GlusterFS 3.7 introduced arbiter volume which it is a 3-way replication where the third brick is the arbiter.
How does one change from 2-way replication to 3-way replication with arbiter?
I could not find any documentation of changing running replica 2 volume to arbiter volume.
Reference: https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/
Just sent the patch http://review.gluster.org/#/c/14502/ to add this functionality. If everything goes well, it should make it to the 3.8 release.