Creating VMKernel connections that support Jumbo Frames

[coldaddy]

I already have VMKernel connections in a particular vSwitch that support NFS connectivity, and I have datastores already mounted. Is there anyway to alter the existing connections or must I recreate them with the esxcfg-vmknic -a command to change the MTU value?

Supposing the answer is that I must recreate the connections, how do I specify which vSwitch to place the VMKernel connections in. The command syntax I have is:

esxcfg-vmknic -a -i <ip address> -n <netmask> -m <MTU> <port group name>

But the esxcfg-vmknic command does not seem to allow have an option to specify which vSwitch to create in.

Thanks,

Steve

[coldaddy]

Looks like the answer is to create a vSwitch that supports the MTU size I want then when I create vmkernel connections within that vswitch they will use the vswitch's MTU size. Is this correct?

[jhanekom]

I get what you're trying to do; however, while ESX 3.5 does have support for Jumbo frames for VMs, the Software iSCSI initiator and NFS module included with ESX does not support jumbo frames.

See this blog post on the topic: http://www.yellow-bricks.com/2008/01/03/what-about-those-jumbo-frames/

[coldaddy]

...bummer. Any ideas what VMWare's plans are concerning support for this?

Steve

[RenaudL]

But the esxcfg-vmknic command does not seem to allow have an option to specify which vSwitch to create in.

That's because the PortGroup name is sufficient to know which vSwitch the vmknic should be connected to (PortGroup names are unique among a host).

Looks like the answer is to create a vSwitch that supports the MTU size I want then when I create vmkernel connections within that vswitch they will use the vswitch's MTU size. Is this correct?

Well, that's a requirement for JFs to work properly, but it won't automatically change the MTU of your vmknics/VMs/etc, so you still have to provide the "-m" argument when creating your vmknic.

...bummer. Any ideas what VMWare's plans are concerning support for this?

Yeah, we know 😕 iScsi over JFs is trickier than it looks like, and we definitively don't want to be too adventurous with storage. Be assured that we're working on it though.

[coldaddy]

RenaudL, I am concerned about NFS and JF, not iSCSI...but I'm sure many out there are concerned about that. Sounds like you are an "insider"...can you confirm JF does not work with NFS (vmkernel connections supporting NFS connectivity)?

[RenaudL · Accepted solution]

AFAIK, even though we do not support it, NFS over Jumbos seems to work fine.

I wasn't involved in this decision at all, so I don't know the precise reasons leading to its lack of support in ESX 3.5. It may just be insufficient QA or scary data corruption issues, I don't know.

I know some of the adventurous people around here tried it, maybe they can relate their experience.

[RenaudL]

I'm correcting myself here: I may have hastily repeated what other people were saying, and it seems that we indeed support NFS over Jumbo Frames.

Sorry about the confusion.

[jhanekom]

Hi Renaud

The ESX 3.5 Release notes (http://www.vmware.com/support/vi3/doc/whatsnew_esx35_vc25.html) specifically state the following...:

"Jumbo frames are not supported for NAS and iSCSI traffic. They are limited to data networking only."

[RenaudL]

That's why I said "it seems": I couldn't find any reference to the non-support of NFS with Jumbo Frames in the ESX Server Configuration Guide (http://www.vmware.com/pdf/vi3_35/esx_3/r35/vi3_35_25_3_server_config.pdf).

Our documentation is misleading & inconsistent, I'll open an internal ticket to track this and maybe fix the documentation.

I just tried to help at first, and ended up confusing everybody...

[jhanekom]

I often have the same problem, especially with my wife!