12.2. Selecting the backup medium
The most important decision regarding backups is the choice
of backup medium. You need to consider cost, reliability, speed,
availability, and usability.
Cost is important, since you should preferably have
several times more backup storage than what you need for the data.
A cheap medium is usually a must.
Reliability is extremely important, since a broken
backup can make a grown man cry. A backup medium must be able
to hold data without corruption for years. The way you use the
medium affects it reliability as a backup medium. A hard disk
is typically very reliable, but as a backup medium it is not
very reliable, if it is in the same computer as the disk you
are backing up.
Speed is usually not very important, if backups can be done
without interaction. It doesn't matter if a backup takes two
hours, as long as it needs no supervision. On the other hand,
if the backup can't be done when the computer would otherwise
be idle, then speed is an issue.
Availability is obviously necessary, since you can't
use a backup medium if it doesn't exist. Less obvious is the
need for the medium to be available even in the future, and on
computers other than your own. Otherwise you may not be able
to restore your backups after a disaster.
Usability is a large factor in how often backups are made.
The easier it is to make backups, the better. A backup medium
mustn't be hard or boring to use.
The typical alternatives are floppies and tapes.
Floppies are very cheap, fairly reliable, not very fast,
very available, but not very usable for large amounts of data.
Tapes are cheap to somewhat expensive, fairly reliable, fairly
fast, quite available, and, depending on the size of the tape,
quite comfortable.
There are other alternatives. They are usually not very
good on availability, but if that is not a problem, they can
be better in other ways. For example, magneto-optical disks
can have good sides of both floppies (they're random access,
making restoration of a single file quick) and tapes (contain
a lot of data).
