From: Other

See the comp.compression FAQ, and don't believe everything a vendor
tells you. 2x compression is the standard going rate for lossless
compression of arbitrary data, though some vendors claim 2.5 or 3x.
Your mileage will vary with your data type.

Compressing tape drives are common, but for disks and other block
devices I don't know of anything being done. The unpredictability of
the compression ratio generally makes it inappropriate for devices
that need fixed capacities and addresses.

Online compression of files can be accomplished by hand using
utilities such as gzip and Unix compress. Some systems support
software compression of files in the file system software, and will
transparently compress and decompress files as needed. Stacker for PCs
is one example; for Unix-like systems this seems to be common research
for object-oriented file systems (including the GNU Hurd), but I don't
know of any production versions offhand (SHMO).

Compression may make your data more vulnerable to errors. A single
error early in a compressed stream of data can render the entire data
stream unreadable.