High latency to MD3000i when using jumbo frames

"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.
"

"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?"