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[Jim Fehlig <jfehlig@xxxxxxxx>]

Hi,

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

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?

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.

Regards,
Jim

[1]
https://github.com/openstack/nova/commit/bd30eb36bbf2c5164ac47256355c543f6b77e064
[2]
https://github.com/openstack/nova/commit/bd30eb36bbf2c5164ac47256355c543f6b77e064