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SSD Benchmark Hypothesis

Now that the issues surrounding SSD in an Oracle environment have been covered, one can form a hypothesis about the results of the benchmark tests. 

The central question of this study is to determine the benefit of SSD for Oracle under these conditions:

• No SSD with a super large Oracle data buffer: This can minimize PIO with only one physical read into the data buffer, but LIO overhead will be revealed.

• SSD with a tiny Oracle data buffer: This makes PIO faster but will have repeated LIO overhead as data blocks are aged out of the Oracle data buffer.  The LIO overhead would be expected to be huge, with high latch contention, buffer busy waits , and free buffer waits.

• SSD for files with large Oracle Data buffer cache: This is duplicating RAM, but it would relieve the LIO issue.

It is now possible to postulate about the benchmark results. If it is assumed that SSD is far faster for PIO, the issue of duplicitous Oracle activity remains.  At PIO time, the data block must still be read into the db_cache_size , and then a consistent get is required to deliver the data block to the requesting program.

RAM resources, therefore, appear to be better allocated directly to the db_cache_sizeinstead of using the RAM with SSD.

The above book excerpt is from:

Oracle RAC & Tuning with Solid State Disk

Expert Secrets for High Performance Clustered Grid Computing

ISBN 0-9761573-5-7

Donald K. Burleson & Mike Ault

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