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Usage, Structure and Geometry of Plain Disks

Plain Disk Space Usage

Unlike a virtual disk, a plain disk is allocated at its full size. This ensures that, if the disk is used in persistent mode, the virtual machine does not unexpectedly run out of space while trying to write to the disk.

Be aware that if you are running with nonpersistent or undoable plain disks, additional disk space is needed on the host file system to hold the redo log. This additional space is not allocated in advance so it is possible to run out of space for the redo log file. If you run out of space for the redo log, VMware Workstation warns you and suggests that you free some additional space. Additionally, the redo log directory may be modified using the Configuration Editor, but only for disks in undoable mode.

The Structure of a Plain Disk

A plain disk consists of a description file that is a text file that describes the geometry of the disk, and the files that hold the data for that disk. Here is an example of a plain disk description file. This sample plain disk description file defines a disk that is about 200MB in size.

# Plain Disk
# Type of disk drive (ide or scsi)
DRIVETYPE ide

# Number of heads
HEADS 16

# Sectors per cylinder
SECTORS 63

# 200 Megabytes is about 400 cylinders
CYLINDERS 400

# Allowed access type (ACCESS or RDONLY), filename, and
# starting offset
ACCESS "C:\vmware\plain\plaindisk.dat" 0

Plain Disk Geometry

The plain disk geometry (cylinders, heads and sectors) is filled in automatically by the Configuration Editor when the plain disk is created. If the plain disk is based on disk partitions, then the geometry matches that of the underlying disk. Otherwise a default geometry is used.

Note: The geometry of a disk can be determined by using the hdparm command. For example, type the following command to obtain information about the disk /dev/hda:
hdparm -i /dev/hda

The field RawCHS shows the actual disk geometry.

For an IDE plain disk, the number of heads defaults to 16 and the number of sectors per cylinder to 63. The number of cylinders depends on the size of the plain disk you wish to create. The sector size is always 512 bytes. The total size of the plain disk in this case is 16 * 63 * 512 * CYLINDERS bytes.

For a SCSI plain drive, the geometry varies with the size of the plain drive.

Disk size

Heads

Sectors

<1GB

64

32

>=1GB but <2GB

128

32

>=2GB

255

63

The number of cylinders is determined by taking the total disk size in sectors and dividing by the number of heads times the sectors per cylinder.

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