In its simplest form, shared storage is achieved through the use of delta disk backings. A delta disk backing is a virtual disk file that sits on top of a standard virtual disk backing file. Each time the guest operating system on a virtual machine writes to disk, the data is written to the delta disk. Each time the guest operating system on a virtual machine reads from disk, the virtual machine first targets the disk block in the delta disk. If the data is not on the delta disk, the virtual machine looks for it on the base disk.
Linked virtual machines can be created from a snapshot or from the current running point. After you create a set of linked virtual machines, they share the base disk backing and each virtual machine has its own delta disk backing, as shown in Linked Virtual Machines with Shared Base Disk Backing and Separate Delta Disk Backing.
