[pcp] mmv and qa/648 questions

[Ken McDonell]

Guys,

I've been looking at the mmv pmda and QA coverage.

What is the expected behaviour when more than one mmv stats file
mentions the _same_ cluster?

If it is OK, the code is wrong as it keeps registering metrics with the
same pmids as existing metrics, but only the first registered is
fetchable.  Also forming the union of the instance domains is not
happening either.

If it is not OK, then the PMDA does not detect this properly, and
mmv_instances.c in the QA suite needs to be changed.

I'll chase it down, if you can give me an indication of what was
supposed to happen.

I think for the Aconex-style semantics, forming the union of the mmap'd
files over a common cluster is probably correct, but I don't think was
within the scope of the original mmv implementation.