Re: [Qemu-devel] QMP: RFC: I/O error info & query-stop-reason

[Anthony Liguori]

On 06/02/2011 01:57 PM, Luiz Capitulino wrote:
> On Thu, 02 Jun 2011 13:32:25 -0500
> Anthony Liguori<address@hidden>  wrote:
>
> > On 06/02/2011 01:01 PM, Luiz Capitulino wrote:
> > > On Thu, 02 Jun 2011 09:02:30 -0500
> > > Anthony Liguori<address@hidden>   wrote:
> > >
> > > > On 06/02/2011 08:24 AM, Jiri Denemark wrote:
> > > > > On Thu, Jun 02, 2011 at 08:08:35 -0500, Anthony Liguori wrote:
> > > > > > On 06/02/2011 04:06 AM, Daniel P. Berrange wrote:
> > > > > > > > > B. query-stop-reason
> > > > > > > > > --------------------
> > > > > > > > >
> > > > > > > > > I also have a simple solution for item 2. The vm_stop() accepts a reason
> > > > > > > > > argument, so we could store it somewhere and return it as a string, like:
> > > > > > > > >
> > > > > > > > > ->      { "execute": "query-stop-reason" }
> > > > > > > > > <- { "return": { "reason": "user" } }
> > > > > > > > >
> > > > > > > > > Valid reasons could be: "user", "debug", "shutdown", "diskfull" (hey,
> > > > > > > > > this should be "ioerror", no?), "watchdog", "panic", "savevm", "loadvm",
> > > > > > > > > "migrate".
> > > > > > > > >
> > > > > > > > > Also note that we have a STOP event. It should be extended with the
> > > > > > > > > stop reason too, for completeness.
> > > > > > > >
> > > > > > > >
> > > > > > > > Can we just extend query-block?
> > > > > > >
> > > > > > > Primarily we want 'query-stop-reason' to tell us what caused the VM
> > > > > > > CPUs to stop. If that reason was 'ioerror', then 'query-block' could
> > > > > > > be used to find out which particular block device(s) caused the IO
> > > > > > > error to occurr&     get the "reason" that was in the BLOCK_IO_ERROR
> > > > > > > event.
> > > > > >
> > > > > > My concern is that we're over abstracting here.  We're not going to add
> > > > > > additional stop reasons in the future.
> > > > > >
> > > > > > Maybe just add an 'io-error': True to query-state.
> > > > >
> > > > > Sure, adding a new field to query-state response would work as well. And it
> > > > > seems like a good idea to me since one already needs to call query-status to
> > > > > check if CPUs are stopped or not so it makes sense to incorporate the
> > > > > additional information there as well. And if you want to be safe for the
> > > > > future, the new field doesn't have to be boolean 'io-error' but it can be the
> > > > > string 'reason' which Luiz suggested above.
> > > >
> > > >
> > > > String enumerations are a Bad Thing.  It's impossible to figure out what
> > > > strings are valid and it lacks type safety.
> > > >
> > > > Adding more booleans provides better type safety, and when we move to
> > > > QAPI with a queryable schema, provides a way to figure out exactly what
> > > > combinations are supported by QEMU.
> > >
> > > To summarize:
> > >
> > >    1. Add a 'io-error' field to query-status (which is only present if
> > >       field 'running' is false)
> >
> > It may or may not be present.  Lack of presence does not tell you anything.
> >
> > It is only true when running is false AND the guest was stopped because
> > of an io error.
>
> Right.
>
> > >    2. Extend query-block to contain error information associated with the
> > >       device. This is interesting, because this information will be available
> > >       even if the error didn't cause the VM to stop
> >
> > Well we need at least some way to indicate that a block device is in a
> > failed state.  For instance, if you have two block device, but you miss
> > the IO_ERROR event, you need to figure out which of the two devices is
> > giving errors.
>
> Can't query-block be used for that? The 'io-error' key will only be present
> for the failing device(s).

Yes, I think we're in violent agreement.

Regards,

Anthony Liguori

> > But I was thinking of something that had the semantics of, last_iop_failed.
> >
> > Regards,
> >
> > Anthony Liguori
> >
> > > Seems good enough to me, comments?