data centers is limited―often metropolitan or campus environments.
A vMSC configuration requires what is in effect a single storage subsystem that spans both sites. In this design, a specified data store must be accessible (that is, be able to be read from and written to) simultaneously from both sites. In addition, when problems occur, the ESXi hosts must be able to continue to access data stores from either array transparently and with no impact to ongoing storage operations.
The storage subsystem for a vMSC must be able to write to and be read from both locations simultaneously. All disk writes are committed synchronously at both locations to ensure that data is always consistent, regardless of the location from which it is being read. This storage architecture requires significant bandwidth and very low latency between the sites in the cluster. Increased distances or latencies cause delays in writing to disk and a dramatic decline in performance. They also preclude successful vSphere vMotion migration between cluster nodes that reside in different locations.