Re: [Qemu-devel] [PATCH] block: extend BLOCK_IO_ERROR event with nospace indicator

[Kevin Wolf <kwolf@redhat.com>]

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Am 09.09.2014 um 14:53 hat Eric Blake geschrieben:
> On 09/09/2014 06:43 AM, Luiz Capitulino wrote:
> 
> >> Enhancing query-block in addition to the event makes sense, if it is
> >> easy enough to do.  At this point, we are talking about debugging aids,
> >> so as long as they are documented appropriately, I won't be too fussy.
> > 
> > OK, but I'm wondering if we need to add the string field to both,
> > BLOCK_IO_ERROR and query-block, or only to one to the other.
> > 
> > In my opinion, we should only add it to BLOCK_IO_ERROR if libvirt is
> > going to consume. Otherwise, it makes more sense to add it to query-block
> > because that's where we'll meet the user.
> > 
> > Btw, by "consume" I mean read it and make it available to libvirt clients
> > so that they can print it to their users. If we don't want libvirt to
> > consume that field then I think we should only add it to query-block and
> > info block.
> 
> [For those not aware, qemu built for downstream RHEL already has an
> error string in the __com.redhat_ namespace; we're trying to figure out
> what upstream should have so that downstream doesn't have to perpetually
> maintain an extension]
> 
> Downstream libvirt does not currently consume any error string.  With
> downstream qemu, the _only_ way to get the error string in the event is
> to parse libvirt logs, or use upstream libvirt with its backdoor of
> 'virsh qemu-monitor-event' (through the explicitly unsupported
> libvirt-qemu.so) to get at the raw event information.  Changing libvirt
> to expose such an error string to the end user would require a new
> libvirt event number (the existing libvirt event is not extensible), so
> existing clients would not be able to get at the information without
> being recompiled to a new libvirt.
> 
> Since the whole point of this field is for debugging, I think that it is
> sufficient to add it to JUST query-block, and not to the event.  That
> is, if the app on top of libvirt gets an error, in the common case, they
> won't care about the message (it won't change how they act), and in the
> debug case, a developer trying to learn more about what happened can do
> their own query-block directly (via 'virsh qemu-monitor-command', also
> in libvirt-qemu.so) rather than trying to wire up libvirt to pass
> through an error string through a new event.

You mention libvirt logs and I think that's an important point: If we
move it to query-block, will we still get a log entry so that we can
check this after the fact when only a log file is attached to a bug
report, but we don't have a running guest?

Kevin

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