SMB Direct With RoCE in a Mixed Switches Environment

I’ve been setting up a number of Hyper-V clusters with  Mellanox ConnectX3 Pro dual port 10Gbps Ethernet cards. These Mellanox cards provide a nice amount of queues (128) for DVMQ and also give us RDMA/SMB Direct capabilities for CSV & live migration traffic.

Mixed Switches Environments

Now RoCE and DCB is a learning curve for all of us and not for the faint of heart. DCB configuration is non trivial, certainly not across multiple hops and different switches. Some say it’s to be avoided or can’t be done.

You can only get away with a single pair of (uniform) switches in smaller deployments. On top of that I’m seeing more and more different types of switches being used to optimize value, so it’s not just a lab exercise to do this. Combine this with the fact that DCB is an unavoidable technology in networking, unless it get’s replaced with something better and easier, and you might as well try and learn. So I did.

Well right now I’m successfully seeing RoCE traffic going across cluster nodes spread over different racks in different rows at excellent speeds. The core switches are DELL Force10 S4810 and the rack switches are PowerConnect 8132Fs. By borrowing an approach from spine/leave designs this setup delivers bandwidth where they need it a a price point they can afford. They don’t need more expensive switches for the rack or the core as these do support DCB and give the port count needed at the best price point.  This isn’t supposed to be the top in non blocking network design. Nope but what’s available & affordable today in you hands is better than perfection tomorrow. On top of that this is a functional learning experience for all involved.

We see some pause frames being sent once in a while and this doesn’t impact speed that very much. It does guarantee lossless traffic which is what we need for RoCE. When we live migrate 300GB worth of memory across the nodes in the different racks we get great results. It varies a bit depending on the load the switches & switch ports are under but that’s to be expected.

Now tests have shown us that we can live migrate just as fast with non RDMA 10Gbps as we can with RDMA leveraging “only” Multichannel. So why even bother? The name of the game low latency and preserving CPU cycles for SQL Server or storage traffic over SMB3. Why? We can just buy more CPUs/Cores. Great, easy & fast right? But then with SQL licensing comes into play and it becomes very expensive. Also storage scenarios under heavy load are not where you want to drop packets.

Will this matter in your environment? Great question! It depends on your environment. Sometimes RDMA is needed/warranted, sometimes it isn’t. But the Mellanox cards are price competitive and why not test and learn right? That’s time well spent and prepares you for the future.

But what if it goes wrong … ah well if the nodes fail to connect over RDAM you still have Multichannel and if the DCB stuff turns out not to be what you need or can handle, turn it of and you’ll be good.

RoCE stuff to test: Routing

Some claim it can’t be done reliably. But hey they said that for non uniform switch environments too Winking smile. So will it all fall apart and will we need to standardize on iWarp in the future?  Maybe, but isn’t DCB the technology used for lossless, high performance environments (FCoE but also iSCSI) so why would not iWarp not need it. Sure it works without it quite well. So does iSCSI right, up to a point? I see these comments a lot more form virtualization admins that have a hard time doing DCB (I’m one so I do sympathize) than I see it from hard core network engineers. As I have RoCE cards and they have become routable now with the latest firmware and drivers I’d love to try and see if I can make RoCE v2 or Routable RoCE work over different types of switches but unless some one is going to sponsor the hardware I can’t even start doing that. Anyway, lossless is the name of the game whether it’s iWarp or RoCE. Who know what we’ll be doing in 5 years? 100Gbps iWarp & iSCSI both covered by DCB vNext while FC, FCoE, Infiniband & RoCE have fallen into oblivion? We’ll see.

SMB Direct: Choosing A Flavor

I often get asked what to buy for implementing SMB Direct. It’s a non trivial question actually and I’m not an expert, nor do I play one on TV.  All joking aside, it’s a classical consulting answer: it depends. I don’t do free consulting in a blog post, even if that was possible, as there are many factors such as the characteristics and futures of your organization. There’s also a lot of FUD & marketing flying around. Basically in real life you only have two vendors: Cheslio (iWarp) and Mellanox (Roce/Infiniband). Hard to say which one is best. You make the best choice for your company and you live with it.

There is talk about other vendors joining the SMB Direct market. But it seems to be taking a while. This is not that strange. I’ve understood that in the early days of this century iWarp got a pretty bad reputation due to the many issues around it. Apparently offloading the TCP/IP stack to the NIC, which is what iWarp does is not an easy endeavor. Intel had some old Net card a couple of years ago but has gotten out of the game. Perhaps they’ll step back in but that might very well take a couple of years.

