This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Golden Triangle's TimesTwo was a unique hard disk driver backed by a
misleading advertising campaign. Unlike the file-level compressors
discussed in the previous section TimesTwo is not an init that
patches the file system. Rather it is a hard disk driver similar to
Drive7 or HardDisk Toolkit. After a disk is formatted with TimesTwo
the Finder will report the disk as twice the size it actually is;
e.g. a forty megabyte disk will seem to be an eighty megabyte disk.
TimesTwo then uses compression to try to fit eighty megabytes of data
into the forty megabytes that's really there. If it can't compress
well enough to fit the eighty megabytes of data it promises (and it
generally can't), it creates a phantom file to take up the space it
overestimated. All data written to the disk will be automatically
compressed. This is the exact opposite of the marketdroid promises
that TimesTwo works without compressing anything. In fact it
compresses everything. It's reassuring to know that the market does
sometimes punish such sleazy advertising. Golden Triangle is out of
business and TimesTwo is no longer either sold or supported.
Stacker ($95) and eDisk ($62) work similarly to Times Two, the
main difference being that they are added on top of your current
hard disk driver rather than in place of it. This may allow you to
retain the partitions and other features of your current driver if
it's one Stacker or Edisk is compatible with. However both are
incompatible with a number of other driver level programs including
several disk formatters and security programs, most notably the
latest Apple driver for asynchronous mode on the 68040 Macs.
Alysis has made a very functional demo version of eDisk available
with the only restriction that it compresses at most three to two.
Driver level compressors allegedly increase disk savings by
compressing everything whereas file level compressors exclude certain
frequently accessed files like the desktop file, most things in the
System Folder, and the hard disk data structures from compression.
However the existing file-level compressors use more efficient
compression algorithms than existing driver level compressors so they
normally save you as much or even more space. Furthermore the
exclusion of frequently accessed files from compression vastly
improves the speed of file-level compressed disks. Under driver
level compression since every file needs to be decompressed when read
or compressed when written, a driver-level compressed disk is
noticeably slower than the same Mac with a non-compressed disk or
even a Mac whose disk has been compressed with a file level
compressor. As one Apple VAR put it, "installing TimesTwo is like
dipping your drive in molasses." Stacker and eDisk have equally high
coefficients of virtual viscosity.
Driver level compressors are more popular in the PC world where it's
common to find a fast 486 CPU driving a slow IDE hard disk so that
the time savings from reading fewer physical blocks outweigh the time
lost doing decompression. In the Macintosh world the opposite
situation, a fast SCSI disk coexisting with a slow 68000 CPU, is more
common so driver level compression doesn't work as well. This may be
changing though. Stacker is now PowerPC native and may soon be able
to decompress files so quickly that disk access speed may actually
improve when it's installed. I haven't seen any benchmarks to show
this yet, but I expect that if current PowerPC chips aren't quite
fast enough to make this a reality, the next generation will be.
Using a file-level compressor on a disk already compressed by one of
these products will gain little if any space and will probably cut
your disk access speed in half again so you should use either
driver-level or file-level compression, not both.
All the transparent compression programs have had a number of bugs
and incompatibilities in their initial releases; and TimesTwo
Stacker, and eDisk are no exceptions. Unlike the file-level
programs, however, there have been a number of reports that the first
releases of all three of these utilities have caused data loss and
even corruption of entire hard disks. It is as yet unknown whether
these bugs are fixed in more recent versions. Given the known
incompatibilities, probable speed loss, and significant risk of data
corruption associated with driver level compression, I recommend that
you do not use any of these products at this time.