Re: [Qemu-devel] [PATCH REPOST 0/2] Add basic "detach" support for dump-guest-memory

[Fam Zheng]

On Tue, 11/24 06:49, Eric Blake wrote:
> On 11/24/2015 04:37 AM, Fam Zheng wrote:
> 
> >> I think the patch should be dropped, and periodic progress reports
> >> should be emitted from within the dump loops that do the heavy lifting.
> >>
> >> For the ELF format dumps, that loop appears to reside in dump_iterate()
> >> [dump.c].
> >>
> >> For the compressed format dumps, the loop seems to live in
> >> write_dump_pages() [dump.c].
> > 
> > This is a good idea!
> > 
> > What I'm not sure is where to report the progress. Can it be the monitor 
> > where
> > the dump-guest-memory command was issued? In other words, do we support 
> > raising
> > events before the previous command returns? If yes, can libvirt handle this
> > correctly? (But the worst case is using another channel to communicate the
> > progress, it is ad-hocery but it must be better than all the risk and 
> > effort to
> > enable multi-threaded dump.)
> > 
> > Eric, Markus, have any idea with the progress reporting?
> 
> I'm fairly certain we support raising events prior to completion of a
> synchronous command; what I'm not sure of is whether the event hits the
> wire right away or whether it piles up waiting for the next synchronous
> command completion.  If the latter, then we need to rework it (since the
> whole point of this exercise is that we are trying to give progress of a
> long-running synchronous command that hasn't completed yet).

So in that case we may want some "flush" operation of events. That sounds OK to
me.

> But we
> only have the one monitor connection for libvirt - the only way to pass
> events through a second channel is to open a second monitor connection,
> but that feels wrong to make libvirt have to track two monitors.

OK, that's a fair point, but FWIW I was thinking about adding an optional
argument:

    "*progress": "fd:dump-progress"

into which dump.c talks in a mini-protocol, to send progress information. It's
just an crazily hacky idea, not anything I'm advocating.

Thanks,

Fam