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https://issues.apache.org/jira/browse/CASSANDRA-13645?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16070608#comment-16070608
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Jason Brown commented on CASSANDRA-13645:
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To be clear, though, a user will have to know that they must use different CLs
in order to gain the optimization as proposed by this ticket. Meaning, you
write at {{CL.QUORUM}} and read at {{CL.HALF}}; you can't write and read at
{{CL.HALF}} and get strong consistency properties.
As much as I don't want to open another can of worms, but do we need a
corresponding {{CL.LOCAL_HALF}}, as well?
> Optimize the number of replicas required in Quorum read/write
> -------------------------------------------------------------
>
> Key: CASSANDRA-13645
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13645
> Project: Cassandra
> Issue Type: Improvement
> Components: Coordination
> Reporter: Dikang Gu
> Assignee: Pengchao Wang
> Fix For: 4.x
>
>
> Currently, for C* read/write requests with quorum consistent level, number of
> replicas required for quorum write is W=N/2+1, and number of replicas
> required for quorum read is R=N/2+1 as well.
> It works fine in odd number of replicas case, which R + W = N + 1, but in
> even number of replicas case, like RF=4, 6, 8, the R+W = N + 2, which means
> we are having two overlapping nodes in read/write requests, which is not
> necessary. It can not provide strong consistency, but will hurts P99 read
> latency a lot (2X in our production cluster).
> In a lot of other database, like Amazon Aurora, they use W = N/2 + 1 and R =
> N/2 for quorum requests, which will provide enough strong consistency, but
> talk to one less replica in read path. "We use a quorum model with 6 votes (V
> = 6), a write quorum of 4/6 (Vw = 4), and a read quorum of 3/6 (Vr = 3)."
> I propose we do the same optimization, change read quorum to talk to N/2
> replicas, which should reduce the read latency for quorum read in general.
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