Re: [pcp] PCP libvirt PMDA

[Ken McDonell <kenj@xxxxxxxxxxxxxxxx>]

On 22/07/16 09:06, Marko Myllynen wrote:
> ...
> On 2016-07-20 23:43, Nathan Scott wrote:
> > ...
> > I forgot another biggie yesterday - related to the persistence of PMIDs.
> > Its important to a number of the client tools (pmlogger, pmie, pmchart,
> > hmmm pretty much all actually) that if pmcd is restarted, metrics return
> > with the same PMID as before (this is pretty much part of the protocol
> > over-the-wire, and its also necessary on-disk between archives of the
> > same host).
> >
> > This becomes very difficult to ensure in the case of dynamic metrics -
> > it was the main reason we switch cgroups to using this model IIRC.  And
> > the pmdaCache interfaces support persisting instance identifiers well of
> > course.
Just a note in passing ...

With only a slight bending of the semantics, the pmdaCache routines can 
be used to maintain a persistent mapping between the tail of a metric 
name and the low-order bits of a PMID.  Just choose an (otherwise 
unused) instance domain for the PMDA, then ...
pmns name -> external instance name
pmid -> internal instance name