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 > >
