Windows Server 2016 RDMA and the Hyper-V vSwitch – Part II


In part I this article I demonstrated that some of the rules in regards to SMB Direct and the Hyper-V vSwitch as we know them for Windows Server 2012 R2 have changed with Windows Server 2016. We focused on the fact that you can expose RDMA to a vNIC exposed to the management OS created on a vSwitch. This means that while in Windows Server 2012 R2 you cannot expose RDMA capabilities via a vSwitch, even when you are using a non-teamed RDMA capable NIC, this is no longer true with Windows Server 2016.

While a demo with a vSwitch on a single NIC as we did in part I is nice it’s unlikely you’ll use this often if at all in the real world? Here we require redundancy and that means NIC teaming. To do so we normally use a vSwitch created on a native Windows NIC team. But a native NIC teaming does not expose RDMA capabilities. And as such a vSwitch created against a Windows native NIC team cannot leverage RDMA either. Which was the one of the reasons why a fully converged scenario in Windows Server 2012 R2 was too limited for many scenarios. Loss of RSS on the vNIC exposes to the management OS was another. The solution to this in Windows Server 2016 Hyper-V comes with Switch Embedded Teaming (SET). Now using SET in each and every situation might not be a good idea. It depends. But we do need to know how to configure it. So let’s dive in.

Switch Embedded Teaming (SET) exposes RDMA to the vSwitch

Switch Embedded Teaming (SET in Windows Server 2016 allows multiple identical (make, model, firmware, drivers to be supported) NICs to be used or “teamed” within the vSwitch itself. The important thing to note here this does not use windows NIC teaming or LBFO (Load Balancing and Fail Over).

SET is the future and is needed or use with the Network Controller and Software Defined Networking in Windows. SET can also be used without these technologies. While today it supports a good deal of the capabilities of native Windows NIC teaming it also lacks some of them. In general SET is meant for full or partial converged scenarios with 10GBps or better NICs, not 1Gbps networking in a (hyper)converged Hyper-V scenario.

Please see New Windows Server 2016 NIC and Switch Embedded Teaming User Guide for Download for more information as there is just too much to tell about it.

Setting it up

We start out with a 2-node cluster where each node has 2 RDMA NICs (Mellanox ConnectX-3) with RDMA enabled and DCB configured. Live migration of VMs between those nodes works over SMB Direct works. All NIC are on the same subnet (thanks to Window Server 2016 Same Subnet Multichannel) and are on VLAN 110. In Failover Cluster Manager (FCM) that looks like below.


We’ll now use the rNICs to create a Switch Embedded Team.


Note that the teaming mode is switch independent, the only option supported with SET in Window Server 2016.


This also gives us a vNIC exposed to the management OS (default)


This is also visible as a vNIC in the mamagement OS called “vEthernet (RDMA-SET-vSwitch)”


This will be used to manage the host and to make its purpose clear we’ll rename it.

We’ll create 2 separate management OS vNICs for the RDMA traffic later. For now, we want the HOST-MGNT vNIC to have connectivity to the LAN and for that we need to tag it with VLAN 10.


The vNIC actually “inherited” the IP configuration of one of our physical NICs and we need to change that to either DHCP or a correct LAN IP address and settings.


You can use the code below to set the HOST-MGNT vNIC to DHCP

To finalize the HOST-MGNT vNIC configuration we enable priority tagging on it. If we don’t we won’t see any traffic other than SMB Direct tagged at all!


Before we go any further we’ll remove the VLAN tag from the rNICS as we don’t want it interfering with egress traffic being tagged by them or ingress traffic being filtered because it doesn’t match the VLAN ID on the rNICs.

From here on we’ll focus on the RDMA capable vNICs well create and use for SMB traffic.

We create 2 vNIC on the management OS for SMB Direct traffic.


Now these vNIC need an IP address, this can be in the same subnet because we have Windows Server 2016 SMB multichannel.

We than also need to put the vNICs in the correct VLAN. Remember that DCB / PFC priority tagging needs tagged VLAN so carry that priority. Right now, we can see that these are untagged.


So we tag them with VLAN ID 110


We enable jumbo frames on the vNICs. Remember that the physical NICs in the SET have jumbo frames enabled as well.


Normally all traffic that is originated from vNICs gets any QOS values set to zero. There is one exception to this and that’s SMB Direct traffic. SMB Direct traffic gets tagged with its QoS priority and that is not reset to 0 as it bypasses the vSwitch completely. But if we set other priorities on other types of traffic for DCB PFC and or ETS that passes over these vNICs we must enable priority tagging on these NICs as well or they’ll be stripped away.


