I had a ticket open with support on this but they could not explain what would happen so trying here.
Single index, multiple primaries spread across multiple nodes, 0 replicas. Do writes continue if 1 of the primaries of an index is offline or does the index become un-writeable?
For example in my test, I created an index with 2 primary shards (P0/1) and 0 replicas. The primaries are distributed across node A and B.
While indexing data I stopped the service on B, thus P1 becomes unassigned/unavailable. As there are no replicas to become primaries the cluster goes red. Search results immediately show half the data as expected, but it looks like writes also stopped which is surprising.
I would have assumed all writes would have gone to the remaining active primary shard that was still available?