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Re: [PATCH RFC] migration: warn about non-migratable configurations unle

From: Daniel P . Berrangé
Subject: Re: [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified
Date: Thu, 15 Apr 2021 17:04:13 +0100
User-agent: Mutt/2.0.5 (2021-01-21)

On Thu, Apr 15, 2021 at 05:44:02PM +0200, Vitaly Kuznetsov wrote:
> When a migration blocker is added nothing is reported to the user,
> inability to migrate such guest may come as a late surprise. As a bare
> minimum, we can print a warning. To not pollute the output for those, who
> have no intention to migrate their guests, introduce '--no-migration'
> option which both block the migration and eliminates warning from

I wonder how this is actually going to work in practice ?

At the time libvirt starts a guest, it has no idea whether the guest
is likely to need migration 3, 6, 12, 24 months in to the future.

IOW, we can't use a --no-migration flag and will be stuck with these
warnings no mtter what.

Is it possible to query the migration blockers via QMP ?

Libvirt recently introduced a new API 'virDomainGetMessages' which
lets us report a list of human targetted message strings against
a guest. We use it for reporting when an operation has tainted
a guest, and also use it for reporting when a deprecated QEMU
feature is used.  We could use it to report any migration
blockers that exist.

These are visible from 'virsh dominfo $guestname' and could also
be displayed by a mgmt application.

NB, the messages are intentionally declared opaque strings, so
mgmt apps shouldn't try to parse them. They merely know whether
the count is non-zero for any given message class.

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