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ESX Server can use disks in four different modes: persistent, nonpersistent, undoable and append.

• Persistent: Persistent disks behave exactly like conventional disk drives on a computer. All writes to a persistent disk are written out permanently to the disk as soon as the guest operating system writes the data.
• Nonpersistent: All changes to a nonpersistent mode disk are discarded after the virtual machine is powered off.
• Undoable: When you use undoable mode, you have the option later of keeping or discarding changes you have made during a working session. Until you decide, the changes are saved in a redo-log file. When you power off the virtual machine, you are prompted to commit the changes, keep the log by continuing to save changes to the redo log or discard the changes.
• Append: VMware ESX Server supports an additional append mode for virtual disks stored as VMFS files. Like undoable mode, append mode maintains a redo log. However, in this mode, no dialog appears when the virtual machine is powered off to ask whether you want to commit changes. All changes are continually appended to the redo log. At any point, the changes can be undone by removing the redo log. You should shut down the guest operating system and power off the virtual machine before deleting that virtual machine's redo log. You can also commit the changes to the main virtual disk file using the commit option in vmkfstools. See Using vmkfstools for details.

To change the disk mode for a virtual disk, see Configuring a Virtual Machine's Virtual Disks.