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Follow the Quick Start guide from the Storage SIG to prepare your hosts. You can create the GlusterFS volumes on your own, and then instruct flexVDI to use them when you create a Gluster Volume, or you can just mount the bricks and create the GlusterFS volume with flexVDI Dashboard. In this last case, take into account that flexVDI requires that bricks belonging to the same volume are mounted in the same path in all the participating hosts. Follow the section on Storage to know more about managing Gluster Volumes with flexVDI Dashboard.
On RHEL, TODOfollow the Red Hat Gluster Storage installation guide.