UDP Connection tuning questions regarding the -b and -l options
I'm new to Iperf and looking at using it to test fibre connections where
there may be problems on one of the pair of fibres between two switches.
I have FreeBSD servers also connected to the switches that I am using to
perform the testing.
I have some questions that I hope are more related to Iperf
specifically, but I'm afraid some seem to me to be more networking in
general. Pointers to other sources of information, or useful lists that
will be friendly to an old sys admin would be appreciated if answers are
not possible because I'm too OT.
1. Using the -b option
I have the testing servers on 100 Mb links. As I increase the -b option
to near this figure I see increased datagram loss. I assume this is due
to the addition of IP packet and ethernet frame information causing
datagrams to be split into packets. Can someone confirm/clarify my
understanding?
iperf -u -c 172.18.1.1 -l 1470B -b 95m -t 5
0.0- 5.0 sec 57.0 MBytes 95.5 Mbits/sec 0.032 ms 21/40652 (0.052%)
iperf -u -c 172.18.1.1 -l 1470B -b 98m -t 5
0.0- 5.0 sec 57.4 MBytes 95.4 Mbits/sec 0.029 ms 1065/42019 (2.5%)
iperf -u -c 172.18.1.1 -l 1470B -b 100m -t 5
0.0- 5.0 sec 57.4 MBytes 95.4 Mbits/sec 0.053 ms 1783/42737 (4.2%)
1. Using the -l option
In the user guide I see in the example both the server and client have
the -l option for a test. Is this necessary on the server, or will it
accept any size (this seems to be the case from my investigation so
far)? I wanted to leave the server running without having to change its
parameters each time.
If I lower the -l option from the default of 1470B I see an increase
data loss the lower I go.
iperf -u -c 172.18.1.1 -l 1470B -b 90m -t 5
0.0- 5.0 sec 53.9 MBytes 90.5 Mbits/sec 0.017 ms 0/38463 (0%)
iperf -u -c 172.18.1.1 -l 1000B -b 90m -t 5
0.0- 5.0 sec 54.2 MBytes 90.9 Mbits/sec 0.046 ms 0/56820 (0%)
iperf -u -c 172.18.1.1 -l 100B -b 90m -t 5
0.0- 5.0 sec 35.1 MBytes 58.8 Mbits/sec 0.025 ms 230978/599009 (39%)
If I remove the -b option I see no datagram loss, but a considerably
slower bandwidth.
iperf -u -c 172.18.1.1 -l 100B -t 5
0.0- 5.0 sec 641 KBytes 1.05 Mbits/sec 0.025 ms 0/ 6563 (0%)
Can someone explain this? I have seen a previous post this month that I
think asks the same question but didn't follow the answer :-(. As I
understand it I am stressing the server nics and switches more by trying
to achieve the same bandwidth throughput with many more (and less
optimal from the hardware's point of view) packets. Hence more are
dropped by the hardware (or OS).
If this is the case I suppose I need to test the servers back to back
and then with the switches in between to see the difference the network
devices. As an alternative I can get the NIC cards/switches to report
packet loss via something like SNMP or interfaces counters available
from the command line. Is this a typical approach?
Thanks,
Jim Mozley