From Fedora Project Wiki

< Features

Revision as of 19:02, 29 January 2013 by Crobinso (talk | contribs) (Add myself as owner, bunch of mini edits)

KVM Device Failover


Support for transparent failover between an assigned and an emulated device, allows enabling the migration and overcommit dynamically, while still gaining the performance benefits of device assignment and without disrupting the guest operation.


Current status

  • Targeted release: Fedora 19
  • Last updated: Jan 29, 2013
  • Percentage of completion: 50%

Detailed Description

For virtual machines, device assignment is the best option for performance. However, when a device is assigned to a VM, both migration and memory overcommit are currently disabled.

This feature aims at removing the performance/features tradeoff, by switching to an emulated device in a way that is almost transparent to users, for configurations where both host and guest are Fedora.

Fedora should detect that the emulated device serves as a failover for the assigned device. When requested by the hypervisor, it will stop and eject the assigned device, switching to failover. After this point, migration and memory overcommit are possible, while device configuration is preserved. Once e.g. migration completes, the reverse switch can take place.

At the kernel level, for networking, this can be done by and creating a bond in a failover configuration, and for storage, using multipath, on top of both the assigned and the emulated device.

Benefit to Fedora

Complex virt setups now have less operational caveats, which makes things simpler for users.


Work left to do:

  • kvm needs to be extended to notify the guest that the two devices are setup in a fallback configuration
  • For networking, udev and/or network manager needs to be extended to detect this and setup bonding
  • For storage, need to setup device-mapper-multipath to autodetect this configuration
  • libvirt has to be extended to specify this configuration
  • libvirt has to be extended to request failover, and ack on guest ack of the failover
  • above covers linux guests
 if possible, guest agent for windows should be extended to add this
 support in windows guests as well

How To Test

Two systems with device assignment (IOMMU) support are required to test this feature. To test the feature, specify an assigned device, start guest and migrate.

XXX: Explicit test steps here for test day

User Experience

User will see that they can specify an assigned network or storage device and still migrate the guest seamlessly.


For networking,

Contingency Plan

None necessary, revert to previous release behaviour.


Links to related upstream documentation:

Release Notes

  • KVM guests with assigned host devices can now be migrated across hosts. The assigned device will be replaced during migration with an emulated device in a transparent manner.

Comments and Discussion