Hi
I was talking about a potential ceph setup, and a question arose about
how ceph handles a potential split brain situation. I had my own ideas
on how that would be handled, but I want to consult with the wider
knowledge base here to verify my understanding.
So, let's say we have two data centres. Replication is configured so
there are 3 replicas, and at least one copy in each data centre.
Also, there are an odd numer of MONs in the cluster.
If we now get a net split, so we end up with 2 replicas in one dc (A), 1
in the other (B). In theory we should be good, as no data is lost and if
there are more than one OSD in B it will re-balance.
But what happens now when it comes to writes? If we write to both sides
of the split, we've lost.
If there are 1 MON in B, that cluster will have quorum within itself and
keep running, and in A the MON cluster will vote and reach quorum again.
In that case we have two clusters, both accepting writes to the same
objects in those 2 and 1 replicas.
So, how does the praxos, crush and other protocols make sure not both
sides of the split is active?
Pointers in the documentation appreciated, as well as other
explanations.
/andreas
--
"economics is a pseudoscience; the astrology of our time"
Kim Stanley Robinson
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