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Implementing database tuning checks (2.1.29/db18.104.22.168/bdb)
I have implemented parts of the tuning recommendations in
http://www.openldap.org/faq/data/cache/191.html using a script to report
the suggested minimum cache size.
Hoever, the section described by this paragraph has been giving me
"Unlike the B-trees, where you only need to touch one data page to find an
entry of interest, doing an index lookup generally touches multiple keys,
and the point of a hash structure is that the keys are evenly distributed
across the data space. That means there's no convenient compact subset of
the database that you can keep in the cache to insure quick operation, you
can pretty much expect references to be scattered across the whole thing.
My strategy here would be to provide enough cache for at least 50% of all
of the hash data. (Number of hash buckets + number of overflow pages +
number of duplicate pages) * page size / 2."
How does one determine (ie which options to db_stat and which resulting
value) the number of hash buckets, overflow pages and duplicate pages for
an index file?
BTW, I did some simple benchmarks a while back, and performance
differences with cache size settings varying from 256kB up to 50MB on a db
with ~250MB total in database files were marginal. Do others really see
huge differences (objective ones).