I've raised a ticket with VMware for these 3 settings and I'll post the response, but I'd also appreciate any comments anyone makes.
We have two Dell R620 servers with ESXi 5.0 U1, directly connected to a Dell MD3220i iSCSI SAN. The SAN has 2 controllers and each server has 2 NICs connected to each controller. Round Robin and jumbo frames are enabled.
Performance isn't great, and I'm wondering if these settings would help...
Reducing IOs per path:
The default is 1000, I've read people saying they changed it to 1, or 3, or 8, or 8800 bytes, also that VMware don't support 1.
Disabling Delayed ACK:
It's enabled by default and sounds helpful, but lots of people say they turned it off, some saying it didn't help. VMware KB 1002598 says it may be necessary to do this with some SANs.
Disabling Large Receive Offload:
I found less info but some people advise it.
These steps aren't included in Dell's build doc but it is basic: http://i.dell.com/sites/content/shared-content/data-sheets/en/Documents/PowerVault_MD_iSCSI_Deployment_Guide_for_VMware_ESX50_Server_Software.pdf