Do you have your SAN already (is it mountable?), or you would like to create one?

If you are looking for solution you dont have, project sheepdog is what we (imho) need to integrate -- see blueprint: https://blueprints.launchpad.net/nova/+spec/sheepdog-support (https://blueprints.launchpad.net/nova...)

Well then http://Lustre.org is pretty fast and stable.

GlusterFS is becoming nice now, but still its not very fast.

I think CEPH might be very nice, but its still highly experimental.

Openstack is now not coming with that solved for you. Swift (Openstack Storage) is for cold object storage, its not high performace distributed FS.

But still i think that generic filesystem will not be better than sheepdog.