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From "Dikang Gu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-13645) Optimize the number of replicas required in Quorum read/write
Date Thu, 29 Jun 2017 00:17:00 GMT
Dikang Gu created CASSANDRA-13645:
-------------------------------------

             Summary: 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: Dikang Gu
             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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