Other vendors like Broadcom, Emulex & QLogic might be working on solutions but I’m not holding my breath. Broadcom has DCB and has been hinting at RDMA in it’s NICs for many years but as of the writing of this post there is nothing functional out there yet. But bar the slowness (is complexity slowing the process?) it will be very interesting to see what they’ll choose: RoCE or iWarp. That choice might be the most public statement we’ll ever see about what technology seems like the best bet for these companies. But be careful, I have seen technology choices based on working/living with design choices at at another level due to constrictions in hardware & software that are no longer true today. So don’t just do blindly what others do.

Infiniband will remain a bit more of a niche I think and my guess is that RoCE is the big bet of Mellanox for the long term. 10Gbps and higher Ethernet switches are sold to everyone in the world. Infiniband, not so much. Does that make it a bad choice? Nope, it all depends. Just like FC is not a bad choice for everyone today, it depends.

Your options today

The options you have today to do SMB Direct are rather limited and bound to the different flavors and their vendor. Yes vendor not vendors.

  1. iWarp: Chelsio
  2. RoCE: Mellanox (v2 of RoCE has brought routability into the game, which counters one of iWarps biggest advantages, next to operational ease but the no fuss about DCB story might not be 100% correct, the question is if this matters, after all many people do well with iSCSI which is easy but has performance limits).
  3. Infiniband: Mellanox (Qlogic was the only other remaining one, but Intel bought it form them. I have never ever seen Intel Infiniband in the wild.

Note: You can do iWarp (and even RoCE in theory) without DCB but in all realistic high traffic situations you’ll want to implement PFC to keep the experience and results good under load. Especially the ports connecting to the SOFS nodes could other wise potentially drop packets. iWarp, being TCP/IP, will handle dropped packets but possibly at the cost of deteriorated performance. With RoCE you’re basically toast if you lose packets, it should be losses. I’m not too convinced that pure offloaded TCP/IP scales. Let’s face it, what was the big deal about lossless iSCSI => DCB Smile I would really love to see Demartek testing these things out for us.

If you have a smaller environment, no need for routing and minimal politics I have seen companies select Infiniband which per Gbps is very cheap. Lots of people have chosen iWarp due to it simplicity (which they heavily market) and routability. The popularity however has dropped due to prices hikes that came with increased demand and no competition. RoCE  is popular (I see it the most) and affordable but for this one you MUST do at least PFC. DCB support on switches is not an issue, even budget friendly DELL PowerConnect N4000 series supports it as did it’s predecessor the PC8100 series. Meaning if you have bought switches in the past 24 months and did your home work you’re good to go. Are routability and distance important? Well perhaps not that much today but as the trend in networking is heading for layer 3 down to the rack which will be more acceptable when we see a lot of the workload goodness in hypervisors (Live Migration, vMotion,yes there is work being done on that) being lit up in layer 3 it might become a key feature.

Adventures In RDMA – The RoCE Path Over DCB To Windows Server 2012 R2 SMB 3.0 Glory


On gloomy day, it was dark, grey and cold, we gave battle with RoCE & DCB (PFC/ETS). The fight was a long one, the battle field uncharted and we had only our veteran attitude towards adversity to guide us through the switch configurations. It seemed that no man had gone that far to the edges of the Windows Server 2012 empire. And when it came to RoCE & DCB meets Didier, I needed to show it that it had been conquered and was remembered of a quote in Gladiator:

Quintus: People RoCE/DCB configs should know when they are conquered.
Maximus: Would you, Quintus? Would I?


After many, many lonely & unsuccessful hours dealing with Performance monitor, switch configurations, reloads, firmware, drivers & Windows we got results:

… “it’s working” … “holey s* look at those numbers” …

On that dark day in a scarcely illuminated room, in the faint glare of the monitors even the CLI  of the switches in PUTTY felt like a grim cold place. But all that changed at as the impressive results brightened up the day and made all efforts seem worthwhile. “Didier Victor” I thought as I looked away from the screen, ‘”Once more”.image

But it has been a hard won victory. And should you fight this battle? We’ll let’s discuss this a bit now we’ve got your attention. RDMA is a learning process for many of us and neither Infiniband,  iWARP or RoCE are the one that need to win at this game. It’s you, via the knowledge you’ll gain working with RDMA technologies.

SMB Direct or SMB over RDMA comes in flavors

Infiniband (Mellanox)

That’s been here for a while. Has high cost associated (depends on where you come from) and also has a psychological barrier to it. Try discussing buying 10Gbps versus Infiniband with semi technical managerial types. You’ll know what I mean.

