Currently mongotune uses setUserWriteBlockMode to block writes on a replica set when the primary is above a certain disk usage threshold.

This policy is not yet supported on sharded clusters because setUserWriteBlockMode only supports running against mongos to block writes to the entire cluster. This is not ideal for customers who may have queries that target shard A fail if shard B is the one that is over the disk threshold.

This ticket is to track the proposed work of allowing setUserWriteBlockMode to be issued against a shard server primary that block writes to individual shards