How Backup Levels Work

Backup levels work in conjunction with a client's backup schedule. The way you define the backup levels directly affects how long the recovery from a disk crash takes and how many backup volumes you need.

The following paragraphs, accompanied by graphics, illustrate the concept of how Backup backup levels work and the data requirements for recovery in the event of data loss.

On October 2, a full backup runs. On October 3, the incremental backup saves everything that changed since the full backup. On October 4, the incremental backup backs up everything that changed since the 3rd. On October 5, the level 7 backup backs up everything that changed since the full backup. To fully recover from a disk crash on October 5, you need only two volumes: the full volume and the level 7 volume. You no longer need the data on the volumes from October 3 and 4, because the level 7 volume includes that information. (See Figure 3-5.)

Figure 3-5 Example: Backups for October 2 through October 8

On October 6, 7, and 8, the incremental backup backs up everything that has changed since the level 7 backup. On October 9, as shown in Figure 3-6, the level 5 backup backs up everything that changed since the full backup. To fully recover from a disk crash on October 9, you need only two volumes: the full volume and the level 5 volume. You no longer need the data on the volume from the level 7 backup or the subsequent incremental backups because the level 5 volume includes that information.

Figure 3-6 Example: Backups for October 2 through October 15

On October 12, the level 7 backup backs up all the data that changed since the last lower numbered backup, in this case the level 5 backup from October 9. To recover from a disk crash on October 12, you need three volumes: the full volume, the level 5 volume, and the new level 7 volume. (See Figure 3-6.)

On October 16, the level 5 backup backs up all the data that changed since the last lower numbered backup. Because no lower numbered level backup has been performed (for example, levels 1 - 4), the level 5 backup backs up all the data that changed since the full backup. To recover from a disk crash on October 16, you need two volumes: the full volume and the new level 5 volume. (See Figure 3-7.)

Figure 3-7 Example: Backups for October 2 through October 16

Level 1-9 backups help you maintain control over the number of volumes you use. A carefully thought-out backup strategy enables you to recover everything to disk with a minimum number of volumes. The fewer volumes you need to recover from a disk crash, the less time you must spend restoring the disk.

You can also control the size and time it takes to back up your data by using directives, which compress and eliminate unnecessary data from your backups. For example, you can use a directive that tells Backup to skip certain files or filesystems when performing a backup. For more information on directives, see "What Are Directives? ".