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On Sep 6, 2011, at 9:57 AM, Alex Lyakas wrote:

> Greetings everybody,
> I have been reading the nova code (pretty close to the latest version) and I have a couple of questions regarding it.
>  
> Live migration:
> I see that VIR_MIGRATE_NON_SHARED_INC flag is used, meaning that at destination, the same backing file should be available. (Indeed, the images that are created for instances use backing files in '_base' folder).
> Question 1: how it is ensured that the backing file on the destination really exists? I.e., '_cache_image' has to be called there for the same image. It looks like this is not ensured at all. If the same image was never spawned earlier on destination, the backing file will not exist there. I verified this and indeed there was no backing file on the destination.

live migration should have _base in shared storage so it doesn't need to move.  If you are talking about block_migration, then this could definitely be a bug.

> Question 2: even when the backing file does exist, the image created at the destination looks incorrect, it is not using the backing file! When querying the file on the destination, after live migration, I don't see that it's backed by the cached file:
> dst# qemu-img info /var/lib/nova/instances/instance-00000441/disk
> image: /var/lib/nova/instances/instance-00000441/disk
> file format: qcow2
> virtual size: 3.0G (3221225472 bytes)
> disk size: 50M
> cluster_size: 65536
>  
> While on the source it is backed:
> src# qemu-img info disk
> image: disk
> file format: qcow2
> virtual size: 3.0G (3221225472 bytes)
> disk size: 50M
> cluster_size: 65536
> backing file: /var/lib/nova/instances/_base/af3e133428b9e25c55bc59fe534248e6a0c0f17b (actual path: /var/lib/nova/instances/_base/af3e133428b9e25c55bc59fe534248e6a0c0f17b)

again, if you are referring to block_migration, this could be a bug.

>  
> And indeed, on destination, when I log in into the VM, its image disk is corrupted. Various tools like 'less','vi',sudo' fail to run.
> When I changed the flag to VIR_MIGRATE_NON_SHARED_DISK, then the whole image was copied, and this time the instance on the destination was fine. So it looks there is some bug on that path when using VIR_MIGRATE_NON_SHARED_INC flag.
>  
> add_fixed_ip_to_instance path:
> From the code it looks like per each network, an instance has exactly one virtual interface, meaning exactly one MAC address. However, it looks like this is possible to have multiple fixed_IPs per same virtual_interface. This is reflected in NetworkManager.get_instance_nw_info(), when adding the
> info['ips'] = [ip_dict(ip) for ip in network_IPs]
> entry.
> (On the other hand, in LibvirtBridgeDriver._get_configurations() only the first IP is taken from this list. )
> Although I see that NetworkManager._setup_network() is called on that path, and even generates a new MAC address (in case of FlatDHCP), but a new virtual_interface is not created.
> What is the purpose of this ability? On the instance we still see a single network interface per each network, correct?

tr3buchet.  Comments here?  Not sure there is a need to have multiple ips on the same nic.
>  
> Offline migration ("resize"):
> I see that when using libvirt, this path is not implemented; it is implemented only for Xen. It looks like the following methods need to be implemented: migrate_disk_and_power_off() & finish_migration(). Is there any special reason for not implementing those methods with libvirt?

Just that no one has done it.  We would love to have this supported in libvirt.

>  
> Thanks everybody,
>   Alex.
>  
>  
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