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I've recently been researching Xen, and this has come up as a quistion for me.

Is it possible to have more than 1 dom0 running on a single Xen host?

The reason I ask is that this is very similar technology to IBM's Power VM and in particular the VIOS used to own the hardware and manage it on the other LPARs behalf. In our company using LPARs and VIOS we always have 2 VIOS per host so that if one VIOS fails for some reason the other continues to provide access to the hardware for the other LPARs.

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As far as I am aware, it is not possible to have two Xen Dom0 kernel's running simultaneously on one physical host. The Xen Dom0 kernel takes over the host completely. Not only is it controlling I/O, but all aspects of the host machine. Xen Dom0 is the hypervisor.

If you're concerned about fail-over, it is possible to connect multiple physical Xen Dom0 hosts together in a Xen resource pool tied to a SAN for storage. That way if one Xen host fails, you can use xenMotion to move the virtual machine to another host automatically.

XenMotion is available in Citrix XenServer or the Xen Cloud Platform (free).

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  • @Sean: The Hypervisor starts first. Next is the Dom0. Look at the console during boot...
    – Nils
    Commented Oct 25, 2011 at 20:47
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Basically the Dom0-functionality is provided through a set of python-scripts. These scripts control the resource-pool and provide an api for the hypervisor.

If these control scripts hang or crash a "reload/restart" should revive them (in theory). I had some of these cases in the past (after a bad xen-patch from SLES).

The reload/restart did NOT work as expected and left one of these conditions:

  • The DomUs were still running and accessible, but I could not control them any more
  • All DomUs were shut off
  • Everything seemed all right, but I could not create new DomUs

In the last case I was able to live-migrate the DomUs to the other node (I use XEN with a 2-node cluster) in the other cases I had to failover, moving the DomUs offline.

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