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Donald K. Burleson

Oracle Tips  

Oracle Rollback Segments

Most systems are not using the parallel instance option of Oracle (called real application clusters, or RAC, in Oracle.) Because of this, private, rather than public, rollback segments should be used. This will allow a single rollback segment to be taken offline for consolidation. If you will have large transactions, such as batch updates with numerous updates and adds between commits, a second INIT.ORA file should be created, which brings online a single large rollback segment to be used during batch operations. These limitations may be reduced or mitigated by using the new undo tablespaces in Oracle.

Oracle disk Raw Devices 

In Unix and NT/Windows systems there are several types of disk formats used. Generally, the highest performance comes from programs that can directly access the disk. In order to be directly accessed a disk must be configured in what as known as raw format meaning no OS buffering or access control is used.

While raw disks provide performance gains over many traditional disk formats they have several limitations that make their use difficult. An example is that only one file may be placed in a raw disk partition at one time, another is that raw disk partitions may require special backup commands. Finally, raw devices can be easily overwritten if the system administrator is not careful.
If you have tuned your application, I/O, and all applicable SGA parameters and still cannot get the performance you want on UNIX or NT, then consider using raw devices. Oracle is capable of reading and writing directly to raw devices. This can increase Oracle performance for disk I/O by over 50 percent and ensures that data integrity is maintained. But when raw devices are used, Oracle datafile names are restricted to a specified syntax. Another limitation is that the entire raw partition has to be used for only one file, which can lead to wasted disk space unless the areas are carefully planned. This will require the DBA to keep an accurate map of which devices belong to which tablespaces, log files, and so on.

This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.




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