Alex Turner <armtuk@gmail.com> writes:
> Infact the cache hit ratio that Oracle suggests is the minimum good
> value is 95%. Anything below that is bad news.
Well that seems very workload dependent. No amount of cache is going to be
able to achieve that for a DSS system chugging sequentially through terabytes
of data. Whereas for OLTP systems I would wouldn't be surprised to see upwards
of 99% hit rate.
Note that a high cache hit rate can also be a sign of a problem. After all, it
means the same data is being accessed repeatedly which implicitly means
something is being done inefficiently. For an SQL database it could mean the
query plans are suboptimal.
On several occasions we found Oracle behaving poorly despite excellent cache
hit rates because it was doing a sequential scan of a moderately sized table
instead of an index lookup. The table was small enough to fit in RAM but large
enough to consume a significant amount of cpu, especially with the query being
run thousands of times per minute.
--
greg