September 1998
A Word About Compression
Many tape drives include fast and convenient hardware based compression.
Tape drive manufacturers, reasoning that tape is used primarily for
backup, have found that hardware compression is a great way to
efficiently pack data onto tape. Users like compression because it saves
money and backup time.
Tape manufacturers have learned to quote two capacities for their
products, the actual capacity, called native capacity, and a second,
higher figure called compressed capacity. Most companies quote compressed
capacity at twice that of native capacity.
When it comes to hardware compression, users should be aware of two
issues:
Hardware compression is usually proprietary. Even if a tape
standard is open, the hardware compression used by one tape drive
manufacturer may be completely incompatible with that of another.
Therefore, tapes compressed on one drive may be completely indecipherable
on another drive, even though they may use the same form factor.
The solution to this problem is to use software compression built into
your backup software. Then the compression is the same regardless of
which tape drive you use. With software compression, however, you lose some of the speed and
efficiency of hardware compression.
Not all files compress equally well. This is an issue of
special importance to managers of imaging systems where scanned images
are already compressed.
Compressed image file formats like JPEG, GIF, Group3 and Group4 don't
compress well. You can see this by running a few sample files through a
software compression utility like Winzip or Stuffit.
If you're buying tape drives with the intention of backing up compressed
image files, don't expect to get much more than native capacity on a
tape.
--Lowell Rapaport
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