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  Oracle Tips by Burleson

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 at work 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 by Oracle to maintain read consistency and data concurrency.

If Oracle logical I/O is expensive, can this expense be avoided when reading directly from disk?

The answer is important to the information presented about the most appropriate placement for SSD in an Oracle environment.

Super large disks are an issue to be noted. With 144 gigabyte disks becoming commonplace, I/O intensive database 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.

If one accepts that LIO expense is going to happen regardless of whether or not a PIO is performed, valuable insight into the proper placement for SSD in an Oracle environment is achieved.

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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