Juan Quintela <quint...@redhat.com> writes: > 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:-)
Yes. The existence of "detach" in QMP is owed to limitations of early QMP infrastructure. It's flagged as "invalid" and "should not be used" since 2010. Perhaps we should start a section on QMP in <http://wiki.qemu.org/Features/LegacyRemoval>. But I'd like to first have a way to communicate "you're using a deprecated feature" warnings via QMP. >> 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. My impression as a superficial reader is that migration configuration is a historically grown mess. Perhaps we shouldn't try to interpret too much intent into it :) If we redo migration as an instance of the "job" abstraction once we have it, then migration configuration & control should become more less messy. Of course, the old messes will stay with us for a while in the form of backward compatibility messes. I'm not too particular on how we do the tri-state now, as long as it reasonably fits what we have, and is documented clearly. >> 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. It's a compatibility break. Whether it's tolerable is a judgement call, and not for me to make. Compatibility breaks need documentation, including release notes. Say you run migrate with -b by accident (say by recalling a prior command from persistent command history, such as qmp-shell's or rlwrap's or socat READLINE's), immediately realize what you've done and cancel the migration. Are you then stuck with -b forever?
