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Q and A: Can I use SMB3 storage without RDMA?

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Question received via e-mail:

Is it practical use SMB3 storage without RDMA or do we have a use case for production rather than development or test?
I thought RDMA would be essential for production deployment of Hyper-V SMB storage.

Answer:

RDMA is not a requirement for the Hyper-V over SMB scenario.
The most important things that RDMA can give you are lower latency and lower CPU utilization.

To give you an idea, without RDMA, I was able to keep a single 10GbE port busy in a 16-core/2-socket Romley system using a little over 10% of the CPU.
For many, using 10% of the CPU is OK in this case. With RDMA, it dropped to less than 5% of the CPU.
Those become much more important if you are using very high bandwidth, like multiple 10GbE, 40GbE (Ethernet) or 54GbIB (InfiniBand).
In those cases, without RDMA, you could end up using much more of your CPU just to do network IO. Not good.

To make a better estimate of your requirements, you need to consider:

  • Number of VMs per host
  • Number of virtual processors per VM
  • Average number of IOs per VM
  • Average size of the IOs from the VM
  • Number of physical cores and sockets per host
  • Physical network configuration (type/speed/count of ports)

With that we can think of the expected load on the CPU and on the network, and how important using RDMA would be.