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From "Justin Cameron (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-13645) Optimize the number of replicas required in Quorum read/write
Date Thu, 29 Jun 2017 01:28:00 GMT


Justin Cameron commented on CASSANDRA-13645:

Would a CL.HALF be sufficient to meet the requirements here (rather than CL.INTEGER)? 

RF3 - CL.HALF (rounding up) write + CL.QUORUM read > 3
RF4 - CL.HALF write + CL.QUORUM read > 4
RF5 - CL.HALF (rounding up) write + CL.QUORUM read > 5
RF6 - CL.HALF + CL.QUORUM read > 6

Might be a little less opaque due to rounding for odd RFs, but it would probably be simpler
to implement than a CL.INTEGER

> Optimize the number of replicas required in Quorum read/write
> -------------------------------------------------------------
>                 Key: CASSANDRA-13645
>                 URL:
>             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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