Optimization of File Distribution (SAP Library - SAP Database Guide: Oracle (BC-DB-ORA-DBA))

Optimization of File Distribution

Use

This section describes how BRBACKUP optimizes file distribution during an Oracle backup. This is especially relevant to backup of large Oracle databases.

Features

When distributing the files to the tapes, BRBACKUP has different optimization targets:

All files from a disk saved to the same tape volume

Therefore, try to avoid competing disk accesses, and keep the number of read/write accesses to a minimum.

If a Logical Volume Manager (LVM) is used, this target can only be attained if the logical volumes are not scattered over several physical hard disks. For more information, see Optimization with a Logical Volume Manager.

Time-based optimization used

Using the backup times for individual files stored in the database, BRBACKUP attempts to minimize the total backup time by keeping the backup time equal on each of the individual tape units. Note the following factors relevant to time-based optimization:

It helps to avoid the following problem. When using tape units with hardware compression, the backup time cannot be estimated accurately from the file size or the size of the compressed file. Therefore, the optimization is performed from the backup times stored by BRBACKUP.

The tape capacity in general is not used to its maximum due to time-based optimization. That is, if the optimal backup time is reached for a tape, no further files are saved to this tape, though the tape still has plenty of space left. Therefore, the total capacity of all available tapes inserted at the same time must significantly exceed the total size of the files to be saved.
The time-based optimization is cancelled internally if it would cause a tape change, since BRBACKUP always attempts to avoid volume changes. In this case, the entire capacity of the tapes is used. As a result, the backup time can vary for individual volumes.

All mounted tapes used

To minimize the total backup time, BRBACKUP always attempts to use all mounted tape units, even if the backup would fit onto a smaller number of tapes.

The one exception is when the number of files to be backed up is less than the number of tape devices.

Files sorted and distributed by size

BRBACKUP sorts the files to be saved according to their size and distributes the largest files to the available tapes first, followed by the smaller ones. As a result, wastage is reduced on individual volumes, because the tape capacity can never be exceeded and the total tape capacity can be better utilized. For more information, see the tape_size parameter.

Previous backup times used

BRBACKUP is capable of "learning." It stores the backup times of individual files in the database and uses these in the next backup for time-based optimization (if this is performed).

As a result, the changes to backup time are taken into account for individual files. The backup time can change, for example, if the fill level of the individual database files varies and the files are saved with hardware compression, using a tape unit.

Parameter -o|-output with options dist, time used

BRBACKUP or BRARCHIVE use these to:

Display the file distribution performed by BRBACKUP before starting the actual backup
Display the backup times of the individual files after the backup is completed

For more information, see -o|-output (BRBACKUP) and -o|-output (BRARCHIVE).