Hi, On Sat, Jun 13, 2015 at 7:32 AM, Daniel Black <daniel.black@openquery.com.au> wrote:
1. A cluster should have an odd number of nodes to avoid brain split.
Split brain is avoided with 3+ nodes. You won't get split brain with even nodes as the quorum is 1/2 + 1 nodes. i.e. It does mean 1/2 the nodes can't form a quorum.
Just to add a little more to this, "split brain" is when a node (or group of nodes) thinks to itself "was it me or those guys, who left? There's not enough people to concur with". This normally happens with networking problems between two datacenters or if the database server crashes. So in your situation, two nodes running while the third one is being upgraded is fine. However, during the time the third node is "down", you will get a split brain scenario *if* one of the online nodes die. The larger case of split brain is when your networking layer is more prone to have problems. It's unlikely that the connection between nodes on the same switch will lose connectivity, but if the nodes were across regions, your chances of some transient connection issues goes up a lot. -will -- Will Fong, Support Engineer MariaDB Corporation