Tuesday, February 8, 2011

TCPDump of NFS traffic that is using jumbo frames

I had an issue a while ago where I needed to troubleshoot poor NFS performance, so VMWare support asked me to do a tcpdump of the traffic. I was using jumbo frames and the performance was poor after about 5 minutes of not using the NFS.

On the vswitch that has the NFS VMKernel (you ARE using a dedicated vswitch for NFS, right?) create a new service console.

Use an unused IP address from the range that you use on your storage LAN. If you are using a different vlan, don't forget to set that too. You will be using the vswif that is created with this previous action to do the tcpdump. Go to the properties of the vSwitch and set promiscuous mode to "Accept".

Now to do a tcpdump of your traffic.

tcpdump -i vswif2 -w ./dump_1 -s 256 (-s 256 is a buffer of 256MB I believe)

You will get a lovely SMALL file.. Not what you want. This is because vswif2 (the newly created vswif) is not using jumbo frames. To get vswif2 on jumbo frames type the following in the command prompt:

ip link set dev vswif2 mtu 9000
service network restart vswif2

Now do the tcpdump again, and watch your harddrive fill up with data. Stop tcpdump with CTRL-C (and don't wait too long, because it fills up quickly).

No comments:

Post a Comment