Markus Armbruster <arm...@redhat.com> wrote: > Juan Quintela <quint...@redhat.com> writes: > >> Eric Blake <ebl...@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? 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.
