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From "Branimir Lambov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-12922) Bloom filter miss counts are not measured correctly
Date Tue, 22 Nov 2016 06:44:58 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-12922?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15685877#comment-15685877
] 

Branimir Lambov commented on CASSANDRA-12922:
---------------------------------------------

The fix is ok, but we also need a test. Something like [{{SSTableReaderTest.testGetPositionsKeyCacheStats}}|https://github.com/mm-binary/cassandra/blob/845daa181f2a48a1c5c799266ac1205e70c5f351/test/unit/org/apache/cassandra/io/sstable/SSTableReaderTest.java#L294],
using a small bloom filter or {{AlwaysPresentFilter}} and counting the false positives.

> Bloom filter miss counts are not measured correctly
> ---------------------------------------------------
>
>                 Key: CASSANDRA-12922
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12922
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Branimir Lambov
>            Assignee: Mahdi Mohammadi
>
> Bloom filter hits and misses are evaluated incorrectly in {{BigTableReader.getPosition}}:
we properly record hits, but not misses. In particular, if we don't find a match for a key
in the index, which is where almost all non-matches will be rejected, [we don't record a bloom
filter false positive|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/io/sstable/format/big/BigTableReader.java#L228].
> This leads to very misleading output from e.g. {{nodetool tablestats}}.



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