Re: [Qemu-devel] [PATCH 3/8] migration: add new capability test-only

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Denis V. Lunev (den@openvz.org) wrote:
> On 10/07/2015 06:05 PM, Eric Blake wrote:
> >On 10/07/2015 12:20 AM, Denis V. Lunev wrote:
> >>From: Igor Redko <redkoi@virtuozzo.com>
> >>
> >>This patch declares a new migration capability that allows
> >>to distinguish between true migration and the test.
> >>
> >>An alternative is using a custom URI but in this case the
> >>following problems should be solved:
> >>1/ QEMUFile abstraction hides the real transport type
> >>2/ We must be ensured that VM will not be stopped due to
> >>“successful migration”
> >>
> >>Possible solutions:
> >>1/ Using global variable analogous to
> >>s->enabled_capabilities[MIGRATION_CAPABILITY_TEST_ONLY] that
> >>will be initialized according to the migration type at the
> >>beginning of the migration process
> >>2/ Additional hook, presence of implementation of which allows
> >>to decide whether the migration is true
> >>
> >Do we want to name the capability 'x-test-only', since this series is
> >experimental, to make it obvious that production code should not rely on it?
> >
> actually the answer is 'no'. This feature is designated for production.
> May be the name is just wrong or description is bad :) Yep, I know this
> for sure...
> 
> OK. The idea of this feature is to estimate efforts required to
> perform migration and downtime during that migration.
> This estimate is necessary to decide whether we are able
> to automatically move the VM from one node (loaded) to
> different node with less load.

Yes, this is the use we had also been considering.

> Should we rename 'test' to 'estimate' or something like that
> to make the situation clearer?
> 
> On the other hand this 'estimate' state would be very useful
> for testing that is why I have proposed this code to John.
> At least we were able to catch recently fixed hang on migration
> cancel (in pipeline already).
> 
> I am completely open to any idea.

I can see how 'test' might confuse with other test things; but
I'm not too fussy about the name.  I was thinking 'null' if it
was a transport name.

Dave

> 
> 
> >>+++ b/qapi-schema.json
> >>@@ -540,11 +540,14 @@
> >>  # @auto-converge: If enabled, QEMU will automatically throttle down the guest
> >>  #          to speed up convergence of RAM migration. (since 1.6)
> >>  #
> >>+# @test-only: If enabled, QEMU instead of migrating will estimate migration
> >>+#          time basing on given downtime and current dirty page rate
> >If we do decide to add this to mainline, it would need 'since 2.5' (or
> >whatever version) documentation.
> >
> Sure!
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK