KVM guest low network speed

chetzz

1/26/24, 8:43 AM

I have a KVM guest VM running RHEL 8.7 (12vCPU, 24GB RAM) and a hypervisor with 10Gb NICs. I'm getting weird results when using iperf3 on my guest machine as a client and a TrueNAS server as a server. See the image: iperf3 low network speed.

I'm not very familiar with OVS so I could not find out the issue. I have tried running the iperf3 on hypervisor as a client and everything works fine.

[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec  1.03 GBytes  8.85 Gbits/sec    0    625 KBytes       
[  4]   1.00-2.00   sec  1.09 GBytes  9.38 Gbits/sec    0    645 KBytes       
[  4]   2.00-3.00   sec  1.09 GBytes  9.39 Gbits/sec    0    645 KBytes       
[  4]   3.00-4.00   sec  1.09 GBytes  9.34 Gbits/sec    0    682 KBytes       
[  4]   4.00-5.00   sec  1.09 GBytes  9.38 Gbits/sec    0    684 KBytes       
[  4]   5.00-6.00   sec  1.09 GBytes  9.37 Gbits/sec    0    732 KBytes       
[  4]   6.00-7.00   sec  1.09 GBytes  9.40 Gbits/sec    0    732 KBytes       
[  4]   7.00-8.00   sec  1.09 GBytes  9.39 Gbits/sec    0    732 KBytes       
[  4]   8.00-9.00   sec  1.09 GBytes  9.39 Gbits/sec    0    771 KBytes       
[  4]   9.00-10.00  sec  1.09 GBytes  9.39 Gbits/sec    0    814 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  10.9 GBytes  9.33 Gbits/sec    0             sender
[  4]   0.00-10.00  sec  10.9 GBytes  9.33 Gbits/sec                  receiver

MTU = 9000

Any ideas?

Thanks.

1 + 0

networking

iperf

kvm-virtualization

openvswitch

truenas

Score:0

Server

chetzz

1/26/24, 12:54 PM

I found out what was the problem here. Physical interface MTU was still 1500 - changed it to 9000 and everything seems fine. Well, it still does not make any sense that why the bitrate with 1500MTU was only couple hundred Kbits/sec but I don't really care anymore :)

+ 1

vidarlo

1/26/24, 2:10 PM

MTU should be equal on all devices in the same subnet. Otherwise you risk that one end sends a frame that is too large for the receiving device. Different MTU ***will*** give you problems. You found one kind of problem.

Elon Musk

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EN: KVM guest low network speed

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