Today the index builds process uses semantics similar to doing a w:all. The index build sets the index commit quorum to be all data-bearing nodes and waits until all of them have voted to commit the index in order to proceed with the final commit.

However, if during the index build a node suddenly dies then the primary will be stuck waiting for the node to come back online.

This can be overcome if a database administrator issues the setIndexCommitQuorum command to change the quorum size but ideally this should be handled in an automatic fashion if possible.