On 2017-07-26 19:38 (-0700), "Peng Xiao" <2535...@qq.com> 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: BE REALLY CAREFUL WHEN USING RACKS. 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. --------------------------------------------------------------------- To unsubscribe, e-mail: user-unsubscr...@cassandra.apache.org For additional commands, e-mail: user-h...@cassandra.apache.org
