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https://issues.apache.org/jira/browse/CASSANDRA-8413?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benedict updated CASSANDRA-8413:
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Attachment: 8413.hack.txt
> 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
>
> Attachments: 8413.hack.txt
>
>
> 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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