Network Disaster Recovery
One common way to mix these types of backups is to perform a full backup of the
system once a week and perform only incremental or differential backups each day of
the week. Examine the following examples:
Full backup Friday nights and incremental backups on MondayThursday
If the system fails Monday morning before any data is entered, you need to
restore only the full backup from the previous Friday night. If the system fails
on Thursday morning, you need to restore four tapes sequentially in order
to retrieve all of the data: the full backup from the previous Friday, then the
incremental tapes from Monday, Tuesday, and Wednesday nights. Moreover, to
guarantee the integrity of the data, you must be able to restore all of those tapes,
and in their proper sequence. Otherwise, you run the risk of ending up with
mismatched data files. In this scenario, you have four media-based points of
failure, which might entail more risk than you care to take.
Full backup Friday night and differential backups MondayThursday
this scenario, if the system fails Monday morning, you just restore the tape
from the previous Friday night. However, if the system fails on Thursday
morning, you need to restore only two tapes: the last full backup from Friday
night, plus the differential backup from Wednesday night. Because differential
backups back up all changed files since the last full backup, you never need to
restore more than two tapes, thereby reducing the number of possible points of
To determine the best backup scheme for your system, you need to balance the nature
of the data and the amount of risk you're willing to take against the cost of each backup,
the capacity of the tapes, and the amount of time it takes to make each regular backup.
The most common backup rotation scheme is called grandfather-father-son (GFS). A
common way to implement this scheme is to use at least eight tapes. You label four of
the tapes as "Monday" through "Thursday," and four others "Friday 1," "Friday 2,"
"Friday 3," and "Friday 4." Every Monday through Thursday, you use one of those
labeled tapes, replacing the data stored the previous week. Each Friday tape corresponds
to which Friday in the month you are on: for the first Friday, you use Friday 1, and so
forth. Finally, on the last day of each month, you prepare a month-end tape, which you
do not reuse, but instead keep off-site in case an environmental failure destroys the
system and all locally stored tapes.
There are three main variations of the GFS scheme. In the first, you simply make
a full backup of the system each time you perform a backup. This variation offers the
greatest amount of media redundancy and the minimum amount of restoration time.
In the second, you perform a full backup on each of the Friday tapes and the monthly
tape, but perform only incremental backups during the week. In the third, you do
much the same thing, but use differential backups instead of incremental backups.