Poor TCP throughput on Gigabit Network

Hi Gang,

I have two FreeNAS boxes with one replicating Snapshots to the other.

Using IPERF to check I seem to be getting really poor TCP network performance and I noticed the replication is pegged at the speed shown in the picture. However if I do a transfer to the servers via SMB I get much higher transfer speeds. I checked UDP with IPERF and I got 900+ Megabits/sec.

Has anyone any ideas as to what is happening or throttling the network speed?

See attached pic.

What nics are you using? Realtek?

Hi David, thanks for replying.

They are the onboard nic’s in the HP Microserver. What I can find online says it is a NC 107i PCi Express Gigabit 10/100/1000.

The curious thing is that iperf in UDP mode states high 900’s Mbits/sec, a SMB copy shows a transfer speed of about 300+ Mbits per second. But Iperf and particularly SSH snapshot replication are stuck at this low 90’s figure I have 16gb of RAM in both machines and both are running the same version of FreeNas 11.3-U2.1. I checked the CPU and it’s not struggling when the replication is happening.

This is a bit puzzling so today I’m going to get the machines attached to each other via 3m cat6 cables and a single gigabit switch to cut out all the current intervening wiring and any interference they may be getting elsewhere to see if that changes anything. I’ve considered a cross over cable to remove the switch.


Well there you go. I have literally just connected them together with only a single switch between them and TCP Iperf is now showing 940Mb/sec. I wasn’t expecting that! Ok so it looks like some part of the intervening infrastructure is the choke point.

I’ll keep you posted.


David, I’ll also put a couple of big files on the host machine and check the replication speeds this evening.


Hi David, It worked a treat replication back to where it should be. Now the only thing is to find the choke point.