Better QUORUM calculation algorithm
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Key: CASSANDRA-1627
URL: https://issues.apache.org/jira/browse/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:
||RF|||1||2||3||4||5||6||7||8||9||10||
|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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