Poor network throughput in Hyper-V guests

Posted on 2015-01-06
Last Modified: 2015-03-12
Here's the situation...

3 x Windows 2012 R2 hyper-v host machines.

Each host has 4 x Gb NICs connected as follows:

Host A
nic1 - host management subnet
nic2 - vm subnet1
nic3 & 4 - vm subnet2 (teamed)

Host B
nic1 - host management subnet
nic2, 3, 4 - vm subnet2 (teamed)

Host C
nic1 - host management subnet
nic2, 3, 4 - vm subnet3 (teamed)

The teamed nics are aggregated on a layer 3 switch using LACP and setup in Windows using LACP with Dynamic load balancing.

The hosts and switch are not in production - so there is no/negligible background data transfer taking place.

If I copy data between the POSE on any two hosts, transfer speed is around 1Gbps - which is what I would expect as the single NICs connected to the management subnet would be used.

If I copy data between a VM on host B and a VM on host C I would expect speeds of >1Gbps given that LACP and dynamic load balancing are being used across 3 x Gb teamed nics.  However, the transfer speed is very erratic and jumps up and down from 0 to 24Mbps.  Pinging between the same VMs produces equally erratic results - 1ms, 180ms, 5ms etc.

Similar results are gained when copying between VMs on host A and host B that are on the same subnet.

If I copy data between VMs on the same host using the same subnet (which as far as I'm aware should never actually reach the physical switch and so be very fast) - speed is around 150Mbps.

Initially this appeared to me to be something to do with the NIC teaming.  However, if I copy data from a VM on host A connected to subnet 1 using a single NIC I still only get the slow/erratic speeds.  What I would say is this particular VM is multi-homed with subnet 2 using the teamed NIC - so could still be related to teaming.

All VMs are gen1 using synthetic NICs.

Any idea what is going on?
Question by:devon-lad
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LVL 58

Accepted Solution

Cliff Galiher earned 500 total points
ID: 40534531
Regarding copying data between two VMs on the same host, one thing none of your description covers is disk configuration or VM VHD placement. A solid 150MB/s may be perfectly normal if the server has reached disk I/O saturation because of the copy from and to the same physical spindles, number of disks, etc.

As far as the rest, I would be inclined to think it isn't teaming, but something closer to hardware. *MOST* switches LACP implementations leave a lot to be desired and more often than not kills performance. I only recommend it with high end Cisco and Procurve switches and, even at that, only in specific implementations. The cost of a high end core switch tends to be so high that you can get more bang for your buck just by going to 10Gb NICs and switches for a smaller network.

If these are Broadcom NICs, go in and disable VMQ. Windows doesn't use VMQ at 1Gb speeds anyways, but a persistent driver bug still kills performance on Broadcoms. Although usually that'd surface in your POSE copy tests too. Still worth pointing out.
LVL 88

Expert Comment

ID: 40534599
You don't tell us anything about the guests. What OS are they? Are the Integration services installed? Newer m$ OS's have those included, but older ones don't. Besides, it always is a good idea to install them manually, even if they are included with the OS. OS updates can also help. With non m$ OS's they won't be installed by default, so there you must do that manually anyway.

Author Comment

ID: 40535108
Disk configuration -  it's all on a fibre channel SAN - slowest component is 6Gbps.  If I do a copy from one host to itself there are no speed issues - only when using the VMs.
Switch - it's an HP 1910 switch - would you see these as falling under the category of "LACP implementations leave a lot to be desired" ?
NICs - yes these are Broadcom ones, I had wondered about VMQ.  Surely a VMQ issue is related to VMs only and wouldn't affect copies between physical hosts?  I will try disabling to see if there's any effect.

rindi - guests are all Win 2012 R2 with integration services installed and all updates.
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Author Comment

ID: 40535201
Cliff - after disabling VMQ on all NICs there is a marked improvement in transfer speeds.  Still slightly erratic - but getting near 1Gbps most of the time.  The LACP teamed NICs don't appear to be getting anything over this though.

Author Comment

ID: 40535272
Ah hangon, seem to remember that no single transfer process will ever get more than the maximum speed of a single NIC.  Only way to get more is if you have more than one process transferring data.  Is that right?
LVL 58

Expert Comment

by:Cliff Galiher
ID: 40535860
That depends on the LACP implementation (again.) Most LACP switches do load balance based on a hash of packet data that makes it per-stream/flow. So getting above the speed of a single NIC requires multiple flows. Higher end switches have smarter algorithms though so getting better throughput even with a single stream is possible. But if you are seeing the upper limit is 1Gb then you are probably dealing with a basic LACP balancer.

Author Comment

ID: 40660619
Cliff - I have a follow on question if you're able to take a look


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