Can you outline a good backup tape rotation strategy?
A good tape rotation strategy is largely dependent on the type of overall backup strategy in place and if disk is being used. In general, you want to avoid writing to the same piece of media every day and you want to move a set of tapes off-site at least once per week. Optimally, a tape should go off-site once a day.
The terms used within backup applications can have multiple meanings depending on the vendor. For our purposes, we are talking about a backup strategy where a backup of all data (a "full" backup) is executed periodically, typically once per week, and then a smaller backup of the data that has changed since the previous backup is performed on a daily basis (a differential or an incremental backup).
A differential backup is conducted each day and refers to backing up all data that has changed since the most recent full backup. This means that data will be backed up multiple times, even if the data has only changed once since the prior full backup.
Incremental backups only back up the data that has changed since the previous backup -- whether that backup was a full or an incremental backup. This means that a file that only changed once will only be backed up once until the next full backup is executed.
Understanding these differences is important for establishing a good tape rotation strategy. If you are performing differential backups, it means that data, no matter how active, is automatically stored on more than one piece of media. Therefore, you have multiple points of capture in case you need to restore a file that changed since the last full backup. It also means you may consume more tape media and need to move more cartridges off-site each day.
An incremental backup may mean that a file may only be protected once, and that is your only capture point until the full backup is performed.
Disk eases these concerns since most disk appliances that integrate with tape or software -- and can use both types of backup destinations -- will typically copy data to tape as soon as possible, but also hold a copy of that data on disk for as long as possible.
Effectively, you have two copies of the data at a minimum to restore from. For this reason, I am a big proponent of leveraging both disk and tape in the backup process. It not only gives you a second point of recovery, it also can reduce long-term disk backup appliance costs.
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