The commit quorum defaults to "votingMembers". But, an in-progress index build ignores when replReconfig changes a member's votes to 0 until the primary steps down. 

This is a customer pain point: if there is a problem node, and it's votes and priority are set to 0 via replReconfig(), an in-progress index build will stall until a stepdown or explicit call to setCommitQuorum.

Customers have to run setCommitQuorum on each in-progress index builds to prevent stalling.