GlusterFS is an open source distributed file system developed by Gluster Inc in 2005 (http://www.gluster.com) and it has been recently acquired by Red Hat in late 2011 (http://www.redhat.com/promo/storage/press-release.html).
GlusterFS features a completely decentralized architecture meaning that there is not a single point of failure and this is great for a cloud infrastructure.
In small and medium infrastructures, the GlusterFS server components could be installed directly on the OpenNebula hosts along with the hypervisor. Instead, for large deployments, it is advisable to install GlusterFS on dedicated machines and perhaps arrange them in a SAN.
Before installing the GlusterFS, verify ...