Datasheet

overvieW of Protection MechanisMs
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13
Full, Incremental, and Differential Backups
We will cover backup to tape in much more detail as a method in Chapter 3, and in practice within
System Center Data Protection Manager in Chapter 4, as one example of a modern backup solution.
But to keep our definitions straight:
Full Backup Copies every file from the production data set, whether or not it has been recently
updated. Then, additional processes mark that data as backed up, such as resetting the archive
bit for normal files, or perhaps checkpointing or other maintenance operations within a trans-
actional database. Traditionally, a full backup might be done each weekend.
Incremental Backup Copies only those files that have been updated since the last full or incre-
mental backup. Afterward, incremental backups do similar postbackup markups as done by
full backups, so that the next incremental will pick up where the last one left off. Traditionally,
an incremental backup might be done each evening to capture only those files that changed
during that day.
Differential Backup Copies only those files that have been updated since the last full backup.
Differential backups do not do any postbackup processes or markups, so all subsequent differ-
entials will also include what was protected in previous differentials until a full backup resets
the cycle. Traditionally, differential backup might be done each evening, capturing more and
more data each day until the next weekend’s full backup.
If your environment only relies on nightly tape backup, then your company is agreeing to half NOTE
a day of data loss and typically at least one and a half days of downtime per data recovery effort.
Let’s assume that you are successfully getting a good nightly backup every evening, and a
server dies the next day. If the server failed at the beginning of the day, you have lost relatively
little data. If a server fails at the end of the day, you’ve lost an entire business day’s worth of data.
Averaging this out, we should assume that a server will always fail at the midpoint of the day,
and since your last backup was yesterday evening, your company should plan to lose half of a
business day’s worth of data.
That is the optimistic view. Anyone who deals in data protection and recovery should be able
to channel their pessimistic side and will recall that tape media is not always considered reliable.
Different analysts and industry experts may place tape recovery failure rates at anywhere between
10 percent and 40 percent. My personal experience is 30 percent tape failure rate during larger
recoveries, particularly when a backup job spans multiple physical tapes.
Let’s assume that it is Thursday afternoon, and your production server has a hard drive fail-
ure. After you have repaired the hardware, you begin to do a tape restore of the data and find
that one of the tapes is bad. Now you have three possible outcomes:
If the tape that failed is last night’s
•u differential, where a differential backup is everything
that has been changed since the last full backup, then you’ve only lost one additional day’s
worth of data. Last night’s tape is no good, and you’ll be restoring from the evening prior.
If the tape that failed is an
•u incremental, then your restorable data loss is only valid up until
the incremental before this one. Let’s break that down:
If you are restoring up to Thursday afternoon, your plan is to first restore the week-
•u
end’s full backup, then Monday’s incremental, then Tuesday’s incremental, and then
finally Wednesday’s incremental.
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