This backup scheme can be advanced in two ways. First, each incremental
backup can be made with reference to the original full backup. In other words,
a level 0 backup is followed by a series of level 1 backups. The benefit of this
backup scheme is that a restoration requires only two tapes (the full backup
and the most recent incremental backup). But because it references the full
backup, each incremental backup might be large (and could grow ever larger)
on a heavily used system.
Alternatively, each incremental backup could reference the previous
incremental backup. This is a level 0 backup followed by a level 1, followed
by a level 2, and so on. Incremental backups are quicker (less data each time)
but require every tape to restore a full system. Again, it is a classic trade-off
decision.
Modern commercial backup applications such as Amanda or BRU assist in
organizing the process of managing complex backup schedules and tracking
backup media. Doing it yourself using the classic dump or employing shell
scripts to run tar requires that system administrators handle all the
organization themselves. For this reason, complex backup situations are
typically handled with commercial software and specialized hardware
packaged, sold, and supported by vendors.
Mirroring Data or RAID Arrays
Given adequate (and often expensive) hardware resources, you can always
mirror the data somewhere else, essentially maintaining a real-time copy of
your data on hand. This is often a cheap, workable solution if no large
amounts of data are involved. The use of redundant array of independent
disks (RAID) arrays (in some of their incarnations) provides for recovery if a
disk fails.
Note that RAID arrays and mirroring systems just as happily write corrupt
data as valid data. Moreover, if a file is deleted, a RAID array will not save it.
RAID arrays are best suited for protecting the current state of a running
system, not for backup needs.
Making the Choice
Only you can decide what is best for your situation. After reading about the
backup options in this book, put together some sample backup plans; then run
through a few likely scenarios and assess the effectiveness of your choice.
In addition to all the other information you have learned about backup