With Windows Server 2008 R2, Microsoft made some changes to Hyper-V to make it a more worthy competitor. While new features like live migration're getting a lot of hype, storage enhancements're also noteworthy. In a standard Windows cluster, one machine in cluster owns a resource at a time. As a result, any cluster that hosts a set of Hyper-V virtual machines on a storage area network (SAN) has to put each VM on a separate LUN so that a cluster node fails over all of its appropriate cluster resources. This puts a severe configuration management hurdle in way of highly-available Hyper-V model. Cluster Shared Volumes (CSV) is a new built-in feature in Hyper-V R2 that allows VMs access to their virtual disk -- even when they’re all stored on same LUN. This allows you to take advantage of storage configuration in place without having to ask storage administrators to rearrange their SAN in order to add a new virtual machine. CSV requires at least two LUNs. One LUN is primary volume that has VMs with configuration and virtual hard disk files, while other LUN is witness disk configured at hardware level, but its not a visible resource to use as a disk.