Making a new feature request to Implement libgfapi so XenServer can use GlusterFS as a 'free' HA storage solution.
Currently XenServer can use GlusterFS over NFS, however performance is terrible. Using libgfapi or a FUSE mount could eliminate the NFS storage bottlenecks.
QEMU on KVM implements libgfapi and I'm told it works great... very fast, stable, high IOPS, high thruput, and also HA is fully supported.
GlusterFS is a nice storage solution because it runs under regular Linux on commodity hardware. The storage pools can grow to hundreds of servers and several petabytes of storage.