I have recently upgraded much of our infrastructure (VMware) to Fedora 12. One totally unanticipated side effect has been a dramatic drop in throughput, particular from the web servers.

The issue seems to be to do with MTU. Small webpages work fine, large ones are VERY slow but do eventually arrive. I am seeing ICMP messages relating to fragmentation. It seems, although it is difficult to tell, that the web servers are sending packets that are bigger than 1500 bytes. Almost as though the MTU is set for jumbo frames even though ifconfig reports 1500 as does esxcfg-nic.

We already have talked here about it, check MTU size on every router/interfaces, somewhere could be less then 1500.

Well, after some experimentation, the culprit has turned out to be open-vm-tools. It would seem the implementation of the vxnet driver in this software takes no notice of the interfaces MTU and send packets without fragmentation. Removing open-vm-tools and reverting to the pcnet32 driver fixes the issue. Seems I will need to use the VMware tools package after all :-(