Add columnfamily option optimize_filters_for_hits to optimize for key hits only
Summary: Summary: Added a new option to ColumnFamllyOptions - optimize_filters_for_hits. This option can be used in the case where most accesses to the store are key hits and we dont need to optimize performance for key misses. This is useful when you have a very large database and most of your lookups succeed. The option allows the store to not store and use filters in the last level (the largest level which contains data). These filters can take a large amount of space for large databases (in memory and on-disk). For the last level, these filters are only useful for key misses and not for key hits. If we are not optimizing for key misses, we can choose to not store these filters for that level. This option is only provided for BlockBasedTable. We skip the filters when we are compacting Test Plan: 1. Modified db_test toalso run tests with an additonal option (skip_filters_on_last_level) 2. Added another unit test to db_test which specifically tests that filters are being skipped Reviewers: rven, igor, sdong Reviewed By: sdong Subscribers: lgalanis, yoshinorim, MarkCallaghan, rven, dhruba, leveldb Differential Revision: https://reviews.facebook.net/D33717
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