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From Juho Mäkinen (JIRA) <>
Subject [jira] Created: (CASSANDRA-1627) Better QUORUM calculation algorithm
Date Mon, 18 Oct 2010 09:35:24 GMT
Better QUORUM calculation algorithm 

                 Key: CASSANDRA-1627
             Project: Cassandra
          Issue Type: Bug
          Components: API, Core
            Reporter: Juho Mäkinen

The current QUORUM calculation algorithm is a bit problematic on some setups, especially when
using ReplicationFactor 3 (RF=3).

As the current algorithm is "N / 2 + 1" the result is rounded to nearest integer, resulting
that on RF=3 the QUORUM is also 3. Discussion with ntelford and ron_r resulted a better suggestion
to use FLOOR(N / 2 + 1), resulting QUORUM 2 on RF=3, but also decreasing the QUORUM value
on odd RF numbers above RF=4, resulting faster cluster operation but still maintaining the
QUORUM requirement.

Here's a table showing current method and the new suggestion:
|round(N / 2 + 1)|2|2|3|3|4|4|5|5|6|6|
|FLOOR(N/2 + 1)|1|2|2|3|3|4|4|5|5|6|

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