You can have obscure network topology issues where each (master eligible) node does not see all other (master eligible) nodes, in this case there may be overlaps. So it is possible to have a split brain even if a (n/2)+1 quorum is active. The risk is very low but the probability of this insane situation rises with the number of master eligible nodes.
Because of this insanity, there is good reason why it is preferable to keep just 3 small data-less nodes as only master eligible with minimum master node of 2. To keep an eye on these 3 nodes should be manageable. To these 3 nodes, you can add as many data nodes as you want, each of them should be visible by at least one master node - that is enough to join. The cluster I operate is in two racks, it hasn't split since mid of 2010. Yes, I have tested network disruptions to cause split brains. Jörg On Tue, Feb 18, 2014 at 8:13 PM, Tony Su <[email protected]> wrote: > Robert, > I think the "(n/2)+1" formula should always work, and work reliably if > applied to > discovery.zen.minimum_master_nodes > > It's absolutely impossible to establish a quorum and elect a new leader if > >50% of total nodes. When and if quorum is established, the conditions to > elect a new leader in the remaining nodes can never exist (because would be > less than 50%) > > Yes, the consequence is that the cluster becomes inoperable, but that is > the current result when a quorum is not achieved. Whether the cluster > should become read-only instead of inoperable is an interesting idea. > > IMO, > Tony > > > > On Tuesday, February 18, 2014 3:54:07 AM UTC-8, Robert Stupp wrote: > >> Am Dienstag, 18. Februar 2014 11:24:34 UTC+1 schrieb Jörg Prante: >>> >>> With using the quorum of (n/2)+1 you are safe the cluster can always >>> elect a single leader. >>> >> Hm.. not really. 3*2 nodes --- quorum=6/2+1=4 --- a 4 + 2 nodes "split >> brain" is still possible. >> And ES stops working, if #nodes<minimum_master_nodes . >> It would be better to switch ES to a "read only" mode - maybe by >> introducing a new configuration option similar to minimum_master_nodes. >> > -- > You received this message because you are subscribed to the Google Groups > "elasticsearch" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elasticsearch/d1f63278-e7e0-4014-9c14-3d833a75f301%40googlegroups.com > . > > For more options, visit https://groups.google.com/groups/opt_out. > -- You received this message because you are subscribed to the Google Groups "elasticsearch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/elasticsearch/CAKdsXoFMSWRnsN3ng7kxrov4XQt3NkqHYjp%3DrH-_xhYfCC0_cg%40mail.gmail.com. For more options, visit https://groups.google.com/groups/opt_out.