The association of the vNIC to pNICs is random. This also changes during creation and destruction (disabling NICs, restarting the OS). We can map a vNCI to a particular pNIC. This prevents suboptimal use of the available pNICs and provides for a well know predictable path of the traffic. We do this with the below PowerShell commands.

Finally, last but not least, we should enable RDMA on our two vNICs or SMB Direct will not kick in at all.

Right now, we have it all configured correctly on one node of our 2-node cluster. The SMB network look like this now:


The cluster now looks like below.


We can live migrate VMs over SMB Direct in this mixed scenario where one node has pNICs RDMA NICs, 1 node has SET with vNICs for RMDA.


When looking at this in report mode we clearly see Node-A send SMB Direct traffic (tagged with priority 4, green) over its RDMA enabled SET vNICs to Node-B which still has a complete physical rNIC set up (blue).


As you can see in the screen shots above we now have RDMA / SMB Direct working with SET / RDMA vNICs on one node (Node-A) and pure physical RDMA NICs on the other (Node-B). This gives us bandwidth aggregation and redundancy. To complete the exercise, we configure SET on the other node as well. But it’s clear SET and RDAM will also work in a mixed environment.

We’ll discuss some details about certain aspects of the vNIC configuration in future articles. Things like the why and how of Set-VMNetworkAdapterTeamMapping and the use of -IeeePriorityTag. But for now, this is it. Go try it out! It’s the basis for anything you’ll do with SDNv2 in W2K16 and beyond.

Windows Server 2016 RDMA and the Hyper-V vSwitch – Part I


With Windows Server 2012 R2, using both RDMA and the Hyper-V vSwitch on the same host required separate physical network adapters (pNICs). There are 2 reasons for this.

  • First a vSwitch is generally created with a native Windows NIC team. Such a NIC team does not expose RDMA capabilities.
  • Second is that in Windows Server 2012 R2 you cannot expose RDMA capabilities via a vSwitch, even when you are using a non-teamed RDMA capable NIC.

As a result, the need for RDMA required more NICs on the Hyper-V hosts and/or a fully converged had some serious drawbacks. As servers have been quite capable and our VMs serve ever more intensive workloads this was not dramatic. Leveraging 2*10Gbps for a vSwitch and 2*10Gbps for redundant RDMA / SMB Direct traffic have long been one of my favorite designs. It leaves room for other traffic, such as backups, and it allows for high VM density. But with 40Gbps NICs that is overkill and a tad expensive in many scenarios, even when connecting to a SOFS share for Hyper-V storage, so 4*40Gbps on a Hyper-V host is not something I ever saw in real life.

Windows Server 2016 can expose RDMA capabilities via a vSwitch even without SET

What many people seem to have missed is that reason 2 has gone in Windows Server 2016 Hyper-V. Reason 1 still holds true. But that has been solved by Switch Embedded Teaming (SET). This means that you actually do not need SET to leverage RDMA with an vSwitch in Windows Server 2016 Hyper-V. You can do this as follows:

Below is what this looks like. We have one vNIC on the management OS leveraging RDMA/SMB Direct consuming all 10Gbps if the NIC we connected to the vSwith. This is a nice lab demo but you can see this isn’t perhaps the best idea in real life.


Other things to note

Do realize this still requires the pNIC to be RDMA capable. This is not some sort of soft RoCE or other software RDMA magic as of today. The pNIC also has to have RDMA enabled or virtual NIC won’t be able to leverage RDMA but fall back to SMB (Multichannel only) instead of SMB Direct. Likewise, RDMA has to be enabled on the vNIC as well. So don’t forget, RDMA must be enabled on both the pNIC and the vNIC for this to work.clip_image004

DCB’s PFC/ETS requires a tagged VLAN to carry the priority, do don’t forget to tag the vNIC. There is actually no need to tag the pNIC as long as the switch port has the tagged VLAN set – most likely as a trunk or in general mode. If you don’t tag consistently across the entire network stack you’ll have network issues anyway and RDMA performance will be bad if it works at all.

Finally, don’t forget this is example is not using VMM /Network Controller and as such is using Set-VMNetworkAdapterVLAN and not Set-VMNetworkAdapterIsolation.

In real life, we need better and more than a single NIC vSwitch

The caveat here is that, while you have a converged setup, you have no redundancy for the vSwitch (there is no team). This also means that you’re are limited to a single NIC in regards to throughput for that vSwith. Depending on the needs of the solutions that might be perfectly fine. It it’s not – in most real-world scenarios you’ll need redundancy – you have to use SET in a converged scenario. That’s what we’ll take a look at in part 2. Then there is the question about QoS as you don’t want SMB Direct traffic to consume to much bandwidth at will. That’s still another issue to discuss and address.

Musings On Switch Embedded Teaming, SMB Direct and QoS in Windows Server 2016 Hyper-V

When you have been reading up on what’s new in Windows Server 2016 Hyper-V networking you probably read about Switch Embedded Teaming (SET). Basically this takes the concept of teaming and has this done by the vSwitch. Which means you don’t have to team at the host level. The big benefit that this opens up is the RDMA can be leveraged on vNICs. With host based teaming the RDMA capabilities of your NICs are no longer exposed, i.e. you can’t leverage RDMA. Now this has become possible and that’s pretty big.


With the rise of 10, 25, 40, 50 and 100 Gbps NICs and switches the lure to go fully converged becomes even louder. Given the fact that we now don’t lose RDMA capabilities to the vNICs exposed to the host that call sounds only louder to many.  But wait, there’s even more to lure us to a fully converged solution, the fact that we now do no longer lose RSS on those vNICs! All good news.

I have written an entire whitepaper on convergence and it benefits, drawback, risks & rewards. I will not repeat all that here. One point I need to make that lossless traffic and QoS are paramount to the success of fully converged networking. After all we don’t want lossy storage traffic and we need to assure adequate bandwidth for all our types of traffic. For now, in Technical Preview 3 we have support for Software Defined Networking (SDN) QoS.

What does that mean in regards to what we already use today? There is no support for native QoS  and vSwitch QoS in Windows Server 2016 TPv3. There is however the  mention of DCB (PFC/ETS ), which is hardware QoS in the TechNet docs on Remote Direct Memory Access (RDMA) and Switch Embedded Teaming (SET). Cool!

But wait a minute. When we look at all kinds of traffic in a converged Hyper-V environment we see CSV (storage traffic), live migration (all variations), backups over SMB3 all potentially leveraging SMB Direct. Due to the features and capabilities in SMB3 I like that. Don’t get me wrong about that. But it also worries me a bit when it comes to handling QoS on the hardware side of things.

In DCB Priority Flow Control (PFC) is the lossless part, Enhanced Transmission Selection (ETS) is the minimum bandwidth QoS part. But how do we leverage ETS when all types of traffic use SMB Direct. On the host it all gets tagged with the same priority. ETS works by tagging different priorities to different workloads and assuring minimal bandwidths out of a total of 100% without reserving it for a workload if it doesn’t need it. Here’s a blog post on ETS with a demo video DCB ETS Demo with SMB Direct over RoCE (RDMA .

Does this mean a SDN QoS only approach to deal with the various type of SMB Direct traffic or do they have some aces up their sleeves?

This isn’t a new “concern” I have but with SET and the sustained push for convergence it does has the potential to become an issue. We already have the SMB bandwidth limitation feature for live migration. That what is used to prevent LM starving CSV traffic when needed. See Preventing Live Migration Over SMB Starving CSV Traffic in Windows Server 2012 R2 with Set-SmbBandwidthLimit.

Now in real life I have rarely, if ever, seen a hard need for this. But it’s there to make sure you have something when needed. It hasn’t caused me issues yet, but I’m a performance & scale first, in “a non-economies of scale” world compared to hosters. As such convergence is a tool I use with moderation. My testing when traffic competes without ETS is that they all get part of the cake but not super predictable/ consistent. SMB bandwidth limitation is a bit of a “bolted on” solution => you can see the perf counters push down the bandwidth in an epic struggle to contain it, but as said it’s a struggle, not a nice flat line.

Also Set-SmbBandwidthLimit is not a percentage, but hard max bandwidth limit, so when you lose a SET member the math is off and you could be in trouble fast. Perhaps it’s these categories that could or will be used but it doesn’t seem like the most elegant solution/approach. That with ever more traffic leveraging SMB Direct make me ever more curious. Some switches offer up to 4 lossless queues now so perhaps that’s the way to go leveraging more priorities … Interesting stuff! My preferred and easiest QoS tool, get even bigger pipes, is an approach convergence and evolution of network needs keeps pushing over. Anyway, I’ll be very interested to see how this is dealt with. For now I’ll conclude my musings On Switch Embedded Teaming, SMB Direct and QoS in Windows Server 2016 Hyper-V