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

[Eric Blake]

On 12/17/2012 03:13 AM, Dietmar Maurer wrote:
>> Next commit will re-enable balloon stats with a different interface, but this
>> old code conflicts with it. Let's drop it.
> 
> I don't really see any conflicts here?
> 
>> It's important to note that the QMP and HMP interfaces are also dropped by
>> this commit. That shouldn't be a problem though, because:
>>
>>  1. All QMP fields are optional
>>  2. This has never been really used

Libvirt has been using it when available (although reluctantly, as it
risks hanging on an uncooperative guest); and while libvirt can be
patched to call 6 QOM commands in a row to query six different QOM
stats, I still think it would be nicer to add a command that provides
all the stats at once.  In particular, when calling 6 commands in
series, you no longer have an atomic picture of the guest (the polling
interval could hit between two QOM queries, resulting in a combined set
of statistics that has no counterpart to the transition of states that
the guest actually went through).  On the other hand, since the stats
are already polling-based, and since it requires cooperation from the
guest, not having a guarantee of an atomic set of stats is not really
much of a loss.

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

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