The switch at hand is a Dell PowerConnect 2716 which does support both VLANs and Jumbo Frames but, as far as I can understand from its manual, it only has a global Jumbo enable/disable setting and no per-port or per-vlan MTU settings.
I have two VLANs on that switch:
- a generic LAN for internal traffic, untagged on ports 1-4, which must be kept at MTU=1500
- an iSCSI-dedicated LAN, untagged on ports 5-8, which could take advantage of Jumbo Frames
The four servers (two as a DIY iSCSI HA cluster made with pacemaker and IET that will provide storage for virtual machines, two that will run the virtual machines) each have three gigabit interfaces: one for the LAN (connected to ports 1-4), one for iSCSI (to ports 5-8) and a third one for the public network / WAN (connected to a different switch)
Is it possible to enable Jumbo Frames globally on the switch (ports 1-16), and on every SAN device (the four dedicated ethernet cards in the four servers), but keep LAN devices at standard MTU?
In theory the switch should not create / rearrange frames so the generic LAN will not get exposed to big/jumbo frames.
And yes, we plan to rework the infrastructure with something better. We'll probably throw away the DIY iSCSI cluster and put an MD3220i or similar thing, keeping the PC2716 as a generic LAN switch and connecting the iSCSI links directly from the servers to the SAN… but not today. Probably 2013Q4 or 2013Q4 if everything goes well, but for now I have to live with this hardware and get the most out of it.
Best Answer
Well, first off, Jumbo Frames are very useful in iSCSI network but mostly if you are using software initiators or software targets (or both).
Your iSCSI network will still work even if the frames are smaller (1500 bytes). What you will notice is that with smaller frames, the CPU usage on both the initiator side and the target side will be higher than if Jumbo Frames where used.
So if you will change your switch for a dedicated one with Jumbo Frames enabled soon, you could simply wait.
On the other hand, I believe your assumption is correct that your regular LAN will not be affected.