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On host 'abc' I run:

iperf -s -w 256k

On host 'xyz' I run:

iperf -c -w 256k

And I get 900Mb. When I reverse the commands, making 'abc' the client, I get 200Mb. This means that abc's upload speed maxes at 200Mb.

There are no devices between these hosts, and they have no firewalls or rate limiting of any kind in place. They both have 1Gb ethernet ports.

Running iftop on abc shows very little Tx activity (so there isn't anything being uploaded). These are Debian hosts. Any ideas?

Ian Ling
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  • Have you verified the TSO (and other) offloading settings on the nics on both ends? Are the sysctl settings the same on both ends? Have you looked at ifconfig/netstat -s/ethtool stats on the nics on both ends to look for retransmits, errors, etc? Is the transmit queue the same on both ends? Nic vendor the same on both? Nic modprobe options customized? CPU load same on both? – Aaron Sep 10 '15 at 18:20
  • They are the same model rack server, so I assume the nics would be identical. ifconfig shows no errors on either host. sysctl configs are identical. TCO is on for both hosts. CPU load is <5% on both hosts. txqueuelen is identical. – Ian Ling Sep 10 '15 at 18:52
  • At that point, I would start looking at access port settings and ensuring they are also identical for both servers and that the bonding configuration; if any, are also the same. Also that they are on the same kernel. Beyond that, perhaps there is a problem with a nic that isn't registering as a fault. Perhaps re-seat the cards in both servers. It would all be speculation beyond this point for me. – Aaron Sep 10 '15 at 19:11

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