[libvirt] [PATCH v2] lib: Add note that bulk stats API queries may overrun RPC buffers

[Peter Krempa]

On Wed, May 24, 2017 at 12:32:05 +0200, Martin Kletzander wrote:
> On Tue, May 23, 2017 at 05:29:52PM +0200, Peter Krempa wrote:
> > On Tue, May 23, 2017 at 17:19:53 +0200, Martin Kletzander wrote:
> > > On Tue, May 23, 2017 at 05:07:40PM +0200, Michal Privoznik wrote:
> > > > On 05/23/2017 04:35 PM, Martin Kletzander wrote:
> > > > > On Tue, May 23, 2017 at 04:23:30PM +0200, Peter Krempa wrote:
> > 
> > [...]
> > 
> > > > > > + * Note that this API is prone to exceeding maximum RPC if querying
> > > > > > too many VMs
> > > > > > + * with lots of statistics. It's suggested to query in batches of
> > > > > > 10VMs, which
> > > > > > + * should be good enough for VMs with 3000 disks + networks.
> > > > > > + *
> > > > >
> > > > > Coming to think about it... Why don't we just batch this ourselves under
> > > > > the hood and just return the merged result?
> > > >
> > > > Because:
> > > >
> > > > https://www.redhat.com/archives/libvir-list/2017-May/msg00088.html
> > > >
> > > 
> > > Not on the RPC level, the API would just be syntax sugar to
> > > virDomainListGetStats() if a flag was passed
> > > (e.g. VIR_DOMAIN_GET_ALL_STATS_I_DONT_REALLY_CARE_IF_THIS_IS_DONE_IN_ONE_LIBVIRT_CALL)
> > 
> > Also compared to a full fragmentation of the returned data, this would
> > result into a worst-case-scenario memory usage of MAX_SIZE *
> > NVMS_QUERIED_IN_ORIGINAL_CALL, when compared to an unbounded memory use
> > of the full fragmentation approach.
> 
> If I get what you are saying, then the same would happen if the mgmt app
> (or client) implemented it themselves.  We would basically just provide
> the guessing logic.

Yes. I don't really think it's worth doing this. Besides the "logic"
would be that we'd call it one-by one, to get the full buffer size for
every single VM.
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