Tuning full-scan I/O in Oracle

To properly tune the Oracle I/O subsystem we must carefully distinguish between Oracle's perception of disk latency and the reality.  The back-end disk devices are "black boxes" and the only "real" information we have about disk latency are the salient metrics collected by Oracle.  The Oracle metrics can easily be skewed by the back-end devices but Oracle only knows the end-to-end latencies:

 

- physical write waits, physical read waits

- "db file scattered reads" waits

- "db file sequential reads" waits

 

Traditionally, multiblock disk I/O operations (e.g. table access full, index fast full scan, index range scans) were considered faster because the access to physically contiguous blocks required only a single seek delay, the largest component of platter latency. 

 

For scan operations (index range scans, index fast full scans, and full-table scans), a read-ahead cache can be very useful for speeding up these "scattered read" operations.

Click below to learn how to properly configure your disk I/O subsystem to maximize multi-block read performance:

http://www.dba-oracle.com/t_disk_access_full_scan_scattered_reads.htm

For the whole story, I recommend my book "Oracle Tuning: The Definitive Reference".  You can get it directly from the publisher below and save 30%:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

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