Because backups are normally made on a per-RDAREA basis, the backup includes pages that have been updated as well as pages that have not been updated since the last backup. By using the differential backup facility, only the pages that have been updated since the last time a backup was made are backed up. Making a backup of only the differences since the last time a backup was made allows you to significantly reduce the time required to make a backup. Use of the differential backup facility should be considered in situations where the database is large and the amount of data updating is relatively small. Figure 7-5 provides an overview of the differential backup facility.

Figure 7-5 Overview of the differential backup facility

Explanation

1. On Sunday, backups are made of RDAREAs 1 through 3. At this time, a backup is made of the used pages in RDAREAs 1 through 3. This kind of backup is called a full backup, and the grouped RDAREAs are called a differential backup group.
2. Updates are performed with Monday's work.
3. At the end of Monday's work day, a backup is made of RDAREAs 1 through 3. This time, a backup is made only of the pages in RDAREAs 1 through 3 that were updated. This kind of backup is called a differential backup.
4. Updates are performed with Tuesday's work.
5. At the end of Tuesday's work day, a backup is made of RDAREAs 1 through 3. At this time, a backup is made only of the pages in RDAREAs 1 through 3 that were updated.

Database recovery procedure

To recover a database up to the point the differential backup was made on Tuesday, as the input information for the database recovery utility, you use the full backup made on Sunday, the differential backup made on Monday, and the differential backup made on Tuesday. For details about how to operate the differential backup facility, see the HiRDB Version 8 System Operation Guide.

Note

A differential backup cannot be made of a LOB RDAREA.

Reference note

The differential backup facility can also be used during operation without unloading the system log.