Deploying Windows Server 2012 with SMB Direct (SMB over RDMA) and the Mellanox ConnectX-2/ConnectX-3 using InfiniBand – Step by Step

iWARP (Chelsio / Intel)

RDMA but it’s TCP/IP offloaded to the card. It can leverage DCB but doesn’t require it.

Deploying Windows Server 2012 with SMB Direct (SMB over RDMA) and the Chelsio T4 cards using iWARP – Step by Step

RoCE (Mellanox)

“Infiniband over Ethernet” > so you “NEED” (no not a real hard requirement) DBC with PFC/ETS (DCBx can be handy) for it to work best. No need for Congestion Notification as it’s for TCP/IP but could be nice with iWARP (see above). Do note that you’ll need to configure your switches for DCB & that’s highly dependent on the vendor & even type of switches.

Deploying Windows Server 2012 with SMB Direct (SMB over RDMA) and the Mellanox ConnectX-3 using 10GbE/40GbE RoCE – Step by Step

Here’s an older overview of RDMA flavor’s pros & cons:image

Please see Jose Barreto’s excellent work on explaining SMB 3.0 over RDMA in his presentations at SNIA, TechEd and on his blog.

While I have heard of two people I have in my network working with Infiniband for SMB Direct and Windows Server 2012 (R2) most of us are doing 10Gbps. Pricing for Infiniband has a bad reputation. Not because Infiniband is super costly compared to 10/40Gbps (I’m told by most people who ask quotes are positively surprised) but when you can’t afford a Porsche you’re not shopping for a Ferrari either.  Especially not when a mid size sedan will serve al of your needs above and beyond the call of duty. On top of that you might have bought all that nice “converged network ready” 10Gbps gear some years ago. Some of us may be working towards 40Gbps but most are 10Gbps shops. My 40Gbps is “limited” to the inter links & uplinks. Meaning that we either go for iWarp or RoCE.


Which one is best of those two? Well, as the line is drawn between vendors. RoCE today equals Mellanox (yes the Infiniband vendor, RoCE is sometimes called “Infiniband on layer 4 over Ethernet layer 2”) and iWarp means Chelsio or Intel (their cards look a bit old in the teeth however).

You’ll find comparisons by both vendors claiming superiority for varied reasons. Here’s the Mellanox side & here’s Chelsio’s take & It’s good to look at your needs and map them. But I cannot declare a winner. I did notice that at least one vendor of SOFS/CiB uses iWarp. Is that a statement? And if so about what? Price? Easy of use? Perfomance/Cost?

What I do find is that Chelsio is really hacking into RoCE as you can see here,, So that begs the question are the right or are the scared of RoCE, as the Infiniband boys are out to eat their lunch?

My take on this for now

iWarp is way easier to get started. That’s for sure. RoCE  is firmware sensitive (NIC, Switches), driver sensitive (NIC). Configuring your switches (DCB) now is usually followed by a rebooting that switch (so you might not do that so easily in production and depending on where in the stack those switches live you really need to Force10 VLT or Cisco vPC, Arista MLAG  or a independent redundant switches to get away with it. RoCE loves green field. Stacking I hear you say? I don’t like stacking on that spot of the stack as firmware updates will get you to suffer through a single point of failure.

Disclaimer: RoCE in itself does not  DEMAND/REQUIRE DCB but the consensus is that it will work better, especially under heavy load. Weather SMB Direct over RoCE requires DCB is another question. For all practical purposes I’m working from the prerequisite it does for a production environment. But as you can do RoCE RDMA between to NIC with no DCB switch in between this indicates that the hard requirement for DCB is not there. Mind you not using DCB might not be smart in regards to QoS & error handling (no TCP/IP goodness handling this for you). But I’m no expert on this subject. Paul Grun however is and he’s involved with RoCE at They tend to know their stuff. Read some of the comments below this article and you’ll know a lot But PFC isn’t Walhalla either and some claim you can just forget about it and build non blocking networks. I guess you could if your pockets are deep enough Smile. And you might go a very long way without the need for RDMA. Many do … and when you talk to some network people & vendors they can’t agree either as everyone is on the same learning curve but from a different perspective. There is no one size fits all & it all depends.

iWarp doesn’t require DCB so you can get away with cheaper switches. Or, not so cheap switches that don’t support DCB (choose wisely). So cheaper switches is probably true on the low end. But, even very economically priced switches from DELL have good DCB support. Some other vendors who are more expensive don’t.

