The balancer sporadically checks if all the shards in the cluster have the exact same binary version. When this is not the case, the balancer emits a warning log message.

This has two problems:

• Correctness:
  During upgrade/downgrade procedure is totally expected that the shards in the cluster will have a mismatching binary version, thus I believe is incorrect to emit a warning log message.
• Performance:
  Even though the check is actually performed sporadically, the balancer collects the shards binary versions every round during construction of ClusterStatistics (this is actually done multiple times per round). The retrieval of binary version is done by executing the serverStatus command on all shard primaries, this is done serially. Thus, in a cluster with large number of shards, this can slow down significantly the balancer round.