Each hard drive has a disk mode. The disk mode determines if and when changes are written to the disk. All hard disks must be given a disk mode.

You can change the disk mode between VM sessions, but not during a session. If a VM is defined with multiple disks, each disk can have a different mode. If multiple VMs are sharing a disk, the disk mode should be nonpersistent for all VMs.

The three disk modes are described in the next section.

The default disk mode is persistent. You can think of persistent disks as the same as conventional disks; that is, all writes to a persistent disk are "permanent" and changes made in one session carry over to the next session.

The first hard disk defined for a VM should be defined as persistent initially, so that the guest operating system can be installed. Subsequently, that disk can be changed to nonpersistent or undoable, in effect, retaining that disk as a "newly installed" OS.

A persistent disk cannot be accessed by multiple VMs concurrently.

In this mode, all writes and changes to the disk persist only for the duration of the current session. Because no changes are saved after a VM is powered off, you cannot save your work between sessions. Whenever the VM is powered on to start a new session, the disk starts in the same ("clean") state.

Some examples of when nonpersistent mode is useful are situations when you want to use the same environment repeatedly, such as starting up a particular test environment or giving the same application demo.

The fact that the disk is nonpersistent is entirely transparent to the guest operating system. In fact, due to efficient buffering, some disk operations may even be faster using a nonpersistent disk than with a persistent disk.

Nonpersistent disks are the only disks that can be shared or accessed by multiple VMs concurrently.

This disk mode uses a redo log file to record changes to the nonpersistent disk made during one session. For more information about using nonpersistent disk mode and this file, refer to See Using Nonpersistent Disks.

This mode is similar to nonpersistent mode, except that you can choose whether to write, save, or discard any changes made during a VM session. Undoable mode essentially allows you to use a disk as persistent or nonpersistent, making the decision whenever you power on or off.

Examples of when undoable mode is useful include installing new software or performing administrative tasks, when you might encounter a problem and would like the option of either applying changes or reverting to the system before the changes.

As in nonpersistent mode, the VMware application writes a file during each session. When you start the subsequent session you can choose whether or not to apply this file, the redo log. For more information about using this mode, refer to See Using Undoable Disks.

An undoable disk should not be accessed by multiple VMs concurrently.