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From "Nicholas Telford (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1627) Better QUORUM calculation algorithm
Date Mon, 18 Oct 2010 09:52:24 GMT


Nicholas Telford commented on CASSANDRA-1627:

To clarify, this would fix two major availability issues:
- Clusters with RF=1 not being able to use QUORUM (at present, the number of QUORUM nodes
exceeds RF, so would always fail)
- Clusters with an odd RF not being able to run QUORUM operations if N / 2 - 1 nodes are down.

The second point is especially important as clusters with RF=3 currently can't run QUORUM
operations when a single node is down (e.g. during a rolling restart).

> Better QUORUM calculation algorithm 
> ------------------------------------
>                 Key: CASSANDRA-1627
>                 URL:
>             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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