There are three types of disk modes that can be enabled for raw, plain, and virtual disks:
Workstation only reads the disk. Any writes to the disk during the session are actually written to a redo log file that is deleted at the end of the session; when you power off or reset the virtual machine. During the Workstation session, any blocks that have been modified and written to the redo log file are read from the redo log file instead of the disk. At the end of the Workstation session, the redo log file is discarded. The guest operating system is entirely unaware that the disk is nonpersistent. Normal guest operating system file buffering works on top of this mechanism, providing efficient buffered I/O. Some disk operations may even be faster to a Workstation nonpersistent disk than to a physical disk. The redo log file is placed in the same directory as the disk file by default. However, the location of the redo log file can be changed in the Configuration Editor..
Undoable disks are similar to nonpersistent disks in that writes to the disk are stored in a file called a redo log. An undoable disk, unlike a nonpersistent disk, gives you the option later of permanently applying the changes saved in the redo log, so they become part of the main disk.
While the Workstation session is running, disk blocks that have been modified and written to the redo log are read from there instead of the disk. Any disk type can be used in undoable mode. When you power off a Workstation session with an undoable disk, you are given three options: commit the changes in the redo log to the disk, discard the changes in the redo log, or keep the redo log. If you choose to keep the redo log, the next time you power on the virtual machine Workstation detects the redo log file and prompts you to either commit the redo log changes from the previous session, 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.
By default, new virtual machines use an IDE disk in normal (persistent) mode for Windows 95, Windows 98, Windows Me and Windows XP guests. The default for all other guest operating systems is a SCSI disk in normal (persistent) mode.
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