The dosfs filesystem type

The dosfs filesystem type allows users access to DOS-formatted floppies using regular system commands and system calls, just as with other filesystem types such as s5 or ufs. The ``dosfs filesystem layout'' figure shows the dosfs filesystem layout.

dosfs filesystem layout

UnixWare supports the FAT12, FAT16, VFAT, and FAT32 versions of the DOS filesystem type. The FAT12 and FAT16 versions come with DOS versions up to MS-DOS 6.x, and limit filenames to a traditional 8.3 style (an 8 character ``name'' portion and a 3 character ``extension'' portion). Only numeric and uppercase alpha characters are allowed, and the following special characters cannot be used:

   [space] " / [ ] : * < > | + = ; ' ? .

The version of dosfs known as ``Extended FAT'' or ``VFAT'', is essentially the same as the older versions of the dosfs, but removes some of the file naming restrictions. A VFAT filename can be 255 characters long, and can include lowercase alpha characters, any Unicode character, and spaces. Filenames can contain a dot (.), but cannot start with one. Nor can filesnames contain the following special characters:

   " / : * < > | ?

The rules for file names are the same in the FAT32 version of dosfs, and in addition it improves on the performance of the earlier versions of the filesystem in the following ways:

• FAT32 supports filesystems up to 2TB in size (effectively, 128GB because of IDE hard disk limitations). The minimum size of a FAT32 filesystem is 512MB.

• Use of disk space is up to fiften percent more efficient than in FAT16 filesystems because FAT32 uses smaller clusters (4K clusters for drives up to 8GB in size).

• FAT32 filesystems can be mounted faster because the operating/ system can initialize its internal data structures without having to read the entire FAT.

• A mounted FAT32 version of dosfs can be exported as an NFS filesystem.

• FAT32 can relocate the root directory and use the backup copy of the file allocation table instead of the default copy. In addition, the boot record on FAT32 drives has been expanded to include a backup of critical data structures. This means that FAT32 drives are less sucesptible to a single point of failure than existing FAT16 filesystems.

• The root directory on a FAT32 filesystem can be located anywhere on the disk. (A previous limitation on the number of root directory entries no longer exists.)

The dosfs filesystem type, whichever version you have using, does not support UNIX special files like pipes and block/character specials, nor does it support real or symbolic links.