VM Disk Modes

• Nonindependent mode This is the default mode, and the only one that allows the snapshot feature.

• Independent Persistent mode This mode treats the virtual disk as a normal disk—any I/O is committed to the disk immediately, and snapshots are not allowed. Of course, you must still shut down the guest OS properly to flush the contents of memory to the disk. This is due to file-system caching, present in many modern OSs. You can use this mode to disable snapshots on a VM. Of course, a much better way to disable access to the snapshot feature is with permissions.


• Independent Nonpersistent mode This mode marks the virtual disk as volatile. Any changes made after this switch stop any I/O events from entering the disk. Every time you power the VM off and on, your changes are lost. Some customers use this with test and development VMs or with training VMs that always need to be reset to a given state. What actually happens is changes accrue in a delta file, but at power off, they are never merged into the virtual disks. Taken to the logical conclusion, this could be very useful in a VDI environment or kiosk VM. Imagine a situation where you have one only VMDK file of Windows XP wasting valuable space on the SAN, and each user receives a delta version. At the end of the working day, these VMs are powered off and reset to the golden state before the users made changes. 

Changing disk modes requires the VM to be powered off.You can enable the Independent Persistent or Nonpersistent disk mode from the Virtual Machine Properties dialog box. Select Hard Disk in the list of devices. You can then choose your desired mode.

If you enable Nonpersistent mode, you could then make all manner of crazy changes to
the VM. If you shut down the VM and then power it back on, it will be reset to its original
state. In this case, all the changes accrue in a snapshot file called delta_REDO, and these
changes are always discarded.