Managing Backup Windows and Performance: BACKUP... DURATION

A backup window is a period of time during which a backup activity must complete. For example, you may want to restrict your database backup activities to a window of time when user activity on your system is low, such as between 2:00 AM and 6:00 AM.

The BACKUP command supports a DURATION argument which lets you specify how long a given backup job is allowed to run. You could, for example, run the following command at 2:00AM:

BACKUP DURATION 4:00 TABLESPACE users;

RMAN backs up the specified data at the maximum possible speed. If the backup is not complete in four hours, the backup is interrupted. Any completed backupsets are retained and can be used in restore operations, even if the entire backup is not complete. Any incomplete backupsets are discarded.

Controlling RMAN Behavior when Backup Window Ends with PARTIAL

By default, when a BACKUP... DURATION command runs out of time before the backup completes, RMAN reports an error. (The effect of this is that if the command is running in a RUN block, the RUN block terminates.) You can control this behavior by adding the PARTIAL option to the BACKUP... DURATION command, as in this example:

BACKUP DURATION 4:00 PARTIAL TABLESPACE users FILESPERSET 1;

When PARTIAL is used, no error is reported when a backup command is interrupted due to the end of the backup window. Instead, a message showing which files could not be backed will be displayed. If the BACKUP command is part of a RUN block, then the remaining commands in the RUN block will continue to execute.

When using DURATION the least recently backed up files are backed up first. Thus, if you retry a job that was interrupted when the available duration expired, each successive attempt covers more of the files needing backup.

Note also the use of FILESPERSET 1 in this example. With this option, RMAN puts each file into its own backupset. This way, when a backup is interrupted at the end of the backup window, only the backup of the file currently being backed up is lost. All backup sets completed during the window are saved, minimizing the lost work due to the end of the backup window.

Managing Backup Performance with MINIMIZE TIME and MINIMIZE LOAD

When using DURATION you can run the backup with the maximum possible performance, or run as slowly as possible while still finishing within the allotted time, to minimize the performance impact of backup tasks. To maximize performance, use the MINIMIZE TIME option with DURATION, as shown in this example:

BACKUP DURATION 4:00 PARTIAL MINIMIZE TIME DATABASE FILESPERSET 1;

To extend the backup to use the full time available, use the MINIMIZE LOAD option, as in this example:

BACKUP DURATION 4:00 PARTIAL MINIMIZE LOAD DATABASE FILESPERSET 1;

RMAN monitors the progress of the running backup, and periodically estimates how long the backup will take to complete at its present rate. If RMAN estimates that the backup will finish before the end of the backup window, it slows down the rate of backup so that the full available duration will be used. This reduces the overhead on the database associated with the backup.

Note:

Note these issues when using DURATION and MINIMIZE LOAD with a tape backup:

Efficient backup to tape requires tape streaming. If you use MINIMIZE LOAD, RMAN may reduce the rate of backup to the point where tape streaming is not optimal.
RMAN will hold the tape resource for the entire duration of the backup window. This prevents the use of the tape resource for any other purpose during the backup window.

Because of these concerns, it is not recommended that MINIMIZE LOAD be used with tape. See "Factors Affecting Backup Speed to Tape" for more details on efficient tape handling.