There are three types of disk modes that can be enabled for raw, plain and virtual disks:
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. The behavior is the same for all disk types.
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.
Any disk type can be used in undoable mode.
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 GSX 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.
The redo log file is placed in the same directory as the disk file by default. However, you can change the location of the redo log file in the Configuration Editor. Click Misc on the left side of the Configuration Editor, then type in or choose the directory in which the redo log should be stored.
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. Any disk type can be used in nonpersistent mode.
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 GSX 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.
The redo-log file is placed by default in the folder defined by the host operating system’s TMPDIR environment variable. However, the location of the redo-log file can be changed in the Configuration Editor. Click Misc on the left side of the Configuration Editor, then type in or choose the directory in which the redo log should be stored.
By default, new virtual machines use an IDE disk in persistent mode for Windows 95, Windows 98, Windows Me, Windows XP and Windows .NET Server guests. The default for all other guest operating systems is a SCSI disk in persistent mode.
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