I would like to increase my MTU=1500

[Jorge Cocomess]

I am running RHAS 3.0 on HP RX4640 with a Gigabit NIC, connected to a Gigabit Procurve. The current MTU on eth0 is set to 1500 - I would like to improve the performance of my NIC, I would like to increase this value.
1) What would be the right value with the H/W I've posted above.
2)Would I use this command to set the MTU, "ip link set eth0 mtu XXXX".

Please let me know if you have done this.

Thank you in advance.

J

[Ajay Agarwal · Accepted solution]

To change the setting temporarily issue the following command as root:

# ip link set dev eth0 mtu XXXX

To make the setting permanent for eth0, edit the configuration file /etc/sysconfig/network-scripts/ifcfg-eth0 and add the line MTU=XXXX as shown below:

DEVICE=eth0
MTU=XXXX
BOOTPROTO=dhcp
ONBOOT=yes
TYPE=Ethernet

Then, restart the interface (as root):

service network restart eth0

[Jorge Cocomess]

Ajay - Thanks for your response. Would you know how much I should increase/degrease the MTU for testing?

Thanks,
J

[Stuart Browne]

http://sd.wareonearth.com/~phil/jumbo.html

This is a good discussion on Jumbo Frames.

[rick jones]

Before you go to enable "JumboFrames (aka an MTU of 9000 bytes) you should triple-check that the "Gigabit Procurve" you have actually supports JumboFrames.

While many if not most or even all of the "newer" ProCurve switches include support for JumboFrames, not all the older ones do.

Everything in the broadcast domain (systems, switches etc) must support JF if you are going to enable it, and you really need to enable it across the entire broadcast domain or you will start having strange connectivity problems.

If you happen to have occasion to upgrade your OS to one based on a 2.6 kernel (ie SLES 9, 10 RHmumble 4) you can enable something called "TSO" or TCP Segmentation Offload. This is something of a "poor man's" Jumbo Frame - the host gets to send larger segments to the NIC, and the NIC actually resegments them to fit the "real" MSS of the TCP connection. So, you talk to non-TSO remote systems just like normal, and get much of the benefit on the sending side as you would from JumboFrame.

When you upgrade your kernel, you will be able to use ethtool to enable/disable tso on a given interface. Assuming of course the interface in question supports TSO. That brings us to the next question - precisely _which_ gigabit NIC do you have in your rx4640? I know one can enable TSO on the A9900A (dual-port NIC) but I'm not sure if one can enable it on the A7061A (single port) or not.

And finally, backing up a few steps - what about the performance of your NIC do you find lacking and what makes you think that JumboFrame would be the solution? Perhaps there is something else to consider.