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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
http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm |
