... and how to setup vmxnet3 vNIC and Jumbo frames
When I set jumbo frames (MTU=9000) on em0 vNIC in virtual FreeNAS I can't get frames bigger than 1500 between the peers, even configuring mtu=9000 in all network elements between end points. Do not be fooled if you see this:
[root@freenas] ~# ifconfig em0 mtu 9000
[root@freenas] ~# ifconfig em0 em0: flags=8843 metric 0 mtu 9000 options=9b
ether 00:50:56:87:xx:xx options=9b ether 00:50:56:87:xx:xx
I did some unsuccessful tests with em0 (in my case vnic is recognized as intel 82545EM) and MTU=9K. Eventually I could not get real jumbo frames between FreeNAS and other vm even inside vSphere host. Probably exception is being in the E1000 driver used by FreeNAS virtual machine though this also depends of physical NICs. em(4) supports variations of the hardware - some do jumbo frames, some don't. It could easily get a lot quirks in a VM, depending on how the VM emulates the card and the mode of networking used. This should show which of the Intel cards is being emulated.
# pciconf -lv | grep -A3 em0