On 07/26/2013 08:59 PM, John Wilkins wrote:
(d) If you have three monitors, Paxos will still work. 2 out of 3 monitors is a majority. A failure of a monitor means it's down, but not out. If it were out of the cluster, then the cluster would assume only two monitors, which wouldn't work with Paxos.
Although John is mentioning a high-availability scenario, I believe this phrase is a bit ambiguous so let me cover the possibility of it not being as clear as possible :-)
As in any monitor configuration, in a 2-monitor scenario, the only situation in which Paxos wouldn't work (i.e., the monitors wouldn't form a quorum) is when a majority of monitors is down. In a 2-monitor cluster that means 1 monitor. That is why 2-monitor clusters are not advised, as they provide as much availability as a 1-monitor cluster; but Paxos *works* just fine in a 2-monitor cluster as it would on 1-monitor or 3-monitor clusters (once again, as long as both monitors are up).
That's why 3 monitors is the minimum for high availability. 4 works too, because 3 out of 4 is a majority too. Some people like using an odd number of monitors, since you never have an equal number of monitors that are up/down; however, this isn't a requirement for Paxos. 3 out of 4 and 3 out of 5 both constitute a majority.
-Joao -- Joao Eduardo Luis Software Engineer | http://inktank.com | http://ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