DCB is uncharted terrain for SMB Direct purposes & new to many for us. So if you want to do RDMA the easy way  … go iWARP. As said, the use of DCB for PFC/ETS is not mandatory in that case, you’ll get great results and it’s easy.  Mind you, you’ll still be dabbling with DCB if you want to do lossless magic in the switches Smile. Why you say? Well, that “converged network” story makes it kind of interesting to do so and PFC, DCBx/TLV is generic and can be leveraged for other things than iSCSI or FCoE.  And for all practical purposes SMB 3.0 with SMB Direct is a storage protocol since Windows Server 2012 made it so (CSV). Or do you do DCB for iSCSI/FCoE & iWarp for SMB Direct? After all there’s only 2 lossless queues to be had. But hey how many do you need? Choices, choices and no vast pool of experienced practitioners yet.

iWARP routes, it’s not bound by a single Ethernet broadcast domain. That could be useful info depending on your environment & needs. I’ll note that I leverage RDMA for East-West traffic, not north south & as such this could not be an issue. The time that I do “Shared Nothing Live Migration" from on premise to the cloud has not arrived yet.

The Mellanox cards in my neck of the woods were 35% cheaper than Chelsio (SFP+)

What about the scalability? “iWarp doesn’t scale that well” is stated left and right but I think that might often be based on older information. Chelsio makes a strong case for iWARP scalability. Especially when it comes to long distances, multiple hops & routing.

Again, your mileage may vary. But for “the smaller environments” who want to leverage RDMA with SMB 3.0 I’d say that iWarp is the easiest path to go & will do just fine. Now if you’re already into lossless Ethernet for iSCSI or working with FCoE you might have all the hardware you need & the experience to deal with DCB. The latter might not always be true however. Most people have Lossless Ethernet for iSCSI or FCoE set up by the vendor or consultants who’ll use well defined step by step guides. These do not exist for the RoCE variant of SMB 3.0 over RDMA.

The case for RoCE can be made as well.  Some claim that high volume of connections consumes memory when using iWARP and TCP’s flow and reliability controls are less suited for large-scale datacenters & cloud deployments due to performance issues. Where iWARP does not know multicast, RoCE does and that could be important to you.

So why did I or still do RoCE?

So why did I walk the walk? Basically because just talking the talk isn’t enough. We considered it an investment in our education. DCB is not going away (the abstraction isn’t their yet and won’t be for a while) and we need to gain knowledge of it to both handle it and make informed decisions. By the way once you go to lossless you might leverage DCB/PFC with iWarp as well just like you do for iSCSI to make it lossless (leveraging DCBx/TLV). Keep in mind that DCB is key in converged networking and as such deserves your attention. That’s why I chose not to avoid it but gave battle. DCB is all over the place when it comes to converged networking (iSCSI, FCoE), so we need to learn the good, the bad and the ugly. Until that day that perhaps, the hardware stack is that good, powerful & has so much bandwidth TCP/IP never needs it built in protection for packet loss. Hmmmmmm, I remember people saying that about 10Gbps, but then they wanted to send everything over 2*10Gbps pipes and it becomes an issue again?

It’s early days yet but you have to give credit to Microsoft for getting RDMA/DCB on the radar screen of the worlds virtualization & storage admins than ever before. It’s not a well established segment yet and it will be interesting to see how this all turns out. I do know that now that I’ve figured out a thing or two about RoCE, I won’t be intimidated & won’t make choices out of fear. And do remember that if you have plenty of idle CPU cycles & 10Gbps you might not even need RDMA. The value for me and my employers is the knowledge gained. DCB has it’s role to play but we’ll leverage iWARP or RoCE without a preference. Today you have 2 choices. RoCE is the newer one while iWarp has been around longer and both have avid proponents it seems.

I know one thing. If you need or want RDMA in any existing 10Gbps environment with minimal effort & no risk to existing switch infrastructure, you’ll use iWarp it seems.


You sit there staring at a truckload of VMs with 120GB of memory assigned in total being evacuated in +/- 70 seconds seconds, while doing a Shared Nothing Live Migration between the same hosts and without consuming CPU load …  and have DCB for SMB 3.0 running on your switches … Yes!


Remember, “What we do in life, echo’s in eternity” Winking smile You might think now that I’m a bit nutty, but I assure you that in my quest to find someone who had hands on experience configuring DCB on switches for SMB Direct with RoCE I had to turn to myself as no one seems to have done it.  I’ll be sharing more info on our setup and configurations in the future. Once you wrap your head around the concepts, you understand why things are done and how. There in lies the value for me.

Preliminary Results With Live Migration Over RDMA Speed & Useful Number Of NICs


With Windows Server 2012 R2 (Preview) we can leverage SMB to do Live Migrations. That means we can now offload the process to the NIC if they support RDMA, save on CPU cycles and potentially get VMs moves a lot faster without impacting the performance of running VMs on the involved hosts. Perhaps it’s even faster than over TCP/IP. Sounds great so let’s do some testing.

