"We are using a setup involving 2 poweredge R410's, an MD3000i and 2 Allied Telesys AT-GS950/48 gigabit switches.

We are using a dual subnet setup, which should be fully redundant.

This summer we tried enabling jumbo frames on the MD3000i, which resulted in disaster at the ESXi side. The story so far:

Without jumbo frames, latency fro ESXi to the SAN is typically low (

- ESXi 4.1 on the 2 hosts, jumbo frames enabled on the vswitch
- AT-GS950/48, jumbo frames supported and enabled
- MD3000i, iSCSI host ports jumbo frames enabled and set to 9k

When doing this, latency on the ESXi side shoots up to 200+ msec (even up to 20K).

When just using a cross cable between the SAN and the hosts, no switches, and leaving jumbo frames enabled, all is fine, latency is low.

Is there a list of supported switches? The AT switch supports jumbo frames, I have confirmed that with AT support, but somehow the frame dropped count (FDR column in the iSCSI stats on MD3000i) on the MD3000i just keeps on rising when the switch is in between.

Please advise.
"

Classic frame size mismatch somewhere in the chain symptoms. Often times, you have to add some frame above 9000 to some devices as the parameter actually includes the overhead in the frame size.

Okay, bu the switch is already at the max. 9K... So would lowering the size on the SAN end to for example 8000 resolve this, too?

"Okay, bu the switch is already at the max. 9K... So would lowering the size on the SAN end to for example 8000 resolve this, too?"

I would take everthing back to default 1500 and see if the issue goes away first. I'm not sure that is what is happening to you here. Dropped / re-assembled packets are just indicative of network issues in general. But since your messing with frame sizes, that is a good place to start.

I know that at 1500 everything works fine, it was a that setting before and all was ok, no dropped frames. I'll go ahead and do some testing with the frame sizes and keep you pasted, thanks.

There are some command line steps that have to be taken to get ESX(i) to actually do the jumbo frames. I would assume those have been performed? I can probably dig it up if you need me to.