Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * Juan Quintela (quintela@redhat.com) wrote:
>> Markus Armbruster <armbru@redhat.com> wrote:
>> > Juan Quintela <quintela@redhat.com> writes:
>> >
>> >> Eric Blake <eblake@redhat.com> 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)

The variables on MigrationParams:

struct MigrationParams {
    bool blk;
    bool shared;
};


I can move to incremental.  I am not sure which one is clearer.

The advantage of having shared as a parameter is that we forget about
all this dependency bussiness.  Is the same than compression_threads
paramter, you setup to whichever value that you want.  But you don't get
compression_threads until you set the compress capability.

So, in this case we will have:

block capability: Are we using block migration or not
block-incremental parameter: If we are using block migration, are we
      using incremental copying of the block layer?


Later, Juan.