  • We have a dual port 10Gbps Mellanox RDMA card (RoCE) in each host. One pair of the ports are interconnected via a direct attach cable. The other one is connected over a Force10 S4810 switch. We’re using in box Windows Server 2012 R2 preview drivers for everything as we have found drivers not to install properly (or not at all) on this release and cause issues.
  • We are using one VM running Windows 2012 RTM with upgraded Integration Services components. This VM has 4 vCPUs and 55GB of fixed memory assigned. For this purpose we had no workload running in the VM. The servers are standard DELL PowerEdge R720 kit running the Windows Server 2012 Preview bits.


No Performance tweaking

Live Migration over RDMA in action. Here we are using 1 10Gbps RoCE RDMA NIC. Here we are moving via the NIC port that goes over the S4810 Switch.image

As you can see the entire process took 74 seconds. RDMA did not kick in until after 19 seconds had past since the start.

The CPU load remains low, which is where you’ll find the biggest benefit of RDMA  with live migrations.image

No let’s put two RDMA RoCE ports into play and see what that does for us. We now Live Migrate the 55GB memory VM in 52-54 seconds. Not bad. Again we saw over 20 seconds time pass before RDMA kicks in.image

Again we see that CPU usage remains low. This is just a quick screenshot. On a hyper-V node you’ll need to dive into Performance Monitor to get some real info.image

Let’s repeat this exercise and see what happens if we move the traffic over the NOC ports that are directly attached. That will give us an indication about the configuration of the switch. Configuring RoCE DCB  features like PFC/ETS is not exactly a well documented process at the moment and often I feel like a magician’s apprentice.

Once more we see that it takes about 20 seconds for RDMA to kick in and that the time rises to 79 seconds. It fluctuates between 74-79 seconds actually?


The CPU load was low again. So both paths seem to perform comparable.

Live Migrations over SMB seem to function faster using two RDMA ports  but not twice as fast. These are the preview bits so nothing definitive yet. And sorry, I cannot do 40Gbps or 56Gbps Infiniband tests. Unless you want to donate the gear and pay for the power, time  & reporting Open-mouthed smile.

Max Performance Tweaking

As my readers very well know I tweak my nodes for best performance. The savings of energy (power, cooling) have to come from making the most out of every node and shutting them down when not needed (Dynamic Optimization/Power Optimization in System Center). I still have a standing order to tale away any physical limitations possible for the business.

While Windows Server 2012 (R2) has made tremendous strides to better use of the available bandwidth of a 10Gbps pipes out of the box I still dive in to the BIOS to turn of the C/C1E states and set the CPU Power Management and Memory Frequency to Maximum performance. Have a look at this blog post Still Need To Optimizing Power Settings On DELL 12th Generation Servers For Lightning Fast Hyper-V Live Migrations? on how to do this with DELL Generation 12 Servers. It also contains a  link to the older generations guidance.

As you can imagine I was quite interested to see if the settings effect RDMA as well. So let’s have a look with these settings here:


One RDMA NIC used (Mellanox, RoCE, 10Gbps)image

54 seconds for that 55GB memory (fixed) VM. We also note that the delay of 19-20 seconds before RDMA kicks in has dropped to 3-4 seconds, which is quite interesting. Basically this makes it as fast as 2 RDMA NICs without performance tweaking.

Two RDMA NICs used (Mellanox, RoCE, 10Gbps)image

30 seconds flat, in a repetitive manner, for that 55GB memory (fixed) VM. Again we note that the delay of 19-20 seconds before RDMA kicks in is again 3-4 seconds. So this is about 45% better than without the power optimization.

What is the CPU doing during all this? Well taking care of the VM load, not spending it on network interrupts Smile. Again, this is a quick screenshot. On a hyper-V node you’ll need to dive into Performance Monitor to get some real info.image

By now you must all be eager to see how this compares against Live Migration over TCP/IP, Multichannel and with Compression. That’s material for other blogs.

Why am I doing this?

We need to get the most out of every € or $ we spent. It’s not that we don’t have any cash left or so but why buy more servers & higher end gear to get better results when the answer lies in the correct configuration & better choices when designing a solution. It’s going to be a while before this knowledge becomes main stream and widely available. Years probably and why wait. It takes time to experiment but the results & ROI are great. Why spend another 50.000 to another 100.000 Euro on Servers, 10Gbps cards & switch ports if you don’t need to?  Count the cost to host, power & cool them and you’ll see that this time is an investment. You could also conclude to leverage the cloud but wasting VM cycles there is also money you have better uses for, so testing will also be needed.