There are four types of disk modes:
Persistent. Disks in persistent mode are the simplest to use. They behave like conventional disk drives on your physical computer. As soon as the guest operating system writes the data, the data is written out permanently to the disk.
Undoable. Undoable mode lets you decide when you power off the virtual machine whether you want to keep or discard the changes made since the virtual machine was powered on. This is especially useful for experimenting with new configurations or unfamiliar software. Because of the disaster-recovery possibilities this mode offers, many users prefer to set disks in undoable mode as a standard part of their configurations.
When data is written to an undoable mode disk, the changes are stored in a file called a redo log. A disk in undoable mode gives you the option later of permanently applying the changes saved in the redo log, so they become part of the main disk files.
While the virtual machine is running, disk blocks that have been modified and written to the redo log are read from there instead of from the disk files.
When you power off a virtual machine with a disk in undoable mode, you are given three options:
If you choose to keep the redo log, the next time you power on the virtual machine VMware ESX Server detects the redo-log file and prompts you to either commit the redo log changes made from the last time the virtual machine ran, discard the redo log, continue appending changes to the redo log or cancel the power on.
Append. Append mode, like nonpersistent and undoable modes, stores changes in a redo log. It continually adds changes to the redo log until you remove the redo-log file or commit the changes using the commit command in vmkfstools.
Nonpersistent. Changes to disks in nonpersistent mode are not saved to the disks, but are lost when the virtual machine is powered off or reset.
Nonpersistent mode is convenient for people who always want to start with a virtual machine in the same state. Example uses include providing known environments for software test and technical support users as well as doing demonstrations of software.
If your virtual disks are in nonpersistent mode, you can take advantage of the repeatable resume feature, which allows you to save the current state of the virtual machine when you suspend it, then resume from the point at which you suspended it every time you start the virtual machine.
VMware ESX Server only reads the virtual disk file. Any writes to the virtual disk are actually written to a redo-log file that is deleted when you power off or reset the virtual machine.
While you are running the virtual machine, any blocks that have been modified and written to the redo-log file are read from the redo-log file instead of the disk. When the virtual machine is powered off or reset, the redo-log file is discarded.
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