Unknown Per investigation with dmitry.rybakov@mongodb.com, currently Ruby driver connection pool treats the max size of 0 literally. This behavior isn't useful in practice because the pool, not ever having any connections, won't be able to perform any useful work. Additionally this behavior conflicts with cmap spec which in https://github.com/mongodb/specifications/blob/master/source/connection-monitoring-and-pooling/connection-monitoring-and-pooling.rst#id1 requires for max size of 0:
> * A value of 0 means there is no limit.
