DOSD(4)

NAME
dosd - DOS file as a virtual disk

DESCRIPTION
The dosd* devices allow one to use a DOS file as a disk. If for instance the Boot Environment contains one of these pairs of settings:

    dosd = file
    dosd0 = \minix\disk0.mnx

    dosd = fat
    dosd0 = hd1:\minix\disk0.mnx

Then the device /dev/dosd0 addresses all the blocks in the DOS file \minix\disk0.mnx.

The "file" and "FAT" versions of the driver differ radically on how they access the DOS file, the first will use actual DOS calls to read and write the file, the second interprets a FAT file system to find the blocks of the file. One understands that the "file" driver can only run if Minix has been started from DOS using the DOS version of the Boot Monitor. Note that the DOS Boot Monitor will also set dosd0 automatically to the file it is to boot, so no configuration is necessary.

In either case the DOS file is seen as a disk under Minix. The disk can have one primary partition table allowing one to create the partitions dosd1 through dosd4.

One more virtual disk may be created for the FAT driver by setting the dosd5 variable to enable the devices dosd[5-9]. The file driver also supports dosd10 and dosd15 for four virtual disks total. The FAT driver allows one to use any primary partition on a normal hard disk or even on a floppy drive (for testing purposes).

The FAT driver has less then 20% performance overhead compared with a true partition due to the DOS file decoding. The file driver depends on how well DOS does file I/O, which isn't too bad for simple reads and writes to one file. Overhead can be minimized if I/O is aligned to DOS file clusters. For Minix file system use you are advised to start the virtual partitions on even-numbered disk sectors, so that the 2-sector Minix blocks do not span clusters. Minix-vmd swap partitions are accessed in units of the page size, i.e. 4096 bytes = 8 sectors. To minimize overhead you could let partitions start on a cluster boundary. The virtual disk will appear to have a track size equal to the cluster size, so a partition is cluster aligned if it starts on the first sector in a track. It is of course best to also defragment your DOS partition to eliminate any extra disk seeks due to gaps between clusters.

Replacing the "file" driver by the "FAT" driver will help performance if the normal disk driver used by the FAT driver is a native Minix driver. The FAT driver used together with the BIOS disk driver will not make much of a difference.

FILES
/dev/dosd[0-9]

SEE ALSO
hd(4), sd(4).

BUGS
The file driver needs calls to read and write arbitrary DOS files to allow file to be easily moved between DOS and Minix.

The FAT driver doesn't understand long file names.

More testing is needed on FAT-32 file systems.

AUTHORS

  FAT driver
Philip Homburg (philip@cs.vu.nl)

  File driver and manual page
Kees J. Bot (kjb@cs.vu.nl)