I remember exactly this discussion some time ago, where one of the developers gave some more subtle reasons for not using even numbers. The maths sounds simple, with 4 mons you can tolerate the loss of 1, just like with 3 mons. The added benefit seems to be the extra copy of a mon. However, the reality is not that simple. There is apparently some kind of subtlety that has more to do with the physical set-up that makes 4 mons worse than 3 (more likely to lead to loss of service). I do not remember the thread, but it was within the last year. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Janne Johansson <icepic.dz@xxxxxxxxx> Sent: 29 October 2020 22:07:45 To: Tony Liu Cc: Marc Roos; ceph-users Subject: Re: frequent Monitor down Den tors 29 okt. 2020 kl 20:16 skrev Tony Liu <tonyliu0592@xxxxxxxxxxx>: > Typically, the number of nodes is 2n+1 to cover n failures. > It's OK to have 4 nodes, from failure covering POV, it's the same > as 3 nodes. 4 nodes will cover 1 failure. If 2 nodes down, the > cluster is down. It works, just not make much sense. > > Well, you can see it the other way around, with 3 configured mons, and only 2 up, you know you have a majority and can go on with writes. With 4 configured mons and only 2 up, it stops because you get the split brain scenario. For a 2DC setup with 2 mons at each place, a split is still fatal. -- May the most significant bit of your life be positive. _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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