Re: [Qemu-devel] [PATCH 1/3] balloon: drop old stats code & API

[Eric Blake <eblake@redhat.com>]

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On 12/19/2012 04:27 AM, Luiz Capitulino wrote:
>> Libvirt has been using it when available (although reluctantly, as it
>> risks hanging on an uncooperative guest);
> 
> This has always been disabled and qemu never returns the stats info.
> I believe libvirt's code is rotting just like qemu's is.

I agree that the QMP has never been providing it (since we pulled it out
of QMP precisely because of the potential for a hang), but if you go
back far enough to an old qemu that provided the stats via HMP, then
libvirt is indeed exposing those stats to the user.

There are two places in libvirt code that were collecting stats.  One
for determining what to print for simple commands like 'virsh dumpxml',
where we only cared about balloon size and not the rest of the
statistics, and where we do NOT want to hang waiting on the guest; for
that case, the solution that Dan came up with was adding a balloon
event, and using the event instead of a query.  The other usage is that
we have an API where the user can specifically request all the stats;
and there, we DO want to block on the guest to get the stats, and would
prefer getting the stats in a single command (but can tolerate getting
the stats via 6 separate commands, if that's what it takes).  It is this
second usage that has been broken in libvirt ever since we crippled the
stats collection in qemu because of the undesirable hang in the first case.

> Something I have been wondering if whether it's possible to have only
> one property (say balloon-statistics) and return all properties in a
> dict. QOM properties return a visitor, so maybe that's possible.
> 
> I'll check that.

Ah, that would be a nice trick - querying a single QOM property that
turns out to be a dict exposing multiple sub-properties at once.  And if
you can get that to work, you are back to your goal of no new QMP command.

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


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