Jumbo frames - what size?

[sonofdbn]

There's a lot of talk about how having jumbo frames can improve GigE performance, so I tried to set them up on my network and ran into an unexpected problem. It seems that while GigE hardware might support jumbo frames, various pieces don't always support the same size jumbo frames. There is surprisingly little information about jumbo frames (at least based on my Google skills), but I assume that the equipment should use the same jumbo frame size.

 

But in trying to do this on my setup I found that I had an SMC switch which supports "up to" 9K jumbo frames, but the Planex NIC on my PC only supports IIRC up to "7K". Fortunately unRaid lets me set the jumbo frame size, but I have no idea what size in bytes corresponds to "7K" on the Planex, but I tried 7014, having read somewhere that it should be the number of 1000's plus 14 bytes. Then I tried to set my laptop (Thinkpad X40) to the same, but there the Intel network adapter doesn't support "7K", only 9014, 4088 or 16128.

 

Can anyone confirm whether the jumbo frame sizes should in fact be the same size? And if so, are the sizes hardcoded in the hardware or is there some way of setting the frame size from the command line (in WinXP)?

[limetech]

Sizes are not typically hard-coded.  If  you look at the Properties of your network device under Advanced, there's usually a set of configurable parameters there.  For example, with Marvell, I see the Propery "Jumbo Frames" which can have one of "Disabled", "4088", or "9014".  Usually switches that "support" jumbo frames don't care what the frame size is - it's the endpoints you have to worry about.

[sonofdbn]

What I meant by "hard coded" is that there are a limited number of options - as in the example you give. My Planex NIC offers Disabled or 2K, 3K, ... 7K MTU. I wonder if 4K MTU on that is the same size as 4088 on the Intel?

 

I'm using the Starter Kit for my unRaid, and it let me use ifconfig to set the jumbo frame size to 7014 - or at least it didn't seem to object, but a) would I see an error message if I specify an unsupported parameter? and b) is 7014 supported?

 

Would I be correct in inferring from your reply that the endpoints do in fact have to have the same size jumbo frames?

[limetech]

...

 

Would I be correct in inferring from your reply that the endpoints do in fact have to have the same size jumbo frames?

 

That is my understanding.  We have not explored jumbo frames much in the last year - now that switches and controllers are more "mature" we should go back and re-examine this.  I'd be interested in the results of your experimentation.

[lovingHDTV]

Is UnRaid setup to handle jumbo frames if so what size frames?

 

thanks,

 

[sonofdbn]

I'd be interested in the results of your experimentation.

 

OK, I'll have a go and report back, but it will take a few weeks, as I'll be travelling about half the time over the next month.

[limetech]

Is UnRaid setup to handle jumbo frames if so what size frames?

 

thanks,

 

 

Yes.  Attached is the README text file for the Intel GigE linux driver.  There is a section entitled "Jumbo Frames" that talks a bit about it and shows the command to use to set up a non-standard size, ie, something like this:

 

ifconfig eth0 mtu 9000 up

 

Good luck - we have no time at the present to tinker with this.