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Re: [libvirt] [PATCH]: Add a flag to let live migration be persistent



Daniel P. Berrange wrote:
> On Wed, May 20, 2009 at 12:06:34PM +0200, Daniel Veillard wrote:
>> On Wed, May 20, 2009 at 11:15:14AM +0200, Chris Lalancette wrote:
>>> As pointed out by ??ukasz Mierzwa <l mierzwa gmail com>, it would be nice if
>>> there was an option to automatically make a domain persistent on the destination
>>> during a live migration.  The attached patch adds this simple capability.  Note
>>> that this has to be applied on top of my previous secure migration patch,
>>> otherwise you'll have conflicts.
>>>
>>> Signed-off-by: Chris Lalancette <clalance redhat com>
>>   Patch looks fine to me, but
>>
>>> @@ -5291,6 +5292,32 @@ qemudDomainMigrateFinish2 (virConnectPtr dconn,
>>>       * object, but if no, clean up the empty qemu process.
>>>       */
>>>      if (retcode == 0) {
>>> +        if (flags & VIR_MIGRATE_PERSISTENT) {
>>> +            if (vm->persistent)
>>> +                newVM = 0;
>>> +            vm->persistent = 1;
>>> +
>>> +            if (virDomainSaveConfig(dconn, driver->configDir, vm->def) < 0) {
>>> +                /* Hmpf.  Migration was successful, but making it persistent
>>> +                 * was not.  If we report successful, then when this domain
>>> +                 * shuts down, management tools are in for a surprise.  On the
>>> +                 * other hand, if we report failure, then the management tools
>>> +                 * might try to restart the domain on the source side, even
>>> +                 * though the domain is actually running on the destination.
>>> +                 * Return a NULL dom pointer, and hope that this is a rare
>>> +                 * situation and management tools are smart.
>>> +                 */
>>> +                vm = NULL;
>>> +                goto cleanup;
>>> +            }
>>   Yup that's a hard decision here, I wonder if some asynchronous event
>> shoul not be generated, at least make sure an error is propagated in
>> some ways.
> 
> I think it is preferable to always return the VM object in this scenario,
> because it is more important for the mgmt tool to know that the migration
> itself has completed successfully and the VM is now running on the dest. 

OK, yeah, that's fine.  I just wasn't sure.  I'll respin that patch.  I guess
I'll also dump something to the logs, so at least a post-mortem analysis will
have some shot at seeing what happened.

> 
> I think there is an argument for adding an explicit API
> 
>    virDomainIsPersistent(virDomainPtr)
> 
> to allow an app to determine whether it has a config or not. Even if
> the mgmt tool did not set the 'VIR_MIGRATE_PERSISTENT' flag, they 
> may still need to find out whether it is persistent or not.

Yeah, true, although maybe we want a call that is a bit more general.  Something
like "virDomainProperties()", one of which is whether it is persistent or not.

-- 
Chris Lalancette


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