Network Disaster Recovery
know this, then it's probably the most important lesson that you can take away from this
book. Making regular backups is a requirement when using computers--period.
You don't need to work with computers for very long before you observe firsthand
the importance of good backups. Computers can and do fail, and they sometimes fail in
ways that render the data stored on them unrecoverable. Also, some turn of events may
cause certain important files to be deleted or corrupted. In cases such as these, jobs are
saved or lost based on the quality of the backups in place and the ability to restore that
Assessing Backup Needs
Before designing network backup procedures, you must understand the company's
backup and restoration needs. Questions such as the following may help in assessing
the needs that you must meet:
How dynamic is the data stored on the servers? How often does it change, and
in what ways does it change?
How much data needs to be backed up, and at what rate is the amount of data
How much time is available to make the backup? Make sure that you avoid
situations where you need to back up terabytes of data using a system that can
handle only megabytes per hour.
If a partial or complete restoration from a backup is required, how quickly
must it take place? As a rule of thumb, restoring data takes about twice as long
as backing it up, although in some cases the times may be approximately equal.
In other words, if it takes your backup system 10 hours overnight to back up
the entire network, it will take 10 to 20 hours to restore that data--and this
estimate doesn't include the time required to resolve whatever problem made
it necessary to restore data in the first place.
How coherent does the backed up data need to be? In other words, does a
collection of data files need to be handled as a single unit? For example,
a directory containing a bunch of word processing files isn't terribly coherent;
you can restore one, many, or all of them without much concern about how
those restorations will affect other files. On the other hand, a collection of
database files for a high-end database is often useless unless you can restore
all of the files in the set, from exactly the same point in time. (High-end
databases--such as Oracle's--that require this kind of backup will have their
own detailed instructions for how backups must be made.)
What is the required trade-off between cost and recoverability? You can design
backup systems that operate minute to minute so that if something fails, the
systems will not lose any data, and management can place a high degree of
confidence in this fact. (A bank, for instance, requires this kind of high-end
backup system.) However, such backup systems cost a lot of money and