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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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These recommendations should help you design a reasonable backup schedule that will allow you to segment your backups and at the same time achieve reasonable recovery times. Of course, your system may have its own special requirements that must be addressed.

Review of System Enhancements

By carefully examining the characteristics of the backup system, you can enhance the performance in several ways:

  Additional CPU power can increase the performance of compression utilities, reducing the amount of data that must be written to tape or network.
  I/O enhancements, such as disk array, can help performance for both hot and cold backups.
  Network enhancements, such as a high-speed link, can reduce the time it takes to transfer the data between machines.
  Splitting up backups across several days’ off-peak hours can reduce the overall effect of the backup on users while maintaining a reasonable recovery time.
  Other innovative enhancements such as hardware compression devices can be used to improve backup performance.

By taking advantage of enhancements to your system, you can improve the performance of your backup and thus reduce the time the backup takes. In the case of a cold backup, this reduces the time the RDBMS is offline.

Performance Verification

Once you design and build your system, you must have some way of verifying performance. If you build your system for optimal backup/recovery performance, this task is quite simple.

The simplest method of verifying performance is to test the system using the backup process as the standard. It is not necessary to save a test database to test backup performance; simply measure the time it takes to perform your daily backups. Keep in mind that as the amount of data in your database grows, the time it takes to back up those tables also increases.

By periodically documenting the backup performance of the system and trying new parameters such as various block sizes, you may be able to fine-tune your backup process. I don’t recommend that you change any parameters on a production system that might put the backup at risk, but values such as block size incur no risk. The following sections look at what should be tested in the RDBMS and the operating system.

What To Test in the RDBMS

Whether you perform hot or cold backups, there is nothing in the RDBMS itself you can tune to increase the performance of the backup. However, this is not the case if you are using the export utility to back up the system.

The export process does use the RDBMS to perform its function. The function of the export utility is to read data from the database and output that data to an OS file. An export does not require much tuning except in the case of long rows (in this case, you may want to increase the BUFFER parameter).

By increasing the size of the BUFFER parameter, you may see some performance improvement. Perform the same export using different values for the BUFFER parameter and note any differences in performance.

What To Test in the OS

You can perform various tests in the OS to verify and increase the performance of the backup process:

  Data disk read performance. By testing the process of reading from the data disk drives, you can determine an optimal block size. Many disk drives and disk array controllers have a certain block size that is optimal.
  Tape write performance. By writing data to the tape drive using various methods and block sizes, you can determine the optimal configuration.
  Network configuration. It may be possible to tune the network so that larger network packets can be used to cut down on the number of packets that must be sent.
  Optimized backup software. By using the most efficient OS facility or third-party utility, you can increase performance of the backup.
  Optimized compression techniques. Using a modern, high-performance compression algorithm can improve performance and efficiency.

An improvement in any of these areas can add an incremental performance improvement to your backup process. The following sections examine some of these areas in more detail.

Disk Block Size

Many disk drives and disk array controllers have a certain block size that is optimal. By reading the data file and discarding the data, you may be able to find the optimal block size for your particular configuration. With some disk array controllers, having a sufficiently large disk request allows multiple disks to service that request simultaneously.

During a backup operation, very large block sizes (such as 64K) are not unreasonable. With some devices, a block size even larger than this might be acceptable. When the block size gets too large, performance begins to degrade because internal OS buffers have been overflowed or the request is split up by the controller’s device driver into smaller pieces, causing additional overhead. By testing various block sizes, you can determine what is optimal for your specific configuration.

Tape Block Size

Different types of tape drives have different characteristics. In determining the proper block size for your tape drive, the goal is to keep the tape streaming. When the tape drive is streaming, it is continually moving and writing data to the tape. If the tape drive is not fed data quickly enough, the tape stops, rewinds a short amount, and starts up again when more data arrives.

Even though data may be arriving at the tape device at a certain rate, the rate at which it is written to tape may be degraded because the tape is stopping and starting. This phenomenon is known a “shoe-shining” or “rocking” because the tape must move back and forth over the tape head for the data to be written contiguously to the tape.

By feeding data to the tape at a fast enough rate to stream the tape, you can see significant improvements in performance. Any time the tape has to stop and back up, performance is degraded. Providing a large enough block size and buffering data is important in helping improve tape performance.

Try various block sizes when the tape is streaming to see what is optimal for your particular hardware. If the tape is not streaming, this test is not really valid because performance is severely degraded by the shoe-shining effect.


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