High performance live migration done right means using SMB Direct

I  saw people team two 10GBps NICs for live migration and use TCP/IP. They leveraged LACP for this as per my blog Teamed NIC Live Migrations Between Two Hosts In Windows Server 2012 Do Use All Members . That was a nice post but not a commercial to use it. It was to prove a point that LACP/Static switch dependent teaming did allow for multiple VMs to be live migrated in the same direction between two node. But for speed, max throughput & low CPU usage teaming is not the way to go. This is not needed as you can achieve bandwidth aggregation and redundancy with SMB via Multichannel. This doesn’t require any LACP configuration at all and allows for switch independent aggregation and redundancy. Which is great, as it avoids stacking with switches that don’t do  VLT, MLAG,  …

Even when your team your NICs your better off using SMB. The bandwidth aggregation is often better. But again, you can have that without LACP NIC teaming so why bother? Perhaps one reason, with LACP failover is faster, but that’s of no big concern with live migration.

We’ll do some simple examples to show you why these choices matter. We’ll also demonstrate the importance of an optimize RSS configuration. Do not that the configuration we use here is not a production environment, it’s just a demo to show case results.

But there is yet another benefit to SMB.  SMB Direct.  That provides for maximum throughput, low latency and low CPU usage.

LACP NIC TEAM with 2*10Gbps with TCP

With RSS setting on the inbox default we have problems reaching the best possible throughput (17Gbps). But that’s not all. Look at the CPU at the time of live migration. As you can see it’s pretty taxing on the system at 22%.


If we optimize RSS with 8 RSS queues assigned to 8 physical cores per NIC on a different CPU (dual socket, 8 core system) we sometimes get better CPU overhead at +/- 12% but the throughput does not improve much and it’s not very consistent. It can get worse and look more like the above.


LACP NIC TEAM with 2*10Gbps with SMB (Multichannel)

With the default RSS Settings we still have problems reaching the best possible throughput but it’s better (19Gbps). CPU wise, it’s pretty taxing on the system at 24%.


If we optimize RSS with 8 RSS queues assigned to 8 physical cores per NIC on a different CPU (dual socket, 8 core system) we get better over CPU overhead at +/- 8% but the throughput actually declined (17.5 %). When we run the test again we were back to the results we saw with default RSS settings.


Is there any value in using SMB over TCP with LACP for live migration?

Yes there is. Below you see two VMs live migrate, RSS is optimized. One core per VM is used and the throughput isn’t great, is it. Depending on the speed of your CPU you get at best 4.5 to 5Gbps throughput per VM as that 1 core per VM is the limiting factor. Hence see about 9Gbps here, as there’s 2 VMs, each leveraging 1 core.


Now look at only one VM with RSS is optimized with SMB over an LACP NIC team. Even 1 large memory VM leverages 8 cores and achieves 19Gbps.


What about Switch Independent Teaming?

Ah well that consumes a lot less CPU cycles but it comes at the price of speed. It has less CPU overhead to deal with in regards to LACP. It can only receive on one team member. The good news is that even a single VM can achieve 10Gbps (better than LACP) at lower CPU overhead. With SMB you get better CPU distribution results but as the one member is a bottle neck, not faster. But … why bother when we have …better options!? Read on Smile!

No Teaming – 2*10Gbps with SMB Multichannel, RSS Optimized

We are reaching very good throughput but it’s better (20Gbps) with 8 RSS queues assigned to 8 physical cores. The CPU at the time of live migration is pretty good at 6%-7%.


Important: This is what you want to use if you don’t have 10Gbps but you do have 4* 1Gbps NICs for live migration. You can test with compression and LACP teaming if you want/can to see if you get better results. Your mirage may vary Smile. If you have only one 1Gbps NIC => Compression is your sole & only savior.

2*10Gbps with SMB Direct

We’re using perfmon here to see the used bandwidth as RDMA traffic does not show up in Task Manager.


We have no problems reaching the best possible throughput but it’s better (20Gbps, line speed). But now look at the CPU during live migration. How do you like them numbers?

Do not buy non RDMA capable NICs or Switches without DCB support!

These are real numbers, the only thing is that the type and quality of the NICs, firmware and drivers used also play a role an can skew the results a bit. The onboard LOM run of the mill NICs aren’t always the best choice. Do note that configuration matters as you have seen. But SMB Direct eats them all for breakfast, no matter what.

