Referring to Vertica documentation -
"Minimum Subcluster Size for K-Safe Databases In a K-safe database, subclusters must have at least three nodes in order to operate. Each subcluster tries to maintain subscriptions to all shards in the database. If a subcluster has less than three nodes, it cannot maintain shard coverage. Vertica returns an error if you attempt to rebalance shards in a subcluster with less than three nodes in a K-safe database." from https://www.vertica.com/docs/10.0.x/HTML/Content/Authoring/Eon/Subclusters.htm?TocPath=Vertica%20Architecture%3A%20Eon%20Versus%20Enterprise%20Mode|Eon%20Mode%20Concepts|_____3
Why do I need 3 nodes?
Wouldn't things work if Ksafety is 1 and there are only 2 shards? So node 1 has shard1 and shard 2 and so does node 2? If node 2 fails then node 1 serves all queries? Has it got to do with QUORUM that with do nodes, if 1 node gets down then QUORUM is lost and thus the database shuts down?
As @minatmerva put it in his comment:
Working on several nodes is what Massive Parallel Processing (MPP) is all about. Working on 2 nodes when the third is down is still MPP. Working on 1 node when the 2nd is down isn't MPP any more. So working MPP on 2 nodes is not foreseen at all.