I connect some esxi server to OMV via NFS. I did this before with openfiler, worked flawless. in principle it works as well or better with OMV - but as son as I enable the firewall, a df (or any other "checking" of the filesystems) in esxi is very slow (+120 sec). the speed of the NFS storage itself is fine.
my firewall rules are such, that 110 and 2049 and 4045 TCP/UDP are allowed as well as that I completely allow all traffic between OMV and ESXi hosts.
have a look: http://screencast.com/t/Oak7WBtPM
I start the firewall, start df, wait, then I clear the firewall and immediately df returns results.