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On May 9, 2012, at 10:08 PM, Jim Fehlig wrote:

> Hi,
> 
> I've been tinkering with improving Xen support in the libvirt driver and
> wanted to discuss a few issues before submitting patches.

Awesome!

> 
> Even the latest upstream release of Xen (4.1.x) contains a rather old
> qemu, version 0.10.2, which rejects qcow2 images with cluster size >
> 64K.  The libvirt driver creates the COW image with cluster size of 2M. 
> Is this for performance reasons?  Any objections to removing that option
> and going with 'qemu-img create' default of 64K?

As per other email, 64K seems correct.
> 
> In a setup with both Xen and KVM compute nodes, I've found a few options
> for controlling scheduling of an instance to the correct node.  One
> option uses availability zones, e.g.
> 
> # nova.conf on Xen compute nodes
> node_availability_zone=xen-hosts
> 
> # launching a Xen PV instance
> nova boot --image <xen-pv-image> --availability_zone xen-hosts ...
> 
> The other involves a recent commit adding additional capabilities for
> compute nodes [1] and the vm_mode image property [2] used by the
> XenServer driver to distinguish HVM vs PV images.  E.g.
> 
> # nova.conf on Xen compute nodes
> additional_compute_capabilities="pv,hvm"
> 
> # Set vm_mode property on Xen image
> glance update <image-uuid> vm_mode=pv
> 
> I prefer that latter approach since vm_mode will be needed in the
> libvirt driver anyhow to create proper config for PV vs HVM instances. 
> Currently, the driver creates usable config for PV instances, but needs
> some adjustments for HVM.

Agree that this is best. Once general host aggregates[1] is done, the capabilities and the availability zone will move into aggregate metadata and it will just be making sure that we have reasonable image properties to help the scheduler place the guest correctly.

Vish

[1] https://blueprints.launchpad.net/nova/+spec/general-host-aggregates 
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