Convinced yet? People, one of my core highly valuable skillsets is getting commodity hardware to perform and I tend to give solid advice. You can read all my tips for fast live migrations here in Live Migration Speed Check List – Take It Easy To Speed It Up

Does all of this matter to you? I say yes , it does. It depends on your environment and usage patterns. Maybe you’re totally over provisioned and run only very small workloads in your virtual machines. But it’s save to say that if you want to use your hardware to its full potential under most circumstances you really want to leverage SMB Direct for live migrations. What about that Hyper-V cluster with compute and storage heavy applications, what about SQL Server virtualization? Would you not like to see this picture with SMB RDMA? The Mellanox  RDMA cards are very good value for money. Great 10Gbps switches that support DCB (for PFC/ETS) can be bought a decent prices. You’re missing out and potentially making a huge mistake not leveraging SMB Direct for live migrations and many other workloads. Invest and design your solutions wisely!

Jumbo Frame Settings & Slow or Failing Live Migrations over SMB Direct

The Problem

I recently had to trouble shoot a Windows Server 2012 R2 Hyper-V cluster where SMB Direct is leveraged for live migration. It seemed to work, sometime perfectly but at times it but it was in “slow” motion. The VMs got queued for live migration, it took some time for it started and sometimes it would finish or it would fail. This did not happen between all the nodes. I diligently checked out the SMB Direct network but that was OK on all nodes. Basically the LM network was perfectly fine.

To me this indicated that the hosts potentially had issues communicating with each other to coordinate the live migration. But pings and such looked good, there was connectivity, on the surface all seemed well.  In the event log details we saw indications that this was indeed the case. Unfortunately I did not get the opportunity to take screenshots or copies of the events in this particular situation.

The nodes had a separate 2*1Gbps native team LAN access and backups. But diving deeper I noticed that they had set Jumbo Frames on some of those member NICs and not on others. So these setting differed from node to node and that was leading to the symptoms we described above.


You can use Jumbo Frames on your live migration network. Testing has shown this to be beneficial. When you’re doing SMB direct it won’t make such a big difference but it doen not hurt. When SMB Direct fails you’ll fall back to SMB with Multichannel and there it helps more! See Live Migration Can Benefit From Jumbo Frames. While SMB Direct (infiniband, RoCE & iWarp) know Jumbo frames the limited testing I have ever done there indicates only a small increase (2%) in throughput so I’m not sure it’s even worthwhile when doing RDMA.

When you can use Jumbo Frames on you host LAN NIC or team of NICs (handy is you use it to do backups as well)  you need to be consistent end to end. Meaning ALL hosts, ALL NICS & all switches/ switch ports. Being inconsistent in this on the cluster nodes  was what cause the slow to failing live migrations. You need to have good communications between the hosts themselves and AD. Just unplug the LAN from a Hyper-V cluster host to demo this => live migration from to that node and the rest of the cluster won’t work. Mismatching Jumbo Frames or potentially other network settings make this less obvious.  Another “fun” example to trouble shoot is a NIC team where the member NICs are in different VLANs.

I Can’t Afford 10GBps For Hyper-V And Other Lies

You’re wrong

There, I said it. Sure you can. Don’t think you need to be a big data center to make this happen. You just need to think and work outside the box a bit and when you’re not a large enterprise, that’s a bit more easy to do. Don’t do it like a big name brand, traditionalist partner would do it (strip & refit the entire structural cabling in the server room, high end gear with big margins everywhere). You’re going for maximum results & value, not sales margins and bonuses.

I would even say you can’t afford to stay on 1Gbps much longer or you’ll be dealing with the fall out of being stuck in the past. Really some of us are already look at > 10Gbps connections to the servers, actually. You need to move from 1Gbps or you’ll be micro managing a way around issues sucking all the fun out of your work with ever diminishing results and rising costs for both you and the business.

Give your Windows Server 2012R2 Hyper-V environment the bandwidth it needs to shine and make the company some money. If all you want to do is to spent as little money as possible I’m not quite sure what your goal is? Either you need it or you don’t.  I’m convinced we need it. So we must get it. Do what it takes. Let me show you one way to get what you need.

Sounds great what do I do?

Take heart, be brave and of good courage! Combine it with skills, knowledge & experience to deliver a 10Gbps infrastructure as part of ongoing maintenance & projects. I just have to emphasize that some skills are indeed needed, pure guts alone won’t do it.

