HYPER-V Guest shows 10Gig Connection but Transfer Rate to Other Machines on GigE Network Is 100Mb or less

Question

HI -
I have recently moved from VMWare ESX to HYPER-V and love it! However, the network adapter shows 10gig and the Virtual Switch shows 10gig.  My actual host adapter speed is set to 1GIG FULL.  I am attached to a CISCO 4510 Switch.  I have tried disabling certain TCP settings like RSS, TCPOffloading, Chimney, and DisableTaskOffload - set to enable for TCP Params.  I can generally transfer from VM to VM or VM to HOST and ungodly speeds. But, when I transfer from VM to a system or server outside of the VM environment, network speed is right at 100mb.. I have tested this with Bandwidth Meter Pro.  Is there something in the host server I am missing?  I have not noticed any QoS or anything...  Please help  !   ;-)

Answer 1

Hi Ron,
The speed of the files transfer between VM and Host or VM and VM really depends on the route the communication uses and the speed of the processor.
According to the architecture of the Hyper-V, a VM to VM file transfer will go VSC --->VMBus--->VSP--->VMswitch--->VSP--->VMBus--->VSC.  Think of the VSP as a port on the VMSwitch.
As to your question, the file transfer rate on a VM to VM file transfer will be primarily determined by the speed of the processor as the entire event takes place in software. The VMswitch will report its speed as 10Gbps, but the actual rate is completely dependent on the speed of the processors and the load on the machine. It can be much faster than 10Gbps, or it can be quite a bit slower.
For "external" virtual networks, we only use the physical NIC if the traffic is between a virtual NIC and an external node. VM to VM traffic is still only bound by CPU. In other words, some connections are bound by the physical NIC and some only by the CPU.
If the host and the VM are attached to the same Hyper-V switch, then they are just like two VMs attached to the same Hyper-V switch.  You can think of the Hyper-V like a physical switch, but it is actually a software switch with ports just like a physical switch has.  In this case the data transfer speed is limited only by how fast the Hyper-V switch software can execute the instructions. This may be slower or faster than the nominal 10Gb/s. Since no hardware is involved, the hardware doesn't affect the speed of the transfer.
If the host and the VM are attached to different Hyper-V switches, or the host is directly connected to the network (i.e., not connected to a Hyper-V switch), then the VM to Host communication will go via the external network, the physical network that joins the VM’s NIC to the host’s NIC.  In this case the data transfer rate is limited by the slower of the Hyper-V switch(es) involved, the NICs involved, and the physical network components traversed by the data.  Without knowing the details of the data loads, the hardware and software components, the host processors, memory, etc., it isn’t possible to determine what the limiting factor will be.
We run file copy tests between VM's running on External and then on Private and noticed a significant improvement on private where it took ~30mins vs about 4 hours on External.
Best regards,
Vincent Hu