I experienced something interesting the past week for a client. Slow performance on iSCSI, SAN and VMWare when large throughput or heavy network i/o was occurring. They had a setup like this:
VMware > All VM’s on a big fat volume iSCSI to the SAN > SAN < 1 TB volume
Inside the guest (Server 2012 for example), you then make an iSCSI connection to the SAN to run their db’s, file shares, etc. They were using Sage 2014 and it was VERY slow especially when printing or running reports. It took me what seemed like FOREVER to figure it out BUT in the end it was all about the MTU. If you don’t know what MTU is, go look it up :P
In the end make sure your MTU matches this with a Dell setup:
SAN interfaces = MTU 9000 (they had a Dell Equallogix PS4100 – that might be auto configured, I think it is)
Switch where SAN & VMWare connect = MTU 9252 (they had a Dell Force10 S60)
VMware = MTU 9000 (that means ALL vswitches)
Note that you ALSO want MTU on the interface where your VM’s talk to the rest of your network. For me that was the vmkernel port and the physical switch ports on the core switch that connected the physical to the virtual – MTU 9252.
There’s a decent kb from VMWare that pointed me in the right direction. Oh, I also believe I disabled the ACK on the VMware iSCSI connector inside the VMWare server based on another KB but this didn’t really help me as far as I know, I have not turned it back on. Again, check that out online if the MTU confirmation doesn’t work for you it might be the ACK on the iSCSI connector.