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
