All such questions are a bit dated; I'm hoping things have changed in the past year.
What do you guys use to serve a DRS cluster with vSphere 5.1/5.5?
SCSI-3 persistent reservation support would be definitely needed, VAAI is a nice-to-have.
I've seen that LIO should be able to do it; they do list VAAI as something merged into the kernel (3.14 maybe?), but that won't be available easily on Ubuntu for the time being.
What are your experiences?
The target block devices are already highly available: they are RADOS block devices exported from a ceph cluster.