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https://issues.apache.org/jira/browse/CASSANDRA-1627?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Juho Mäkinen updated CASSANDRA-1627:
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Description:
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|
EDIT: as pcmanus pointed out, the N/2+1 calculation indeed returns 2 when N=2,
so the round error doesn't occur here. I'll need to dig the problem up a bit
because this suggestion originated when my cluster returned
UnavailableException (I'm using RF=3) when doing QUORUM operations when one
node was down.
was:
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|
Priority: Minor (was: Major)
> Better QUORUM calculation algorithm
> ------------------------------------
>
> Key: CASSANDRA-1627
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1627
> Project: Cassandra
> Issue Type: Bug
> Components: API, Core
> Reporter: Juho Mäkinen
> Priority: Minor
>
> 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|
> EDIT: as pcmanus pointed out, the N/2+1 calculation indeed returns 2 when
> N=2, so the round error doesn't occur here. I'll need to dig the problem up a
> bit because this suggestion originated when my cluster returned
> UnavailableException (I'm using RF=3) when doing QUORUM operations when one
> node was down.
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