A stretched cluster provides the capability of balancing workloads between two datacenters. Workloads run undisturbed and can be migrated between geographically close sites without the need for sustaining an outage.
A stretched cluster is a single cluster, typically set up over two sites that are separated by a distance of up to 100 kilometers. With a stretched cluster, all disk writes are committed synchronously at both the primary and secondary sites. Synchronous writes ensure that data is always consistent regardless of the site from which the data is being read.
A stretched cluster occupies a single IP address space, so virtual machines can be migrated nondisruptively by vMotion. The underlying technology of a stretched cluster is Metro vMotion. Metro vMotion can be used to move a running virtual machine when the source and destination ESXi hosts have a latency of more than 5 milliseconds round-trip time latency. The maximum supported round-trip time latency between the two hosts is now 10 milliseconds. Metro vMotion is available with the vSphere 5 Enterprise Plus license.