From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Daniel P. Berrangé" <berrange@redhat.com>,
"Juan Quintela" <quintela@redhat.com>,
qemu-devel@nongnu.org, "Markus Armbruster" <armbru@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Vitaly Kuznetsov" <vkuznets@redhat.com>
Subject: Re: [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified
Date: Tue, 20 Apr 2021 15:10:44 +0100 [thread overview]
Message-ID: <YH7g5CdYGGCgHGpR@work-vm> (raw)
In-Reply-To: <20210420134811.o2ypiw3y4fr73udb@habkost.net>
* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Tue, Apr 20, 2021 at 12:51:43PM +0100, Dr. David Alan Gilbert wrote:
> > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > On Mon, Apr 19, 2021 at 07:47:34PM +0100, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > > > On Mon, Apr 19, 2021 at 06:15:56PM +0100, Daniel P. Berrangé wrote:
> > > > > > On Mon, Apr 19, 2021 at 06:11:47PM +0100, Dr. David Alan Gilbert wrote:
> > > > > > > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > > > > > > I would make live migration policy an enum, just to make sure
> > > > > > > > we are explicit about the requirements:
> > > > > > > >
> > > > > > > > - UNKNOWN: this is the current state in QEMU 6.0, where we don't
> > > > > > > > really know what the user expects.
> > > > > > > > This can be the default on existing versioned machine types,
> > > > > > > > just for compatibility.
> > > > > > > > I suggest making this print warnings for every migration
> > > > > > > > blocker (like this patch does).
> > > > > > > > I suggest deprecating this behavior as soon as we can.
> > > > > > > >
> > > > > > > > - PREFERRED: try to make the VM migratable when possible, but
> > > > > > > > don't print a warning or error out if migration is blocked.
> > > > > > > > This seems to be the behavior expected by libvirt today.
> > > > > > > >
> > > > > > > > - NOT_NEEDED: live migration is not needed, and QEMU is free to
> > > > > > > > enable features that block live migration or change guest ABI.
> > > > > > > > We can probably make this the default on machine types that
> > > > > > > > never supported live migration.
> > > > > > >
> > > > > > > I suggest you could do this by adding:
> > > > > > > -warn-none-migratable
> > > > > > > -no-warn-none-migratable
> > > > > > >
> > > > > > > and then argue about defaults another time.
> > > > > >
> > > > > > If we're going to add new args, lets at least future proof our
> > > > > > approach with an extensible option that we can wire into QMP
> > > > > > too later
> > > > > >
> > > > > > -migratable none|preferred|required
> > > > > >
> > > > > > and letting us add extra key/value pairs to tune it if desired.
> > > > >
> > > > > Having said that, we potentially don't need a dedicated arg if we
> > > > > just make 'migratable=none|preferred|required' be a property of
> > > > > the machine type and hook everything off that
> > > >
> > > > I think my only difficulty with that is that I don't find any of those
> > > > 3 words 'obvious'.
> > >
> > > Any suggestions of replacements for those 3 words?
> > >
> > > Would the descriptions below be enough to clarify their meaning
> > > in documentation?
> >
> > I prefer things that are fairly obvious without needing to look at the
> > documentation until you want the detail.
> >
> > > - NONE: live migration is not needed, and device or machine code
> > > is allowed to enable features that block live migration or
> > > change guest ABI.
> > > (Not implemented yet)
> > >
> > > - PREFERRED: machine and device code should try to make the VM
> > > migratable when possible, but won't emit a warning or error out
> > > if migration is blocked.
> > > (Current default behavior)
> > >
> > > - REQUIRED: live migration support is required, and adding a
> > > migration blocker will be an error.
> > > (Implemented today by --only-migratable)
> >
> > How about
> > -migratable blocked
> > Live migration is not allowed; an outbound migration will fail
>
> "none" and NOT_NEEDED above were about letting QEMU automatically
> enable features that block live migration or change guest ABI.
>
> If that's implied by "blocked", I'd like to get that documented
> explicitly. If that's not implied by "blocked", I don't
> understand what's the use case for "blocked".
My 'blocked' is stronger - migration is hard disabled by a blocker
always; it's for (rare) cases where the user wants to stop a migration
happening, even if qemu believes it can do it.
> >
> > -migratable allowed
> > Live migration is allowed, but some devices/options may block
> > it if they're unable to migrate [current default]
>
> "preferred" above was about QEMU trying to keep live migration
> working as much as possible. That's something we all expect QEMU
> to do, but it's not documented anywhere.
>
> If that's implied by "allowed", I'd like to get that documented
> explicitly. If that's not implied by "allowed", then we have a
> problem.
My difficulty by your definition is I don't understand what
'working as much as possible' means - that's the current behaviour
as I understand it. I think mine is more explicit.
>
> >
> > -migratable warn
> > Live migration is allowed, but if some device/option is
> > unable to migrate, migration will be blocked and a warning
> > printed
>
> This makes sense, but I don't think we need to support this use
> case.
I thought that was exactly what Vitaly's patch tried to do?
> >
> > -migratable required
> > Live migration is allowed, attempting to add a device or
> > enable an option that can't migrate will fail. [--only-migratable]
>
> This matches "required" above.
Dave
> --
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
next prev parent reply other threads:[~2021-04-20 14:12 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-15 15:44 [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified Vitaly Kuznetsov
2021-04-15 16:04 ` Daniel P. Berrangé
2021-04-15 16:30 ` Eduardo Habkost
2021-04-15 16:40 ` Daniel P. Berrangé
2021-04-15 17:07 ` Daniel P. Berrangé
2021-04-15 17:28 ` Dr. David Alan Gilbert
2021-04-16 7:33 ` Vitaly Kuznetsov
2021-04-16 16:28 ` Eduardo Habkost
2021-04-17 9:33 ` Markus Armbruster
2021-04-19 16:42 ` Daniel P. Berrangé
2021-04-19 16:48 ` Eduardo Habkost
2021-04-19 17:11 ` Dr. David Alan Gilbert
2021-04-19 17:15 ` Daniel P. Berrangé
2021-04-19 17:17 ` Daniel P. Berrangé
2021-04-19 18:47 ` Dr. David Alan Gilbert
2021-04-19 19:32 ` Eduardo Habkost
2021-04-20 11:51 ` Dr. David Alan Gilbert
2021-04-20 13:48 ` Eduardo Habkost
2021-04-20 14:10 ` Dr. David Alan Gilbert [this message]
2021-04-20 14:15 ` Daniel P. Berrangé
2021-04-20 15:20 ` Eduardo Habkost
2021-04-17 9:35 ` Markus Armbruster
2021-04-19 7:26 ` Markus Armbruster
2021-04-19 15:46 ` Markus Armbruster
2021-04-18 15:53 ` Peter Maydell
2021-04-19 16:28 ` Eduardo Habkost
2021-04-19 16:37 ` Daniel P. Berrangé
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