Virtual Disk Types—Fixed, Dynamic, and Pass-through

When you are creating a virtual machine, options exist to create a hard disk for the VM as a fixed size or dynamically expanding. In VMware, the dynamically expanding disk is referred to as thin provisioning. The difference is that with a fixed-size disk, the space allocated to a virtual machine is immediately accounted for on the host operating system disk volume. For instance, if the host has 500GB of free disk space and a 100GB fixed-size disk is created, the host reflects 400GB of free space. Dynamically expanding disks differ in that a maximum size is specified that the disk can grow to, but the space is not immediately consumed. Continuing the previous example, the virtual machine still believes it has a 100GB hard disk, but space on the host physical disk is consumed only as the virtual machine begins to write data to the disk. The virtual disk is negligible at first, but it might consume 10GB of space after an operating system is installed, and more when applications and data are added.

A third type of option in disk configuration for virtual machines is to use pass-through disks. Pass-through disks present a physical hard disk directly to a virtual machine. This configuration is not as typical for small environments, but where performance must be guaranteed, and resources are not shared with other virtual machines, pass-through disks are an attractive option.

For Lync and SQL Servers, Microsoft recommends using either fixed-size disks or pass-through disks, not to use dynamic disks, due to the potential performance overhead in production.