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From Ken Hancock <>
Subject Re: Replication Factor question
Date Tue, 15 Apr 2014 13:14:30 GMT
Keep in mind if you lose the wrong two, you can't satisfy quorum.  In a
5-node cluster with RF=3, it would be impossible to lose 2 nodes without
affecting quorum for at least some of your data. In a 6 node cluster, once
you've lost one node, if you were to lose another, you only have a 1-in-5
chance of not affecting quorum for some of your data.

In much larger clusters, it becomes less probable that you will lose
multiple nodes within a RF group.

On Tue, Apr 15, 2014 at 4:37 AM, Markus Jais <> wrote:

> Hi all,
> thanks for your answers. Very helpful. We plan to use enough nodes so that
> the failure of 1 or 2 machines is no problem. E.g. for a workload to can be
> handled by 3 nodes all the time, we would use at least 5, better 6 nodes to
> survive the failure of at least 2 nodes, even when the 2 nodes fail at the
> same time. This should allow the cluster to rebuild the missing nodes and
> still serve all requests with a RF=3 and Quorum reads.
> All the best,
> Markus
>   Tupshin Harper <> schrieb am 21:23 Montag, 14.April
> 2014:
> tl;dr make sure you have enough capacity in the event of node failure. For
> light workloads, that can be fulfilled with nodes=rf.
> -Tupshin
> On Apr 14, 2014 2:35 PM, "Robert Coli" <> wrote:
> On Mon, Apr 14, 2014 at 2:25 AM, Markus Jais <> wrote:
> "It is generally not recommended to set a replication factor of 3 if you
> have fewer than six nodes in a data center".
> I have a detailed post about this somewhere in the archives of this list
> (which I can't seem to find right now..) but briefly, the "6-for-3" advice
> relates to the percentage of capacity you have remaining when you have a
> node down. It has become slightly less accurate over time because vnodes
> reduce bootstrap time and there have been other improvements to node
> startup time.
> If you have fewer than 6 nodes with RF=3, you lose >1/6th of capacity when
> you lose a single node, which is a significant percentage of total cluster
> capacity. You then lose another meaningful percentage of your capacity when
> your existing nodes participate in rebuilding the missing node. If you are
> then unlucky enough to lose another node, you are missing a very
> significant percentage of your cluster capacity and have to use a
> relatively small fraction of it to rebuild the now two down nodes.
> I wouldn't generalize the rule of thumb as "don't run under N=RF*2", but
> rather as "probably don't run RF=3 under about 6 nodes". IOW, in my view,
> the most operationally sane initial number of nodes for RF=3 is likely
> closer to 6 than 3.
> =Rob

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