Mons are a paxos quorum and as such want to be in odd numbers. 5 is generally what people go with. I think I've heard of a few people use 7 mons, but you do not want to have an even number of mons or an ever growing number of mons. The reason you do not want mons running on the same hardware as osds is resource contention during recovery. As long as the Xen servers you are putting the mons on are not going to cause any source of resource limitation/contention, then virtualizing them should be fine for you. Make sure that you aren't configuring the mon to run using an RBD for its storage, that would be very bad.
The mon Quorum elects a leader and that leader will be in charge of the quorum. Having local mons doesn't do anything as the clients will still be talking to the mons as a quorum and won't necessarily talk to the mon running on them. The vast majority of communication to the cluster that your Xen servers will be doing is to the OSDs anyway, very little communication to the mons.
On Mon, Jul 10, 2017 at 1:21 PM Massimiliano Cuttini <max@xxxxxxxxxxxxx> wrote:
Hi everybody,
i would like to separate MON from OSD as reccomended.
In order to do so without new hardware I'm planning to create all the
monitor as a Virtual Machine on top of my hypervisors (Xen).
I'm testing a pool of 8 nodes of Xen.
I'm thinking about create 8 monitor and pin one monitor for one Xen node.
So, i'm guessing, every Ceph monitor'll be local for each node client.
This should speed up the system by local connecting monitors with a
little overflown for the monitors sync between nodes.
Is it a good idea have a local monitor virtualized on top of each
hypervisor node?
Did you see any understimation or wrong design in this?
Thanks for every helpfull info.
Regards,
Max
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