3.5.3 Disk Geometry Configuration

Geometry information describes the physical characteristics about the disk. Its has three purposes:

formatting

The geometry information is written into the boot sector of the newly made disk. However, you may also describe the geometry information on the command line. See mformat, for details.

filtering

On some Unixes there are device nodes which only support one physical geometry. For instance, you might need a different node to access a disk as high density or as low density. The geometry is compared to the actual geometry stored on the boot sector to make sure that this device node is able to correctly read the disk. If the geometry doesn’t match, this drive entry fails, and the next drive entry bearing the same drive letter is tried. See multiple descriptions, for more details on supplying several descriptions for one drive letter.

If no geometry information is supplied in the configuration file, all disks are accepted. On Linux (and on SPARC) there exist device nodes with configurable geometry (/dev/fd0, /dev/fd1 etc), and thus filtering is not needed (and ignored) for disk drives. (Mtools still does do filtering on plain files (disk images) in Linux: this is mainly intended for test purposes, as I don’t have access to a Unix which would actually need filtering).

If you do not need filtering, but want still a default geometry for mformatting, you may switch off filtering using the mformat_only flag.

If you want filtering, you should supply the filter flag. If you supply a geometry, you must supply one of both flags.

initial geometry

On devices that support it (usually floppy devices), the geometry information is also used to set the initial geometry. This initial geometry is applied while reading the boot sector, which contains the real geometry. If no geometry information is supplied in the configuration file, or if the mformat_only flag is supplied, no initial configuration is done.

On Linux, initial geometry is not really needed, as the configurable devices are able to auto-detect the disk type accurately enough (for most common formats) to read the boot sector.

Wrong geometry information may lead to very bizarre errors. That’s why I strongly recommend that you add the mformat_only flag to your drive description, unless you really need filtering or initial geometry.

The following geometry related variables are available:

cylinders

tracks

The number of cylinders. (cylinders is the preferred form, tracks is considered obsolete)

heads

The number of heads (sides).

sectors

The number of sectors per track.

Example: the following drive section describes a 1.44M drive:

  drive a:
      file="/dev/fd0H1440"
      fat_bits=12
      cylinders=80 heads=2 sectors=18
      mformat_only

The following shorthand geometry descriptions are available:

1.44m

high density 3 1/2 disk. Equivalent to: fat_bits=12 cylinders=80 heads=2 sectors=18

1.2m

high density 5 1/4 disk. Equivalent to: fat_bits=12 cylinders=80 heads=2 sectors=15

720k

double density 3 1/2 disk. Equivalent to: fat_bits=12 cylinders=80 heads=2 sectors=9

360k

double density 5 1/4 disk. Equivalent to: fat_bits=12 cylinders=40 heads=2 sectors=9

The shorthand format descriptions may be amended. For example, 360k sectors=8 describes a 320k disk and is equivalent to: fat_bits=12 cylinders=40 heads=2 sectors=8