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  1. Cassandra
  2. CASSANDRA-13645

Optimize the number of replicas required in Quorum read/write



    • Type: Improvement
    • Status: Open
    • Priority: Normal
    • Resolution: Unresolved
    • Fix Version/s: 4.x
    • Component/s: Legacy/Coordination
    • Labels:


      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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              • Assignee:
                wpc Pengchao Wang
                dikanggu Dikang Gu
                Pengchao Wang
              • Votes:
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