Hi,
During Hosted Engine setup question about glusterfs volume is being asked because you have setup the volumes yourself. If cockpit+gdeploy plugin would be have been used then that would have automatically detected glusterfs replica 3 volume created during Hosted Engine deployment and this question would not have been asked.
During new storage domain creation when glusterfs is selected there is a feature called 'use managed gluster volumes' and upon checking this all glusterfs volumes managed will be listed and you could choose the volume of your choice from the dropdown list.
There is a conf file called /etc/hosted-engine/hosted-engine.conf where there is a parameter called backup-volfile-servers="h1:h2" and if one of the gluster node goes down engine uses this parameter to provide ha / failover.
Hope this helps !!
Thanks
kasturi