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(a)gmail.com>
Sent: 29 October 2020 22:07:45
To: Tony Liu
Cc: Marc Roos; ceph-users
Subject: [ceph-users] Re: frequent Monitor down
Den tors 29 okt. 2020 kl 20:16 skrev Tony Liu <tonyliu0592(a)hotmail.com>om>:
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.
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