Hi Ken, thanks. Good point.
Markus Ken Hancock <ken.hanc...@schange.com> schrieb am 15:15 Dienstag, 15.April 2014: 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 <markus.j...@yahoo.de> 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 <tups...@tupshin.com> 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" <rc...@eventbrite.com> wrote: >>> >>>On Mon, Apr 14, 2014 at 2:25 AM, Markus Jais <markus.j...@yahoo.de> 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 >>>> >>>> >>> >>> > > > > > > > > > > >
