Disk Compression

Since hard disks have become large enough for most users, disk compression has passed out of vogue. But on older systems with IGB or smaller hard disks, disk compression extends the capacity of a hard disk drive. Disk compression uses data compression techniques to reduce the amount of disk space a file uses. The effect is that more files fit into the same space. Understand that nothing is really happening to the disk; the data is actually being compressed and stored in a special file. The compressed data must be translated in and out of the compressed data store.

A disk compression utility must reside in memory and work between the operating system and the disk controller. The compression utility intercepts any file read and write actions sent to the disk. When the operating system saves a file to disk, the compression utility intercepts the file and compresses it before it's written to the compressed data store. When the operating system reads a file, the compression utility intercepts the file, decompresses it, and then passe's the data on to the system memory. This utility does add some overhead to the process and slows down all file access from the compressed disk.

A number of third party disk compression utilities are available, all of which work essentially the same. Windows has included disk compression software in nearly all of its versions. Windows 3.x featured a routine called DBLSpace. Windows 95 included DriveSpace, which could compress and uncompress data on floppy disks, removable media, or hard disk drives. DriveSpace works by creating a new uncompressed logical drive, called the host drive, where it stores the CVF (Compressed Volume File), a form of VFAT for the compressed drive. The uncompressed drive also contains files that should not or cannot be compressed, such as system files. Any unused space is available to the user. The Windows 95 version of DriveSpace creates compressed drives of up to 512MB. Large disk drives usually can't be compressed as a single volume. The version available in Microsoft Plus! and Windows 98 can compress drives up to 2GB.