Retention of Backups

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Frequency of Backups

Once you understand the general backup requirements for all of the data and the business and legal requirements, you should have a pretty good idea of how much data needs to be backed up and at least a minimum requirement for the frequency. The trick in establishing the ideal frequency policy is to come up with a schedule that gives you adequate protection with minimal media usage. You don't want to back up any more often than needed to get the necessary level of protection, since 'more often' means more tapes, more data being moved, and more administration. When in doubt, however, go with more media.

Establishing the best frequency and retention policy for data that is not covered by business and legal requirements also involves knowing why the data is being backed up and what recovery requirements are. In general, truly static data or static systems should not need very frequent backups. They might be backed up as infrequently as once a week or even once a month. As far as the number of copies required, a normal practice is to keep between two and four copies of the backups.

Data that is more dynamic requires more frequent backups and probably needs one of the incremental types. The decision between differential incremental and cumulative incremental is based on recovery requirements versus media usage. Weekly full backups and daily differential incremental backups could require up to six tapes to restore a directory, filesystem, or database in a worst-case scenario. Each of the incremental backup images might be small, but each day's changes could be on a separate tape, or at least different images on the same tape. If you did the same backup sequence with cumulative incremental backups, no recovery would take more than two images that could reside on two tapes; however, if there were enough changes to the data, the cumulative incremental backups could approach a full backup in size. You must decide whether it is better to use fewer tapes with differential backups but run the risk of having to mount more tapes on a restore or potentially use more tapes on the backups but only have to restore two images to restore an entire backup.

Obviously, another piece of this equation is the anticipated type of recovery activity. If the data is being backed up for DR protection or to protect against hardware failures, the question of differential versus cumulative is important. If the data is being backed up to protect against user deletion or error, you should stay with differential backups. The recovery requirement also comes into play. By using the information you gathered during the interview process with the data owners, you can realistically plan based on the expectations you set during your discovery. If the absolute speed of recovery is important, the use of cumulative incremental backups is desired. The incremental type generally comes into play when you are working with filesystem backups. With databases, you generally work with the application tools that are integrated with the backup application, and this will dictate much of what you do.

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