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Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParam
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams |
Date: |
Mon, 15 May 2017 17:38:41 +0100 |
User-agent: |
Mutt/1.8.2 (2017-04-18) |
* Juan Quintela (address@hidden) wrote:
> Markus Armbruster <address@hidden> wrote:
> > Juan Quintela <address@hidden> writes:
> >
> >> Eric Blake <address@hidden> wrote:
>
>
> >>> Or is the proposal that we are also going to simplify the QMP 'migrate'
> >>> command to get rid of crufty parameters?
> >>
> >> I didn't read it that way, but I would not oppose O:-)
> >>
> >> Later, Juan.
> >
> > I'm not too familiar with this stuff, so please correct my
> > misunderstandings.
> >
> > "Normal" migration configuration is global state, i.e. it applies to all
> > future migrations.
> >
> > Except the "migrate" command's flags apply to just the migration kicked
> > off by that command.
> >
> > QMP command "migrate" has two flags "blk" (HMP: -b) and "inc" (HMP: -i).
> > !blk && inc makes no sense and is silently treated like !blk && !inc.
> >
> > There's a third flag "detach" (HMP: -d), but it does nothing in QMP.
>
> As qmp command is asynchronous, you can think that -d is *always* on in
> QMP O:-)
>
> > You'd like to deprecate these flags in favour of "normal" configuration.
> > However, we need to maintain QMP backward compatibility at least for a
> > while. HMP backward compatibility is nice to have, but not required.
> >
> > First step is to design the new interface you want. Second step is to
> > figure out backward compatibility.
> >
> > The new interface adds a block migration tri-state (off,
> > non-incremental, incremental) to global state, default off. Whether
> > it's done as two bools or an enum of three values doesn't matter here.
>
> Tristates will complicate it. I still think that:
>
> - capability: block_migration
> - parameter: block_shared
>
> Makes more sense, no?
I don't understand what making block_shared a parameter gives you as
opposed to simply having two capabilities.
(And how did we get 'shared'? We started off with block & incremental)
Dave
> If block_migration is not enabled, we ignore the shared parameter. We
> already do that for other parameters.
>
> > If the new interface isn't used, the old one still needs to work. If it
> > is used, the old one either has to do "the right thing", or fail
> > cleanly.
> >
> > We approximate "new interface isn't used" by "block migration is off in
> > global state". When it is off, the migration command needs to honor its
> > two flags for compatibility. It must leave block migration off in
> > global state. Yes, this will complicate the implementation until we
> > actually remove the deprecated flags. Par for the backward compatility
> > course.
> >
> > When block migration isn't off in global state, we can either
> >
> > * let the flags take precedence over the global state (one
> > interpretation of "do the right thing"), or
> >
> > * reject flags that conflict with global state (another interpretation),
> > or
> >
> > * reject *all* flags (fail cleanly).
> >
> > The last one looks perfectly servicable to me.
>
> Yeap, I think that makes sense. If you use capabilities, parameters,
> old interface don't work at all.
>
> We still have a problem that is what happens if the user does:
>
> migrate -b <foo>
> migrate_cancel (or error)
> migrate <bar> (without -b)
>
> With current patches, it will still use -b. Fixing it requires still
> anding more code. But I think that this use case is so weird what we
> should not even care about it.
>
> Later, Juan.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, (continued)
- [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/11
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Peter Xu, 2017/05/11
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/12
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Eric Blake, 2017/05/12
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Eric Blake, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Markus Armbruster, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams,
Dr. David Alan Gilbert <=
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Markus Armbruster, 2017/05/16
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/16
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Peter Xu, 2017/05/15
[Qemu-devel] [PATCH 3/3] migration: Remove old MigrationParams, Juan Quintela, 2017/05/11