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

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jun 02, 2011 at 03:01:24PM -0300, Luiz Capitulino wrote:
> On Thu, 02 Jun 2011 09:02:30 -0500
> Anthony Liguori <anthony@codemonkey.ws> 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)

This isn't really enough. There are many reasons why a VM may have
transitioned to the paused state, of which IO Error is merely one.
The query-status needs to be able to report what the reason for
the transitioning to the paused state is.

>  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

Apps will only look for this data, if the query-status stop reason
indicates that it was stopped due to an I/O error.

Regards,
Daniel
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