First of all you need to realize that you do not need to rip and replace your existing network infrastructure. That’s very hard to get approval for, takes too much time and rapidly becomes very expensive in both dollars and efforts. Also, to be honest, quiet often you don’t have that kind of pull. I for one certainly do not. And if I’d try to do that way it takes way too many meetings, diplomacy, politics, ITIL, ITML & Change Approval Board actions to make it happen. This adds to the cost even more, both in time and money. So leave what you have in place, for this exercise we assume it’s working fine but you can’t afford to have wait for many hours while all host drains in 6 node cluster and you need to drain all of them to add memory. So we have a need (OK you’ll need a better business case than this but don’t make to big a deal of it or you’ll draw unwanted attention) and we’ve taking away the fear factor of fork lift replacing the existing network which is a big risk & cost.

So how do I go about it?

Start out as part of regular upgrades, replacement or new deployments. The money is their for those projects. Make sure to add some networking budget and leverage other projects need to support the networking needs.

Get a starter budget for a POC of some sort, it will get your started to acquire some more essential missing  bits.

By reasonably cheap switches of reasonable port count that do all you need. If they’re readily available in a frame work contract, great. You can get it as part of the normal procedures. But if you want to nock another 6% to 8% of the cost order them directly from the vendor. Cut out the middle man.

Buy some gear as part of your normal refresh cycle. Adapt that cycle life time a bit to suit your needs where possible. Funding for operation maintenance & replacement should already be in place right?

Negotiate hard with your vendor. Listen, just like in the storage world, the network world has arrived at a point where they’re not going to be making tons of money just because they are essential. They have lots of competition and it’s only increasing. There are deals to be made and if you chose the right hardware it’s gear that won’t lock you into proprietary cabling, SPF+ modules and such. Or not to much anyway Smile.

Design options and choices

Small but effective

If you’re really on minimal budget just introduce redundant (independent) stand alone 10Gbps switches for the East-West traffic that only runs between the nodes in the data center. CSV, Live Migration, backup. You don’t even need to hook it up to the network for data traffic, you only need to be able to remotely manage it and that’s what they invented Out Off Band (OOB) ports for. See also an old post of mine Introducing 10Gbps With A Dedicated CSV & Live Migration Network (Part 2/4). In the smallest cheapest scenario I use just 2 independent switches. In the other scenario build a 2 node spine and the leaf. In my examples I use DELL network gear. But use whatever works best for your needs and your environment. Just don’t go the “nobody ever got fired for buying XXX” route, that’s fear, not courage! Use cheaper NetGear switches if that fits your needs. Your call, see my  recent blog post on this 10Gbps Cheap & Without Risk In Even The Smallest Environments.

Medium sized excellence

First of all a disclaimer: medium sized isn’t a standardized way of measuring businesses and their IT needs. There will be large differences depending on you neck of the woods Smile.

Build your 10Gbps infrastructure the way you want it and aim it to grow to where it might evolve. Keep it simple and shallow. Go wide where you need to. Use the Spine/Leaf design as a basis, even if what you’re building is smaller than what it’s normally used for. Borrow the concept. All 10Gbps traffic, will be moving within that Spine/Leaf setup. Only client server traffic will be going out side of it and it’s a small part of all traffic. This is how you get VM mobility, great network speeds in the server room avoiding the existing core to become a bandwidth bottleneck.

You might even consider doing Infiniband where the cost/Gbps is very attractive and it will serve you well for a long time. But it can be a hard sell as it’s “another technology”.

Don’t panic, you don’t need to buy a bunch of Nexus 7000’s  or Force10 Z9000 to do this in your moderately sized server room. In medium sized environment I try to follow the “Spine/Leaf” concept even if it’s not true ECMP/CLOSS, it’s the principle. For the spine choose the switches that fit your size, environment & growth. I’ve used the Force10 S4810 with great success and you can negotiate hard on the price. The reasons I went for the higher priced Force10 S4810 are:

  • It’s the spine so I need best performance in that layer so that’s where I spend my money.
  • I wanted VLT, stacking is a big no no here. With VLT I can do firmware upgrades without down time.
  • It scales out reasonably by leveraging eVLT if ever needed.

For the ToR switches I normally go with PowerConnect 81XX F series or the N40XXF series, which is the current model. These provide great value for money and I can negotiate hard on price here while still getting 10Gbps with the features I need. I don’t need VLT as we do switch independent NIC teaming with Windows. That gives me the best scalability wit DVMQ & vRSS and allows for firmware upgrades without any network down time in the rack. I do sacrifice true redundant LACP within the rack but for the few times I might really need to have that I could go cross racks & still maintain a rack a failure domain as the ToRs are redundant. I avoid stacking, it’s a single point of failure during firmware upgrades and I don’t like that. Sure I can could leverage the rack a domain of failure to work around that but that’s not very practical for ordinary routine maintenance. The N40XXF also give me the DCB capabilities I need for SMB Direct.

