Datasheet

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CHAPTER 1 What Kind of Protection do You need?
If it is Wednesday’s incremental that failed, you can reliably restore through Tuesday •u
night, and will have only lost one additional day’s worth of data.
But if the bad tape is Tuesday’s incremental that failed, you can only reliably recover
•u
back to Monday night. Though you do have a tape for Wednesday, it would be sus-
pect. And if you are unlucky, the data that you need was on Tuesday night’s tape.
The worst-case scenario, though, is when the full backup tape has errors. Now all of your •u
incremental and differentials throughout the week are essentially invalid, because their
intent was to update you from the full backup — which is not restorable. At this point, you’ll
restore from the weekend before that full backup. You’ll then layer on the incrementals or
differentials through last Thursday evening. In our example, as you’ll recall, we said it was
Thursday afternoon. When this restore process is finished, you’ll have data from Thursday
evening a week ago. You’ll have lost an entire week of data. But wait, it gets worse. Remember,
incrementals or differentials tend to automatically overwrite each week. This means that
Wednesday night’s backup job will likely overwrite last Wednesday’s tape. If that is your rota-
tion scheme, then your Monday, Tuesday, and Wednesday tapes are invalid because its full
backup had the error. But after you restore the full backup of the weekend before, the days
since then may have been overwritten. Hopefully, the Thursday evening of last week was
a differential, not an incremental, which means that it holds all the data since the weekend
prior and you’ll still have lost only one week of data. If they were incrementals, you’ll have lost
nearly two weeks of data.
Your Recovery Goals Should Dictate Your Backup Methods
The series of dire scenarios I just listed is not a sequence of events, nor is it a calamity of errors.
They all result from one bad tape and how it might affect your recovery goal, based on what you
chose for your tape rotation.
One of the foundational messages you should take away from this book is that you should be choos-
ing your backup methods and evaluating the product offerings within that category, based on how
or what you want to recover.
This is not how most people work today. Most people protect their data using the best way that they
know about or can believe that they can afford, and their backup method dictates their recovery
scenarios.
Disk vs. Tape
The decision to protect data using disk rather than tape is another of the quintessential debates
that has been around for as long as both choices have been viable. But we should not start the dis-
cussion by asking whether you should use disk or tape. As in the previous examples, the decision
should be based on the question, “What is your recovery goal?”
More specifically, ask some questions like these:
Will I usually restore selected data objects or complete servers?
•u
How frequently will I need to restore data?•u
How old is the data that I’m typically restoring?•u
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