Tape Rotation Schemes For Disaster Recovery

The most basic type of disaster recovery is to copy all of your data to magnetic media and store it in an offsite storage location. In the event of a disaster to the server or the entire building the data will be secure in an offsite location. The data can then be reloaded and business restarted.

The two most important considerations when devising a disaster recovery plan is security and cost. Magnetic tapes are the most common option because they have a large capacity. In addition, they are not-volatile which means they have a very long storage life. However, to get the life you expect out of magnetic tapes they should be stored in a media vault that controls temperature and moisture.

The most basic tape rotation scheme is to backup your data every day for permanent offsite storage. This would work but requires a tremendous amount of tapes and storage space. An incremental step is to only take incremental backups. This only backs up the changes that occur between the last full backup. This will reduce the number of tapes for any backup and reduces the wear on the tapes.

Now since you don't want -- or need -- every backup, you can re-use tapes from prior periods. This is commonly known as tape rotation. Don't forget that tapes will wear out and need to be replaced periodically. You have reduced the number of tapes but there are more efficient and secure rotation schemes. Here are three common tape rotation strategies:

Grandfather – Father – Son (GFS)

This is the most widely used scheme for tape rotation. It combines security with ease of implementation. It's simplicity comes from running on a traditional calendar. On the last day of every month a full backup is performed and labeled ‘grandfather'. This tape is stored permanently offsite. On the last day of every week a full backup is done called the ‘father' and stored offsite. Then daily an incremental backup is done called the ‘son'. Son tapes can be stored onsite or offsite depending on the volume of data changes. Adding up the tapes you will have: 4 son tapes (assuming a five day work week), 3 father tapes and a new grandfather tape every month.

Six Tape

This scheme is the easier to implement but lacks the redundancy of a GFS tape rotation scheme. It is best used by small business with limited data needs. Five tapes are labeled for each day of the week, assuming a five day work week. The sixth tape is also labeled Friday. A full backup is taken each Friday and an incremental on Monday through Thursday. The Friday tapes are rotated and stored offsite.

Tower of Hanoi

This is the most secure tape rotation scheme but also the most complex one. This scheme requires five tapes labeled A, B, C, D, and E. The A tapes are used for a full backup every other day. The B tapes are used for a full backup every 4th day. The C tapes are used for a full backup every 8 th day. The D and E tapes are alternated for a full backup every 16 th day. Or said another way, tape A is used every odd numbered day. Tape B is used every other even numbered day. Tape C is used every third numbered day. Tape D and E are used every fourth even numbered day. The B, C, D and E tapes are stored offsite. Confusion can arise because it doesn't operate on a traditional work week calendar. If you implement this scheme it is wise to have automated reminders to help everyone keep track of what tape is to be used each day.

The most important thing for any tape rotation system is to get started. A poorly designed one is far better than being left with no backup at all. Keep at least one tape stored in a secure offsite location. Make sure the backup tapes are accessible at all times. It is also wise to test your backups at least quarterly. What is critical data and where it is located can change and leave you missing valuable information when you need it most. Do a full reload on a test system to see if you can withstand a disaster to your systems.