Hook it up to the normal core switch of the existing network, for just the client/server.(North/South) traffic. I make sure that any VLANs used for CSV, live migration, can’t even reach that part of the network.  Even data traffic (between virtual machines, physical servers) goes East-West within your Spine/Leave and never goes out anyway unless you did something really weird and bad.

As said, you can scale out VLT using eVLT that creates a port channel between 2 VLT domains. That’s nice. So in a medium sized business you’re pretty save in growth. If you grow beyond this, we’ll be talking about a way larger deployment anyway and true ECMP/CLOS and that’s not the scale I’m dealing with where. For most medium sized business or small ones with bigger needs this will do the job. ECMP/CLOS Spine/leaf actually requires layer 3 in the design and as you might have noticed I kind if avoid that. Again, to get to a good solution today instead of a real good solution next year which won’t happen because real good is risky and expensive. Words they don’t like to hear above your pay grade.

The picture below is just for illustration of the concept. Basically I normally have only one VLT domain and have two 10Gbps switches per rack. This gives me racks as failure domains and it allows me to forgo a lot of extra structural cabling work to neatly provide connectivity form the switches  to the server racks .image

You have a  scalable, capable & affordable 10Gbps or better infrastructure that will run any workload in style.. After testing you simply start new deployments in the Spine/Leaf and slowly mover over existing workloads. If you do all this as part of upgrades it won’t cause any downtime due to the network being renewed. Just by upgrading or replacing current workloads.

The layer 3 core in the picture above is the uplink to your existing network and you don’t touch that. Just let if run until there nothing left in there and you can clean it up or take it out. Easy transition. The core can be left in place or replaces when needed due to age or capabilities.

To keep things extra affordable

While today the issues with (structural) 10Gbps copper CAT6A and NICs/Switches seem solved, when I started doing 10Gbps fibre cabling of Copper Twinax Direct Attach was the only way to go. 10GBaseT wasn’t an option yet and I still love the flexibility of fibre, it consumes less space and weighs less then CAT6A. Fibre also fits easily in existing cable infrastructure. Less hassle. But CAT6A will work fine today, no worries.

If you decide to do fibre, buy OM3, you can get decent, affordable cabling on line. Order it as consumable supplies.

Spend some time on the internet and find the SFP+ that works with your switches to save a significant amount of money. Yup some vendor switches work with compatible non OEM branded SPF+ modules. Order them as consumable supplies, but buy some first to TEST! Save money but do it smart, don’t be silly.

For patch cabling 10Gbps Copper Twinax Direct Attach works great for short ranges and isn’t expensive, but the length is limited and they get thicker & more sturdy and thus unwieldy by length. It does have it’s place and I use them where appropriate.

Isn’t this dangerous?

Nope. Technology wise is perfectly sound and nothing new. Project wise it delivers results, fast, effective and without breaking the bank. Functionally you now have all the bandwidth you need to stop worrying and micromanaging stuff to work around those pesky bandwidth issues and focus on better ways of doing things. You’ve given yourself options & possibilities. Yay!

Perhaps the approach to achieve this isn’t very conventional. I disagree. Look, anyone who’s been running projects & delivering results knows the world isn’t that black and white. We’ve been doing 10Gbps for 4 years now this way and with (repeated) great success while others have to wait for the 1Gbps structural cabling to be replaced some day in the future … probably by 10Gbps copper in a 100Gbps world by the time it happens. You have to get the job done. Do you want results, improvements, progress and success or just avoid risk and cover your ass? Well then, choose & just make it happen. Remember the business demands everything at the speed of light, delivered yesterday at no cost with 99.999% uptime.  So this approach is what they want, albeit perhaps not what they say.

Live Migration Speed Check List – Take It Easy To Speed It Up

When configuring live migrations it’s easy to go scrounge on all the features and capabilities we have in Windows Server 2012 R2.

There is no one stopping you configuring 50 simultaneous live migrations. When you have only one, two or even four 1Gbps NICs at your disposal,  you might stick to 1 or 2 VMs per available 1Gbps. But why limit yourself if you have one or multiple 10Gbps pipes or bigger ready to roll? Well let’s discuss a little what happens when you do a live migration on a Hyper-V cluster with CSV storage. Initiating a live migrations kicks of a slew of activities.

