[libvirt] Execute script on migration

[Daniel P. Berrange]

On Mon, Oct 12, 2009 at 12:37:22PM +0200, Radek Hlad?k wrote:
> Short question: What is the best way to run some script on VM migration?
> 
> Detailed description:
> We use own system for volume management. Its based on md and we found it very
> flexible. Basic idea is that we have md mirror everywhere we can. This suits
> great for libvirt: We have a disk on iSCSI server, its "partioned" using
> device mapper, the resulting "partition" is member of md raid1, that raid is
> exported via iSCSI to the libvirt node, it is on the node included in another
> md raid1 and that "final" md raid is used as storage by the VM. This provides
> high level of abstraction that allows us to migrate  the storage to another
> disk, another iSCSI server, etc...
> The only glitch is on live migration of the VM. It is possible to assemble and
> run the array on two nodes. Also all superblock modifications are done on
> first write to the array. However it would be more secure to stop the array
> after the source vm stops and before destination vm starts.
> What is the best way to run some script in this particular moment? I know we
> could create a new storage driver but I am not sure that it is the best option.

There is no explicit support for running a script upon migration in a
transactional manner.

It ought to be possible to create a small daemon that listens to libvirt
domain lifecycle events, and when an event occurs, it would then invoke
a script corresponding to the event. These events are asynchronous to 
any of the operations though - you'll receive them some fairly short time
after the operation is complete. There are quite a few you'll see during
migration. On the source side you'll see a STOPPED event, possibly with a
detail message of MIGRATED if the hypervisor supports that info (only
QEMU does, not Xen). On the dest side you'll see a STARTED event, again
possibly with a detail of MIGRATED. If doing non-live migration you'll
also see RESUME/SUSPEND events on dest/source hosts.

If you just want to cleanup the volumes on the source after migration,
then you could use the STOPPED event to trigger cleanup. There's no
way to insert any script between the stop on source & start  on dest
sequence.  

NB, storage drivers won't help you here - they are not involved in
migration at all

Daniel
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