Veeam Backup & Replication leverages SMB Multichannel


Is it true that Veeam Backup & Replication leverages SMB Multichannel? That is a question that I was asked recently. The answer is yes, when you have a backup design and configuration that allows for this. If that’s the case it will even happen automatically when possible. That’s how SMB 3 works. That means it’s a good idea to pay attention to the network design so that you’re not surprised by the route your backup traffic flows. Mind you that this could be a good surprise, but you might want to plan for it.

I’ll share a quick lab setup where SMB 3 Multichannel kicks in. Please don’t consider this a reference guide for your backup architectural design but as a demo of how SMB multichannel can be leveraged to your advantage.

Proving Veeam Backup & Replication leverages SMB Multichannel

Here’s a figure of a quick lab setup I threw together.


There are a couple of significant things to note here when it comes to the automatic selection of the best possible network path.

SMB 3 Multichannel picks the best solution based on its logic. You can read more about that here. I’ve included the figure with the overview below.


The figure nicely show the capabilities of the NIC situation. To select the best possible network path SMB 3 uses the following logic:

1. RDMA capable NICs (rNICs) are preferred and chosen first. rNICs combine the highest throughput, the lowest latency and bring CPU offloading. on the processor when pushing through large amounts of data.

2. RSS capable NICs: NIcs with Receive Side Scaling (RSS) improve scalability by not being limited to core zero on the server. Configured correctly RSS offers the second-best capabilities.

3. The speed of the NICs is the 3rd evaluation criteria: a 10 Gbps NIC offers way more throughput than a 1 Gbps NIC.

Following this logic it is clear that Multichannel will select our 2 RDMA capable 10Gbps NICs over the management LBFO interface which does not support RDMA and while supporting RSS can only deliver 2Gbps throughput at best. That’s exactly what you see in the screenshot below.



So yes, Veeam Backup & Replication leverages SMB Multichannel! Please note that this did not require us to set SMB 3 Multichannel constraints or a preferred network for backups in Veeam Backup & Replication. It’s possible to do so when needed but ideally you design your solution to have no need for this and let automatic detection chose the best network path correctly. This is the case in our little lab setup. The backup traffic flows over network even when our Veeam Backup & Replication VM, the Hyper-V host and the backup target have as their management subnet. That’s the one they exist on the Active Directory domain they belong to for standard functionality. But as both the source and the target can be reached via 2*10Gbps RDMA capable NICs on the subnet SMB3 will select those according to its selection criteria. No intervention needed.

SMB Direct Support

Now that we have shown that Veeam Backup & Replication backups in certain configurations can and will leverage SMB Multichannel to your benefit another question pops up. Can and does Veeam Backup & Replication leverage SMB Direct? The answer to that is also, yes. If SMB Direct is correctly configured on all the hosts and switches their networks paths in between it will. Multichannel is the mechanism used to detect SMB Direct capabilities, so if multichannel works and sees SMB Direct is possible it will leverage that. That’s why when SMB Direct or RDMA is enabled on your NICs it’s important that it is configured correctly throughout the entire network path used. Badly configured SMB Direct leads to very bad experiences.

Now think about that. High throughput, low latency and CPU offloading, minimizing the CPU impact on your Hyper-V hosts, SOFS nodes, S2D nodes and backup targets. Not bad at all, especially not since you’re probably already implementing SMB Direct in many of these deployments. It’s certainly something that could and should be considered when design solutions or optimizing existing ones.

More SMB3 and Windows Server 2016 Goodness

When you put your SMB3 file share continuously available on a Windows 2012 (R2) or Windows Server 2016 cluster (it doesn’t need to be on a CSV disk) you’ll gain high availability trough transparent failover with SMB3 and except for a short pause your backups will keep running even when the backup target node reboots or crashes after the File Server role has failed over. Now, start combing that with ReFSv3 in Windows Server 2016 and the Veeam Backup & Replication v9.5 support of this and you can see a lot of potential here to optimize many aspects of your backup design delivering effective and efficient solutions.

Things to investigate further

One question that pops up in my mind is what happens if we configure a preferred back-up network in Veeam Backup & Replication. Will this affect the operation of SMB multichannel at all? By that I means would enabling a preferred network in Veeam prevent multichannel from using more than one NIC?

I my opinion it should allow for multiple scenarios actually. When you have equally capable NICs that are on different subnets you might want to make sure it uses only one. After all, Veeam uses the subnet to configure a preferred path, or multiple subnets for that matter. Now multichannel will kick in with multiple equally capable NICs whether they are on the same subnet or not and if they are on the same subnet you might want them both to be leveraged even when setting a preferred path in Veeam. Remember that 1 IP / NIC is used to set up an SMB session and then it detects capabilities available, i.e. multiple paths, SMB Direct, RSS, speed, within 1 or across multiple subnets.

I’ll leave the combination of Veeam Backup & Replication and SMB multichannel for a future blog post.

New SMB Instances in Windows Server 2016


I’m pretty sure you all know / remember that in Windows Server 2012 R2 one of the improvements we got for SMB 3 was the default and the CSV instance. You can take a peak here at my slide deck from a presentation I gave at the Microsoft Technical Summit in Berlin 2014 Failover Clustering- What’s new in Windows Server 2012 R2


You can also read more on TechNet here.

We have New SMB Instances in Windows Server 2016

I’m happy to see this concept being expanded in the new SMB workloads. When playing  with Windows Server 2016 TPv5 I you’ll notice that we now have gotten extra SMB instances:


So what are these New SMB Instances in Windows Server 2016 about?

Default: This is what we had before, it’s nonspecific default SMB traffic, such as file shares

CSV: Also already known since Windows Server 2012 R2. This is the CSV traffic. This got isolated for better resiliency and isolation of issues. The goal was not to let CSV SMB issues affect default SMB traffic.

SR: This stands for Storage Replica. The purpose of this instance is the same as for CSV, isolate and make SMB on the whole more resilient.

SBL: This stands for “Storage Bus Layer”, which is related to Storage Spaces Direct replication traffic. The Storage Bus Layer is now called the Software Storage Bus (SSB). Again, isolated in its own instance for better resilience and isolation of issues. You can find more on the Software Storage Bus in Storage Spaces Direct and its use of SMB here Storage Spaces Direct – Under the hood with the Software Storage Bus

Picture by Microsoft®

“SSB uses SMB3 and SMB Direct as the transport for communication between the servers in the cluster. SSB uses a separate named instance of SMB in each server, which separates it from other consumers of SMB, such as CSVFS, to provide additional resiliency. Using SMB3 enables SSB to take advantage of the innovation we have done in SMB3, including SMB Multichannel and SMB Direct. SMB Multichannel can aggregate bandwidth across multiple network interfaces for higher throughput and provide resiliency to a failed network interface. SMB Direct enables use of RDMA enabled network adapters, including iWARP and RoCE, which can dramatically lower the CPU overhead of doing IO over the network and reduce the latency to disk devices”

Funny anecdote: while doing some research I stumbled upon the MSDN article MSFT_SmbShare class. As you can see in the screenshot below the name was mistakenly put at “Storage Bus Later” but it should be Storage Bus Layer or as it’s now called the Software Storage Bus:


Now that will be fixed soon or by the time you read this blog post. All fun aside, if you want to see what that Software Storage Bus is capable off look at this blog post and video by Ned Pyle on what Claus Joergenson achieved already in 2015 with this technology. Amazing results!