July 09, 2015

VMDirectPath I/O

VMDirectPath I/O

VMDirectPath I/O allows guest operating systems to directly access an I/O device, bypassing the virtualization layer. This direct connection frees up CPU cycles and improves performance for VMware ESXi hosts that utilize high-speed I/O devices, such as 10 GbE devices.

A single VM can connect to up to four VMDirectPath PCI/PCIe devices.

The disadvantages of using VMDirectPath I/O on a VM include:
  • Unavailability or restrictions of vSphere features such as vMotion and DRS
  • The adapter can no longer be used by any other virtual machine on the ESXi host
A known exception to this is when ESXi is running on Cisco Unified Computing Systems (UCS) through Cisco Virtual Machine Fabric Extender (VM-FEX) distributed switches.

DirectPath I/O allows virtual machine access to physical PCI functions on platforms with an I/O Memory Management Unit.

The following features are unavailable for virtual machines configured with DirectPath:
  • 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
"VMDirectPath I/O enables a virtual machine to directly connect to a physical device such as a network card or storage adapter."

In the case of networking, instead of using an emulated network device, such as E10000, VMDirectPath I/O enables the virtual machine to bypass the hypervisor and directly access a physical NIC.


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