That is generic guidance. Specific guidance is much better.
The big questions you need to have an answer to before you start setting up your backup retention schedule is:
How much data am I willing to lose, and how long am I willing to take to recover what I can?
Tape backup is near the bottom of the backup/disaster-recovery hierarchy. Very roughly, that is (and I'm sure I'll forget a few steps):
- RAID (data-loss prevention)
- Traditional data backup
- Multi-site data backup
- Data replication
- Cold fail-over services
- Hot fail-over services
- Load-balanced replicated services
- Multi-site replication
- Multi-site cold failover services
- Multi-site hot failover services
- Multi-site load-balanced replicated services
We're talking about steps 2 and 3 here. How fast you want your data back depends on several factors:
- How much of it you have
- How many backup sets you have to go through to get it all back
- What those backup sets are stored on
- How fast the hardware supporting all of this (both servers, network, and backup hardware) can run
- Whether or not the backup system can do a 'differential' backup, or is it just Full/Incremental
In case you hadn't run into the term before a Differential backup is defined as "everything that has changed since the last full backup". I think the term originated with BackupExec and has since been adopted elsewhere. But I digress.
In the book's backup scheme, one full a month, net-change daily the rest of it, the worst-case disaster recovery scenario is a data-loss event the day before the full backup is taken. Recovering in that case will require:
- The last Full backup, 29 days ago
- Every single tape since then, all 28 of them.
Depending on the aforementioned variables, this could take a really long time to recover.
Take an alternate scenario, Full on Friday, net-change the other 6 days. The worst-case recovery here is a loss event Friday afternoon. Recovering in that case will need:
- Last Friday's tape
- The other 6 tapes
This should take a lot less time.
One thing that hasn't been covered is what happens when a backup tape is bad. With the 30-day between fulls scenario, a bad tape can cost you anywhere from 1 to 59 days of data-loss. If that's unacceptable, run your full backups more often.
One thing some backup to disk vendors are selling these days is something called a synthetic full backup. How it works is that you take an initial full backup and then do net-change forever more. On a set schedule you do a synthetic full backup which coalesces a week/two-weeks/months worth of net-change with the last full backup to come up with a virtual full backup. This is handy for staying within backup windows.
When doing a hybrid disk/tape system, you do your weekly/monthly backups to disk, and then spool archive sets off to tape to sit on a shelf for 3/5/7/10 years. When used in combination with something that can do a synthetic full, a synthetic full can be spun to tape and sent off-site on a regular schedule. Hybrid systems offer the most flexibility these days, and I recommend them whenever possible. Disk for short-term, tape for long term.