Backup Methods and Rotation Schemes

Here are the most common backup methods:

• Full includes files whether they have been changed or not;
• Differential includes all files changed since the last full backup, whether they have been changed since the last backup operation or not;
• Incremental includes only those files that have changed since the last backup operation of any kind.

To choose a method, you must first weigh three factors: the capacity of your tape format, the period of time or window available for your backup, and the level of urgency experienced by those on your network when a file restoration is necessary. For example, conducting full backups on a daily basis will require both large amounts of tape and a long period of time. But doing so will facilitate rapid and easy restoration because you will need only one tape to pull data from. On the other hand, weekly full backups combined with daily incremental backups will conserve tape and shorten the daily backup period, though data recovery would require the last full backup and each subsequent incremental backup up to the most current—a process that can seem to take forever when there are needy users waiting for a file.

After you select a backup method, you need to pick the most appropriate rotation scheme for your organization and network needs, which can reduce media costs and extend the longevity of your tapes while ensuring that every file is protected. Here are the two most common ones that provide the best compromise between backup window and number of tapes.

Grandfather-father-son (GFS) is probably the most common rotation scheme. The grandfather backup is essentially a monthly full backup that is stored off-site, the father is a weekly full backup that is kept on-site (eventually moved off-site or recycled, depending on your organizations needs), and the son is a daily incremental backup that is kept on-site (possibly moved off-site along with its accompanying father or recycled). Its probably a good idea to preserve grandfather tapes for a full year and fathers and sons for a month before reuse. This type of scheme requires 20 tapes for a single year.

The Tower of Hanoi scheme is a common alternative to GFS that is secure and cost-effective but more complex. This method requires you to perform a full backup on five tapes labeled A, B, C, D, and E. Tape A is used every other backup session, tape B every 4 sessions, tape C every 8 sessions, tape D every 16 sessions, and tape E every 32 sessions, or once a month. This allows for easy file restorations, because you dont have to shuffle through partial backups, and it is more cost-effective than GFS because it uses fewer tapes. The Tower of Hanoi methods chief disadvantages are the need for a large enough backup window to accommodate daily full backups and its complexity, which means you should make sure your backup software can automate tape rotation.