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Oracle Performance Tuning and Optimization
(Publisher: Macmillan Computer Publishing)
Author(s): Edward Whalen
ISBN: 067230886x
Publication Date: 04/01/96

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Use of Temporary Backup Space

Because access to the tablespace is deferred to the SGA for updates during the backup, it is essential that you finish the backup as soon as possible to get the table back to its normal state.

If you use a tape drive for the backup media, you may be constrained by the speed of the tape drive. If you use a network to back up the data to a network backup server, you may be constrained by the speed of the network. In both of these cases, you can minimize the time that the tablespace access must be deferred by backing up to a disk drive or set of disk drives reserved for this purpose. Even though you may not be able to get purely sequential I/Os out of the data files, you will be able to take advantage of sequential access to this temporary space.

Once the data has been written to this temporary space, the tablespace can be brought back online and normal operations resumed. At this point, you can back up the data from the temporary space to the tape or network—or even compress the data—without significantly affecting performance to that tablespace.

Once the data has been transferred off the RDBMS system, you can back up the next tablespace. In this way, you minimize the time that database operations are affected by the backup. Of course, some extra I/Os are associated with the backup files, but these are isolated to a separate disk volume. Compressing the data while it is still on the RDBMS system can significantly affect performance by taking large amounts of CPU time.

Isolate the Network

When performing backups across the network to another server or a dedicated backup server, it is important to keep in mind the large amount of network bandwidth that can be consumed by this backup. Because large amounts of data are being transferred with large block sizes, the load can be significant.

By isolating the backup to its own network segment, you can not only increase the performance of the backup itself but also reduce the effect your backup has on other users on the network. Confining the backup to its own network segment isolates the effect of the backup to the segment used for the backup.

If multiple servers use the same network segment for backups, simple scheduling can reduce the load by spreading out the backups so that each system backs up at different times. If you need very fast backups to multiple systems, you may have to segment the network further.

Hardware/Software Considerations

When selecting hardware and software for your backup tasks, try to get the best-performing hardware currently available. Because backup is such a critical task, it is worth the extra expense to get the very latest hardware and software.

If you can perform only hot, or online, backups, you may want to invest in some extra disks to off-load the data to temporary storage so that you can get the data files back online as soon as possible. If you perform your backups over a network, you should get fast network hardware such as 100Base-T or fiber-optics network hardware.

Your backup media (such as tape or optical storage) should perform at rate necessary to do the backup in the specified amount of time. Upgrading your backup hardware to a faster tape device is always a good investment because both the speed and quality of your backup is essential to your system.


TIP:  Be sure to periodically test your backups to make sure that they function properly. The time of a failure is the wrong time to realize that your backups have been failing.
Review of Design Considerations

By carefully designing a backup plan that isolates sequential I/Os on a per-disk-volume basis, you can greatly enhance the performance of the cold backup. With hot backups, it is more difficult to isolate the I/Os because read activity still occurs on the data files during backup and because other data files may share the same disk volume.

By taking advantage of fast backup media (such as fast tape devices, optical storage, and fast network components), you can enhance the performance of the hot backup as well. If you are innovative and use a temporary disk storage device, you can reduce the time the tablespace is offline and bring the system back to its full performance level as soon as possible.

Tuning Considerations

When tuning for backups, there are no particular Oracle parameters that can help you because Oracle allows you to use your OS or third-party backup utility to perform the backup itself. But there are a few things you can do to improve the performance of the backup program itself.

Test various block sizes. Don’t stop with the default block size that the backup program uses. By increasing the block size used both to read the data from disk and to write the data to tape, you may find enhanced performance. Increasing the value too high may degrade performance.

The optimal block size for backups varies based on your operating system, disk hardware, tape hardware, and OS. A good starting point is a 64K block size. By varying the block size and timing the backups, you should be able to determine the kind of effect different block sizes have on your system.

If the time it takes to perform a backup is critical, and you are not particularly concerned about the overall effect on the system, you can increase the priority of the backup process. Doing so ensures that the process always runs when it is ready; in some OSes, increasing the backup’s priority ensures that the job is never preempted. Although this may help reduce the time it takes to perform the backup, I do not recommend adjusting process priorities. Doing so can sometimes degrade performance.

System Enhancements To Improve Backup Performance

You can enhance the system by adding faster backup devices such as faster tape devices, faster network hardware, or faster software. By using the fastest storage medium, you can significantly reduce the time it takes to perform the backup.

Don’t assume that all backup software performs the same. The way the software reads the data files—as well as the algorithms it uses for compression—can greatly affect performance. By choosing a backup software package that performs well and has all the features you need, you can optimize your backup performance.


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