What is a shared virtual disk and how does it benefit a Windows Server data center?
Normally, one virtual machine is coupled to one virtual disk. With the introduction of Windows Server 2012 R2, multiple virtual machines can share a single virtual disk (such as a VHDX file). This functionality is not intended to support simultaneous access from multiple VMs. Instead, the goal is to provide failover support so that if one VM fails, a duplicate VM can step in and continue using the virtual disk without disruption.
The principal advantage to this approach is that a failover cluster is no longer limited to the existing storage topology, and IT administrators can create a failover cluster using almost any available storage resources, including a shared virtual disk, Fibre Channel storage, server message block storage, Windows Server's Storage Spaces or iSCSI storage. So organizations have greater flexibility in selecting and implementing failover storage. A shared virtual disk is highly recommended for databases (such as SQL Server) or other virtualized file server workloads.
In addition to Windows Server 2012 R2, sharing a virtual disk within a failover cluster will require an available Hyper-V failover cluster with a minimum of two nodes, each sharing the same Active Directory domain. There must be adequate storage resources to host the shared virtual disk (as well as any replication or data protection needed to ensure business continuity). And there must be enough spare computing resources in the failover cluster to handle any virtual machines failed over from other servers.