On 6 Sep 2013, at 13:46, Jens Kristian Søgaard wrote: > You created 7 mons in ceph. This is like having a parliament with 7 members. > > Whenever you want to do something, you need to convince a majority of > parliament to vote yes. A majority would then be 4 members voting yes. > > If two members of parliament decide to stay at home instead of turning up to > vote - you still need 4 members to get a majority. > > It is _not_ the case that everyone would suddenly agree and acknowledge that > only 5 parliament members have turned up to vote, so that only 3 yes votes > would be enough to form a majority.
Perhaps not a great analogy. At least in the case of the UK parliament, if 2 members of a 7 member parliament stay at home and don't vote, you would only need 3 members to pass a resolution. In the UK (and I believe in most other parliaments) you need the number of 'yes' to exceed the number of 'no'. The number of members does not matter. In ceph, you need the number of monitors active and voting yes to exceed (i.e. be strictly greater than) half the number of monitors configured. There is no magic about anything being odd or even, save that the quorum for an n-MON cluster, where n is odd, is the same as the quorum for an n+1 MON cluster (as n+1 is even) - in both cases if at least k=(n+1)/2 devices fail it will take the cluster out (i.e. (n-1)/2 have to survive). This makes deploying even numbers of MON devices wasteful (does not increase quorum) and arguably increases the chance of failure (as now we need k devices of n+1 to fail, as opposed to k devices of n). -- Alex Bligh _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
