Monday, 8 October 2012

DirectPath IO

This feature allows a virtual machine to access the physical PCI/PCIe device directly bypassing the hypervisor using IO Memory Management Unit (MMU). This will offload the processing of IO for this VM from physical host CPU to physical NIC directly which is the main advantage.
Enabling Direct IO for a VM will disable many features:

  • Hot adding and removing of virtual devices
  • Suspend and resume
  • Record and replay
  • Fault tolerance
  • High availability
  • DRS (limited availability. The virtual machine can be part of a cluster, but cannot migrate across hosts)
  • Snapshots

If Cisco UCS FEX is used, those features will be enabled


  • The virtual machine must be on hardware version 7 or later.
  • Intel Virtualization Technology for Directed I/O (VT-d) or AMD I/O Virtualization Technology (IOMMU) enabled in the BIOS of the host.
  • PCI Devices are connected to the host and marked for passthrough


  1. Select the physical host and Navigate to Configuration Tab > Advanced Settings in Hardware Section > Configure Passthrough Hyperlink.
  1. Select the physical NICs which will be used for Direct Path IO
  1. You need to reboot the Host for those NICs to be ready. Note the sign before and after reboot.
  1. Edit VM Settings and Add New PCI devices (the VM should be powered off for adding new PCI device).