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Applies to SUSE Linux Enterprise Server 11 SP4

4 I/O Virtualization

VM Guests not only share CPU and memory resources of the host system, but also the I/O subsystem. Because software I/O virtualization techniques deliver less performance than bare metal, hardware solutions, that deliver almost "native" performance have been developed recently. SUSE Linux Enterprise Server supports the following I/O virtualization techniques:

Full Virtualization

Fully virtualized drivers emulate widely supported real devices, which can be used with an existing driver in the VM Guest. Since the physical device on the VM Host Server may differ from the emulated one, the hypervisor needs to process all I/O operations before handing them over to the physical device. Therefore all I/O operations need to traverse two software layers, a process that not only significantly impacts I/O performance, but also consumes CPU time.


Paravirtualization allows a direct communication between the hypervisor and the VM Guest. With less overhead involved, performance is much better than with full virtualization. However, paravirtualization requires either the guest operating system to be modified to support the paravirtualization API or paravirtualized drivers. See Section 1.2.1, “Availability of Paravirtualized Drivers” for a list of guest operating systems supporting parvirtualization.

Direct Assignment via PCI-Passthrough

Directly assigning a PCI device to a VM Guest (PCI passthrough) avoids performance issues caused by avoiding any emulation in peformance critical paths. With PCI passthough, a VM Guest can directly access the real hardware using a native driver getting almost native performance. This method does not allow to share devices—each device can only be assigned to a single VM Guest. PCI-Passthrough needs to be supported by the VM Host Server CPU, chipset and the BIOS/EFI. The VM Guest needs to be equipped with drivers for the device. See Section 10.5, “Adding a PCI Device with Virtual Machine Manager” or Section 10.6, “Adding a PCI Device with virsh for setup instructions.

Single Root I/O Virtualization (SR-IOV)

The latest I/O virtualization technique, SR-IOV combines the benefits of the aforementioned techniques— performance and the ability to share a device with several VM Guests. SR-IOV requires special I/O devices, which are capable of replicating resources, so they appear as multiple separate devices. Each such "pseudo" device can be directly used by a single guest. However, for network cards for example the number of concurrent queues that can be used is reduced, potentially reducing performance for the VM Guest compared to paravirtualized drivers. On the VM Host Server SR-IOV must be supported by the I/O device, the CPU and chipset, the BIOS/EFI and the hypervisor. . See Section 10.7, “Adding SR-IOV Devices” for setup instructions.

Important: I/O Virtualization and Live Migration

Live migration is currently not supported when using devices with PCI passthrough or SR-IOV. In case live migration needs to be supported, you need to use software virtualization (paravirtualization or full Virtualization).

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