An incremental backup backs up only the data that has changed since the last full or incremental backup. Incremental backups might take less time and media to back up the database; the amount of time you can save depends on the amount of data that has changed and the speed of your backup device. You must use PROBKUP to perform an incremental backup.
In an OpenEdge database, the master block and every database block contains a backup counter. The counter in the master block is incremented each time the database is backed up (online or offline, full or incremental). When a database block is modified, PROBKUP copies the backup counter in the master block to the backup counter in the modified database block. When you perform an incremental backup, PROBKUP backs up every database block where the counter is greater than or equal to the master block counter. The following figure illustrates how the size of an incremental backup varies, depending on the amount of changes to the database between incremental backups.
Figure 2. Incremental backup—size varies
The Overlap qualifier (-io) specifies the amount of redundancy in an incremental backup. When you specify Overlap (-io), PROBKUP backs up every database block where the counter is greater than or equal to the master block counter, less the overlap value. The following figure illustrates how the Overlap qualifier determines the data backed up during an incremental backup.
You must perform a full backup of a database before you can perform the first incremental backup. You should also perform full backups regularly in conjunction with incremental backups. If you do not perform full backups regularly, you will use increasing amounts of backup media for incremental backups and increase recovery time restoring multiple incremental backups in addition to the full backup.
