Benedict created CASSANDRA-8413:
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Summary: Bloom filter false positive ratio is not honoured
Key: CASSANDRA-8413
URL: https://issues.apache.org/jira/browse/CASSANDRA-8413
Project: Cassandra
Issue Type: Bug
Components: Core
Reporter: Benedict
Fix For: 2.0.12, 2.1.3
Whilst thinking about CASSANDRA-7438 and hash bits, I realised we have a
problem with sabotaging our bloom filters when using the murmur3 partitioner. I
have performed a very quick test to confirm this risk is real.
Since a typical cluster uses the same murmur3 hash for partitioning as we do
for bloom filter lookups, and we own a contiguous range, we can guarantee that
the top X bits collide for all keys on the node. This translates into poor
bloom filter distribution. I quickly hacked LongBloomFilterTest to simulate the
problem, and the result in these tests is _up to_ a doubling of the actual
false positive ratio. The actual change will depend on the key distribution,
the number of keys, the false positive ratio, the number of nodes, the token
distribution, etc. But seems to be a real problem for non-vnode clusters of at
least ~128 nodes in size.
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