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From "Jeff Jirsa"<>
Subject Re: 回复: 回复: tolerate how many nodes down in the cluster
Date Thu, 27 Jul 2017 06:19:58 GMT

On 2017-07-26 19:38 (-0700), "Peng Xiao" <> wrote: 
> Kurt/All,
> why the  # of racks should be equal to RF?
> For example,we have 2 DCs each 6 machines with RF=3,each machine virtualized to 8 vms
> can we set 6 racs with RF3? I mean one machine one RAC to avoid hardware errors or only
set 3 racs,1 rac with 2 machines,which is better?

The guarantee you get from racks is that IF you have more racks than replicas, you won't have
2 replicas on the same rack. There's no requirement that # of racks >= # of replicas, you
just leave yourself exposed to losing quorum if you have an outage while # racks < # replicas.

Yes, with a rack == a hypervisor, the snitch would avoid placing 2 replicas on the same physical
machine, and would protect you against hardware errors. There's nothing to gain from having
3 racks instead of 6 in that case (in fact 6 is probably better, as you're less likely to
have to skip a duplicate rack in getNaturalEndpoints()).

All of this said:


If you start with # of racks < RF, and you try to add another rack, you will probably be
very unhappy (when you add that first node in the new rack, it'll take 1/RF of the ring instantly,
which usually crashes everything). For that reason, a lot of people advise not to use racks
unless you have > RF racks, or you REALLY know what you're doing.

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