  1. First it is establish form where (aka the source host) to where we are migrating (aka the target host).
  2. Permissions are checked, are we allowed to do this?
  3. Do we have enough memory on the target to do this? If so allocate that memory.
  4. Set up a skeleton VM on the target host that is a perfect copy of the source VM’s  specifications and configure dependencies on the target host.
  5. Let’s see if we can get a network connection set up and running. If that works, we’re cool and can now transfer the memory.
  6. A bitmap is created to track the changes to the memory pages of the source VM’s pages. Each memory page is copied from the source host to the target host VM during which the memory page is marked clean.
  7. As long as the source VM is running memory is changing, which continues to be tracked in the bitmap and as such that page is mapped as dirty over there. In an iterative process this dirty memory is copied over again and so on. This continues until the remaining dirty memory is minimal. This will take longer if the VM is very memory intensive.
  8. The tiniest amount of not yet copied dirty memory is that part of a VMs state that is copied during “black out”. For this to happen the VM on the source host is paused, the remaining state is copied.
  9. A final check is done to confirm all is well and then the virtual machine is resumed on the target host.
  10. Any remains of the VM on the source host are cleaned up.

That’s actually a lot of work and as you can see copying the state is just part of the process. The more bandwidth & the lower the latency we throw at this part of the process becomes less of the total time spent during live migration.

If you can’t fill of just fill the bandwidth of your 10/40/46Gbps pipe or pipes & you operate at line speed, what’s left as overhead? Everything that’s not actual the copy of VM state. The trick is to keep the host busy so you minimize idle time of the network copies. I.e we want to fill up that bandwidth just right but  not go overboard otherwise  the work to manage a large number of multiple live migrations might actually slow you down. Compare it to juggling with balls. You might be very good and fast at it but when you have to many balls to attend to you’ll get into trouble because you have to spread you attention to wide, i.e. you’re doing more context switching that is optimal.

So tweaking the number of simultaneous live migrations to your environment is the last step in making sure a node is drained as fast as possible. Slowing things down can actually speed things up.  So when you get your 10Gbps or better pipes in production it pays of to test a bit and find the best settings for your environment.

Let’s recap all of the live migration optimization tips I have given over the years and add a final word of advice.  Those who have been reading my blog for a while know I enjoy testing to find what works best and I do tweak settings to get best performance and results. However you have to learn and accept that it makes no sense in real life to hunt for 1% or 2% reduction in live migration speeds. You’ll get one off  hiccups that slow you down more than that.

So what you need to do is tweak the things that matter the most and will get you 99% results?

  • Get the biggest pipe you need & can afford. Bigger pipes are always better than lots of aggregated smaller pipes when it come to low latency & high throughput.
  • Choose the best performance settings Hyper-V offers you. You can choose from TCP/IP,Compression, SMB. Ben Armstrong has a blog post on this Faster Live Migration–Which Option Should You Choose? I’d like to add that you can use NIC teaming for live migration as well and prior to Windows Server 2012 R2 that was the only way to aggregate bandwidth. Now you have more options. I prefer SMB but when I don’t have 10Gbps at my disposal I have found that compression really makes a difference. In my home  lab where I have only 1Gbps, the horror, it stopped me from going crazy Smile (being addicted to 10Gbps).


  • Optimize the power settings for your server BIOS if you want an extra speed & smoothness with 10Gbps (less so with 1Gbps). Look here An Early Look At Live Migration Over TCP/IP & Multichannel In Windows Server 2012 R2 Preview, the network traffic is a lot more stable, i.e. a flat line!  In Windows 2008 R2 this was a real need for 10Gbps or you’d be stuck at 16% max.
  • Enable Jumbo Frames for another 15-20%. Thanks to Multi Channel I can visualize this now. See also this blog post Live Migration Can Benefit From Jumbo Frames. The pictures say it all!
  • Figure out the best number of simultaneous live migrations in your environments. Well you just read this blog, so now you know.  Start at 4 and experiment upwards. Tune it back down if the speed deteriorates. The “best” number depends on your environment.

If you do these 5 things you’ll have really gotten the best performance out of your infrastructure that’s possible for live migration. Bar compression, which is not magic either but reducing the GB you need to transport at the cost of CPU cycles, you just cannot push more than 1.25GB/s trough a single 10Gbps pipe and so on. You might keep looking to grab another 1% or 2% improvement left and right  but might I suggest you have more pressing issues to attend to that, when fixed are a lot more rewarding? Knocking 1 or 2 seconds of a 100 second host evacuation is not going to matter, it’s a glitch. Stop, don’t over engineer it, don’t IBM it, just move on. If you don’t get top performance after tweaking these 5 settings you should look at all the moving parts involved between the host as the issue is there (drivers, firmware, cables, switch configurations, …) as you have a mistake or problem somewhere along the way.