DISK COMPRESSION

2/21/98

Disk Compression: A technique best known with MSDOS wherein files are written to disk in compressed form. In the case of MSDOS, a number a products including STACKER, DOUBLEDISK, and NETROOM were written. Starting with DOS 6, Microsoft included DOUBLESPACE with DOS 6.0 through 6.20. After a lawsuit, DOS 6.21 was released with no compression and DOS 6.22 was released with a modified DBLSPACE compatible compression program -- DRVSPACE. DRVSPACE is also included in Windows 95 with an enhanced version incorporated in the PLUS package.

DRVSPACE2 is limited to 512MB file systems. DRVSPACE3 allows larger (2.1GB?) file systems. DRVSPACE2 requires more than 50K of memory. DRVSPACE3 distributed with the PLUS! package requires 100K. Under WIN95, protected mode drivers remove most Drivespace code from DOS memory space. Note that reinstalling Win95 may also require reinstalling the PLUS! DRVSPACE3 if it was previously installed.

All MSDOS Disk Compression programs operate by allocating a large hidden file on an uncompressed disk. The hidden file is then managed by direct reads and writes -- effectively hiding one or more smaller file systems within the MSDOS file system. The hidden file systems include some simple data compression as well as a smaller "cluster" size than ordinary FAT16 file systems. ["Cluster" size does not seem to be the unit of allocation for compressed files.] The combination of compression and less allocated, but unused storage in clusters generally yields compression factors of about 2 to 1.

Although some CPU time is spent compressing/uncompressing files, it is asserted that this about offset by the time save by reading and writing fewer bytes. Exact timing depends on the operations performed and the data it is performed on.

Compressed drives are slightly less robust than uncompressed drives since errors in either the MSDOS FAT/master directory or the compressed drive control structure will cause information to be lost. The difference in robustness is probably less than most PC advisors think -- about equivalent to the decrease in reliability from using deeply nested directory structures. Most utilities will run on a compressed drive Defragmentation of a compressed drive is multilevel and slow. A few complications exist. For example, Windows 3 swap files should not be allocated to a compressed drive. The major confusion is that a rather obscure renaming process is used when booting a compressed boot drive that results in the compressed drive being seen as C: once the compression program has completed and the actual C: drive that would be seen if one booted from a floppy drive will be allocated to another drive letter. The renaming process is usually transparent to the user, but can be quite confusing when problems occur during boot.

Linux supports MSDOS/Windows compressed files as dmdos file systems.

Updated 021018

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