Re: [Qemu-devel] [PATCH 7/8] migration: new migration test mode

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

* Denis V. Lunev (den@openvz.org) wrote:
> On 10/07/2015 06:08 PM, Eric Blake wrote:
> >On 10/07/2015 07:56 AM, Dr. David Alan Gilbert wrote:
> >>* Denis V. Lunev (den@openvz.org) wrote:
> >>>From: Igor Redko <redkoi@virtuozzo.com>
> >>>
> >>>In this patch the ability to start a migration with test-only
> >>>capability was added. It allows to gather the guest VM’s memory
> >>>usage statistics avoiding time and memory overheads and real
> >>>data transmission.  New MIGRATION_STATUS_TEST_COMPLETED was
> >>>added to distinguish between test migration and true migration
> >>>success states.
> >>Why isn't this just a new transport? i.e. I could do this just by doing
> >>a migrate to test:   ?
> >>
> >>It seems simpler and avoids some of the special casing?
> >Indeed. Even if the new state can only be triggered by using a new
> >option, the fact that the new state exists in introspection means
> >clients have to figure out what to do with it; while a new transport
> >would not require any new states ("running" to completion means that
> >nothing migrated, but the existing states can then be reused to see that
> >the test: transport has finished computing its numbers).
> >
> 
> we need to expose calculated numbers to the caller somehow.
> This could be done in MIGRATION_STATUS_COMPLETED state
> even for ordinary migration. Will it be OK?

I think so; if you look at the way the information is displayed
in info migrate, a lot of the data is only displayed 'sometimes',
e.g. :
        if (blk_mig_active()) {
            info->has_disk = true;
            info->disk = g_malloc0(sizeof(*info->disk));
            info->disk->transferred = blk_mig_bytes_transferred();
            info->disk->remaining = blk_mig_bytes_remaining();
            info->disk->total = blk_mig_bytes_total();
        }

It seems fair to do the same type of thing if you have an extra
block of values.

Dave

> 
> Den
> 
> >>>+++ b/qapi-schema.json
> >>>@@ -432,6 +432,8 @@
> >>>  #
> >>>  # @completed: migration is finished.
> >>>  #
> >>>+# @test-completed: migration time estimation finished.
> >>>+#
> >If this gets added, in spite of our ideas to use a 'test:' transport
> >instead of a new state, it would need a 'since 2.5' marker.
> >
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK