[ceph-users] Re: stretched cluster or not, with mon in 3 DC and osds on 2 DC

Hi, since I just read that documentation page [1] on Friday, I can't tell you anything that isn't on that page. But that particular problem of which monitor gets elected should be solvable simply by using connectivity election mode [2], shouldn't it? Apart from the latency to the mon, the stretch cluster is mainly about the failover characteristics of the OSDs: When DC1 or DC2 fails, without a stretch cluster, the other DC will try to replicate all the data again to reach size=4 again. With a stretch cluster, it will happily live with size=2 until the other DC comes back online. So when it's right to assume that if - god forbid - one of the DCs goes offline, it will come back online not too long after again, so that the cluster can live with size=2 during that phase, then a stretch cluster probably is the better choice. Also, as the documentation states, there are edge cases where even given an appropriate CRUSH rule, size=4 min_size=2 don't necessarily mean you have a live copy of every PG in each of the two DCs. Best regards, Jan-Philipp [1]: https://docs.ceph.com/en/latest/rados/operations/stretch-mode/ [2]: https://docs.ceph.com/en/latest/rados/operations/change-mon-elections/