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Donald K. Burleson

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Why is Oracle Logical I/O so Slow?

Disk latency is generally measured in milliseconds while RAM access is expressed in nanoseconds.  In theory, RAM is four orders of magnitude, 10,000 times, faster than disk; however, this is not true when using Oracle.  In practice, logical I/O is seldom more than 1,000 times faster than disk I/O.  Most Oracle experts say that logical disk I/O is only 15 times to 100 times faster than a physical disk I/O.


Oracle has internal data protection mechanisms that cause a RAM data block access, a consistent get, to be far slower due to internal locks and latch serialization mechanisms.  This overhead is required in order to maintain read consistency and data concurrency.


If Oracle logical I/O is expensive, can this expense be avoided by reading directly from disk?  The answer to this question is important to the discussion about the most appropriate placement for SSD in an Oracle environment.


There are also issues associated with super large disks.  With 144 gigabyte disks becoming commonplace, I/O intensive databases will often see disk latency because many tasks are competing to read blocks on different parts of the super large disk. 


An Oracle physical read must read the disk data block and then transfer it into the Oracle RAM buffer before the data is passed to the requesting program as shown in Figure 14.10.


Figure 14.10 - Physical reads include logical I/O latency


So if the DBA accepts that LIO expense is going to happen regardless of whether or not a PIO is performed, valuable insight can be gained into the proper placement for SSD in an Oracle environment:


In the next section, information will be presented on the KEEP and RECYCLE data buffers and how objects are selected for inclusion.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:




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