5 Apr
2021
5 Apr
'21
8:08 p.m.
Thanks Sage,
I opted to move to an explicit placement map of candidate hostnames and
a replica count rather than using labels. This is a testing cluster of
VMs to experiment before updating the production system.
The only reason I was starting with 2 on the test cluster is that my
production ceph has a few mons that are custom built docker containers
that will give some grief in the adoption process. My plan is to remove
those mons before converting to cephadm. On further thought, it will
probably be safer to add a few normal mons to the OSD servers before
adopting to cephadm, to ensure I have more than 3 before the conversion.
I like the work done with cephadm and orchestration as compared to the
older ceph-deploy. I do have a few questions/challenges on OSD
deployment that I will put in a new post.
Cheers.
Gary
On 2021-03-31 5:00 p.m., Sage Weil wrote:
Hi Gary,
It looks like everything you did is fine. I think the "problem" is
that cephadm has/had some logic that tried to leave users with an odd
number of monitors. I'm pretty sure this is why two of them were
removed.
This code has been removed in pacific, and should probably be
backported to octopus.
There is nothing wrong with an even number of mons. The only number
you might want to avoid is 2 because a failure of either monitor will
cause the cluster to lose quorum and become unavailable (quorum
requires > N/2, which in a 2-mon case means both mons). As far as
availability goes that is probably not ideal, but as far as durability
goes, it's extremely useful to have a duplicate copy of the mon data
so that losing a single disk doesn't destroy the cluster metadata (and
require a complicated recovery process).
In any case, generally speaking, nobody should worry about having an
even number of monitors. Focus instead of getting >2 so you can
tolerate at least one mon failure and keep the cluster running.
On Wed, Mar 31, 2021 at 10:14 AM Gary Molenkamp <molenkam(a)uwo.ca> wrote:
--
Gary Molenkamp Computer Science/Science Technology Services
Systems Administrator University of Western Ontario
molenkam(a)uwo.ca http://www.csd.uwo.ca
(519) 661-2111 x86882 (519) 661-3566
A nautilus cluster with two mons (I know this is
not correct for
quorum), a mgr, and a handful of osds. I went though the adoption
Any number of
monitors is correct. Less than 3 is not recommended.
sage