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  Oracle Tips by Burleson

Oracle and Bandwidth Saturation

A sub-optimal disk configuration can make even a well-tuned Oracle RAC database slow to a crawl. An Oracle clustered environment has multiple database instances all sharing the same set of data files. MetaLink Note: 62172.1 notes that the top remedy for increasing DBWR throughput is increasing the speed of the device.

This example from an AWR report from $ORACLE_HOME/rdbms/admin/awrrpt.sql shows an I/O bound database:

Top 5 Timed Events
~~~~~~~~~~~~~~~~~~ % Total
Event Waits Time (s) Ela Time
-------------------------- --------- ----------- --------
db file sequential read 2,598 7,146 48.54
db file scattered read 25,519 3,246 22.04
library cache load lock 673 1,363 9.26
CPU time 1,154 7.83
log file parallel write 19,157 837 5.68

Since most DBAs are too busy to run disk performance scripts on every node of a large RAC cluster, they can look to the web for examples of scripts. Another source is Mike Ault’s book Oracle Disk I/O Tuning (2004, Rampant TechPress), which also provides a plethora of scripts and examples.

For the vast majority of non-scientific systems, the primary bottleneck is disk I/O. Back in the days before RAID and giant db_cache_size, the DBA had to manually load balance the disk I/O sub-system to relieve contention on the disks and the disk controllers. The next section explores this topic.

The above book excerpt is from:

Oracle RAC & Grid Tuning with Solid State Disk
Expert Secrets for High Performance Clustered Grid Computing

ISBN: 0-9761573-5-7
Mike Ault, Donald K. Burleson  

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