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From "Robert Stupp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8413) Bloom filter false positive ratio is not honoured
Date Sun, 05 Jul 2015 15:17:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8413?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14614284#comment-14614284
] 

Robert Stupp commented on CASSANDRA-8413:
-----------------------------------------

Thanks.
Committed as 23fd75f27c40462636f09920719b5dcbef5b8f36 with a few sentences in NEWS.txt regarding
the new SSTable file version.

> 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
>            Assignee: Robert Stupp
>             Fix For: 3.0 beta 1
>
>         Attachments: 8413-patch.txt, 8413.hack-3.0.txt, 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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