Donald K. Burleson
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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Burleson. All rights reserved.
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