using Dell 6248 with Gigabit ports. You need to make sure if using Jumbo Frames that its supported by your switch. Make sure your packets are not getting fragmented. Use something like this test from your xenservers to your DSS server ISCSI IP's: ping -M do -s 8972 -c 10 10.10.10.1 (where 10.10.10.1 is your dss ip addr) If the packets are fragmenting it will tell you, if they are ok then the repsone will be 10 normal looking ping responses.

Your Xenserver nics have to have MTU=9000 set and the same on the DSS side. On the Dell 6248 we have to set each port that we want to do Jumbo Frames to an MTU of 9016 which kicks it into Jumbo Frame mode. Check the docs for your switch to see if there is anything you have to do to enable JF's.