Re: [Qemu-devel] [PATCH v2 1/5] qapi/block: Add "fatal" to BLOCK_IMAGE_CORRUPTED

[Eric Blake <eblake@redhat.com>]

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On 09/05/2014 08:47 AM, Max Reitz wrote:
> On 05.09.2014 16:40, Eric Blake wrote:
>> On 09/05/2014 08:07 AM, Max Reitz wrote:
>>> Not every BLOCK_IMAGE_CORRUPTED event must be fatal; for example, when
>>> reading from an image, they should generally not be. Nonetheless, even
>>> an image only read from may of course be corrupted and this can be
>>> detected during normal operation. In this case, a non-fatal event should
>>> be emitted, but the image should not be marked corrupt (in accordance to
>>> "fatal" set to false).
>>>
>> Question - what happens if management misses the signal?  For example,
>> if libvirt opens qemu on a read-only image, then goes away, then
>> corruption is detected, then libvirt reconnects.  Does query-block need
>> to also be updated to report whether a read-only BDS is currently
>> detected as fatal, but that an event has already been delivered?
> 
> Well, the obvious problem with that is that corruption currently is a
> strongly format-specific topic, and only reported for qcow2. So, to do
> this, we'd have to move the corruption signalling code into the common
> block layer functions and proceed from there. This actually probably
> isn't too bad of an idea, anyway. But then we'll need a global
> bdrv_mark_corrupt() function (which we probably don't want in case we
> get more flags in the future, so we'll rather want bdrv_set_flag() or
> something) and I don't really want to make these changes now... We could
> postpone this, though. Making this change later shouldn't break anything.
> 
> The other solution would be simply to suppress the stderr message but to
> always deliver the QMP event.

That feels like it would flood the channel, if the image is being
repeatedly read.  The approach of warning exactly once is nice because
it prevents flooding.  We already have a way to report per-format
details, is it sufficient to modify ImageInfoSpecificQCow2 to add a bool
field that reports true if the image is currently known to be corrupted?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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