1. Reinstall XenTools 5.5
2. On the Virtual Machines, Click Start, click Run, type regedit, and then click OK.
3. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
4. In the right pane, make sure that the DisableTaskOffload registry entry exists. If this entry does not exist, follow these steps to add the entry:
a. On the Edit menu, point to New, and then click DWORD Value.
b. Type DisableTaskOffload, and then press ENTER.
5. Click DisableTaskOffload.
6. On the Edit menu, click Modify.
7. Type 1 in the Value data box, and then press ENTER.
8. Exit Registry Editor.
9. Restart all Virtual Machines

Fix poor IO on iScsi targets.