VMware Workstation Disk Types and Disk Modes
VMware Workstation allows users to flexibly configure the disks used by a
virtual machine. Users can choose a type and a mode for each
disk. There are three types and three modes. Each type can be
configured with any one of the three modes and vice versa.
A plain disk is like a virtual disk, but it can be larger than 2GB.
It is composed of several files, called extents. Each
extent is limited to 2GB in length. Unlike a virtual disk,
when a plain disk is created, all the disk space defined for the extents
is allocated and filled with zeros.
A raw disk directly accesses an existing local disk or partition.
Raw disks can be used to allow VMware Workstation to run one or more guest operating
systems from existing disk partitions. Raw disks may be set up on both
IDE and SCSI devices. At this time, however, booting from an operating
system already set up on an existing SCSI disk or partition is
experimental.
Additional information on using raw disk partitions
can be found on the VMware Web site:
Disk modes determine how changes are saved to the disk. Raw,
virtual and plain disks can use any available mode. For example, a user
could have an undoable raw disk, an undoable virtual disk, or an
undoable plain disk.
Persistent
VMware 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 VMware 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 VMware 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 VMware Workstation
nonpersistent disk than to an actual 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 under Misc.
While the VMware 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 VMware Workstation session with an undoable disk, you are
given the option of either:
If you choose to keep the redo log, you are prompted again
(fig 3.) when you power on the next VMware Workstation session to see if
you want to 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 guest operating system is entirely unaware that the disk is
undoable. Normal guest operating system file buffering works on
top of the redo log mechanism, providing efficient buffered
I/O. Some disk operations, including file system creation, repair,
and other operations may be faster on a VMware Workstation undoable disk than
on an actual 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 under Misc.
Plain disks have a .pln extension, and the extents of the plain
disk have a .dat extension.
Log files are used to save blocks that the virtual machine modifies
during a session. The log file for a nonpersistent disk is not saved
at the end of a session, while the log file for an undoable disk is
saved. As described above, the log file for undoable disks is
called the redo log, and the user is able to decide
whether the log file should be saved or not.
The log file for a virtual disk called filename is called
filename.dsk.REDO and is created by default in the same
directory as the .dsk file. In the case of plain disks,
filename.pln.REDO is created by default in the same directory
as the .pln file.
By default, log files for raw disks
are located in the same directory as the virtual machine configuration
file. Using the Configuration Editor, you can change the
log file for nonpersistent and undoable disks using the
Advanced button and specifying a different VMware Workstation Redo Log
Directory.
You may choose to locate the log files in a different directory
to increase available space. For best performance, the log
files for a virtual machine should be local to the machine running the
virtual machine.
Lock files are always created in the same directory as the .dsk
or .pln file. There are two types of lock files: reader and
writer. A nonpersistent disk will be protected by reader lock files, while
persistent and undoable disks use writer lock files. A disk
protected by a writer lock file can be accessed by only one virtual
machine. A disk that has reader lock files can be read by more than
one virtual machine but cannot be written to. The extents of a plain disk
are individually locked, using the same method.
A virtual machine will remove lock files it created when the virtual
machine is powered off. If it cannot remove the lock, a stale lock file
will be left protecting the .dsk or .pln file. For
example, if the machine crashes before the virtual machine has a chance
to remove its lock file, there will be a stale lock.
If a stale lock file remains when the virtual machine is started again,
the virtual machine will try to remove the stale lock. To make sure that
no virtual machine could be using the lock file, the virtual machine
checks the lock file to see if:
Raw disk partitions are also protected by locks. However, if the
host operating system is installed on the same raw disk that the virtual
machine is using, the locks are ignored. VMware strongly recommends that
the raw disk for the virtual machine not be installed on the same raw
disk as the host operating system for this reason.
Disk Types
A virtual disk is a file on the host file system
that appears as a physical disk drive to a guest operating
system. The file system can be on the host machine or on a remote
computer. When you configure a virtual machine with a virtual disk, you
can install a new operating system onto the disk file without
the need to repartition a physical disk or reboot the host.
Virtual disks are limited to a maximum of 2GB. The actual file used by the
virtual disk starts out small and grows to the maximum size as needed.
Disk Modes: Persistent, Nonpersistent
and Undoable
In the VMware Workstation configuration menu
(fig. 1) disks can be configured in one of three modes:
Persistent, Nonpersistent, and Undoable.
Figure 1.
Configuration Editor showing an IDE hard disk configured as
undoable
Persistent
disks are the simplest type of disks that VMware Workstation
supports. Persistent disks behave like conventional disk drives on
your computer. All writes done to a persistent disk are written
out permanently to the disk. The behavior is the same for all disk types.
Nonpersistent
Changes to
nonpersistent disks are not saved during the VMware Workstation session and
are lost at the end of the session (that is, when the virtual machine is
powered off or reset). Nonpersistent disks are 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.
Undoable
Undoable disks are similar to nonpersistent disks in that writes
to the disk are logged elsewhere in a file called a redo
log. An undoable disk, unlike a nonpersistent disk, gives the user
the option later of permanently applying the changes saved in the redo
log to the disk. Undoable disks are convenient in that errors or problems
can be undone by simply discarding the redo log. Examples of uses for
undoable disks include installing software or doing administration
tasks that may need to be undone if there are problems.
Figure
2. When you power off with an
undoable disk, you are prompted to either commit the redo log
changes, discard the changes, or keep the redo log.
Figure
3. During power on, if VMware Workstation detects a redo log
file it prompts you to either commit the redo log changes,
discard the changes, append new changes to the redo log, or cancel
the power on.
File locations
The Configuration Editor allows you to specify where the disk
file is located on the file system. A disk file has a .dsk
extension for virtual disks.
Lock files
A running virtual machine tries to create lock files to prevent
consistency problems on disks. If the virtual machine didn't use locks,
multiple virtual machines might read and write to the disk, causing users
to lose data.
If those two conditions are true, the virtual machine can safely remove the
stale lock. If either of those conditions are not true, a dialog will
be shown explaining what the user can do about the lock.