Re: [pcp] RFC: derived metrics in PCP archives

[Ken McDonell <kenj@xxxxxxxxxxxxxxxx>]

G'day Mark.

On 04/12/15 09:46, Mark Goodwin wrote:
> On 12/04/2015 07:20 AM, Ken McDonell wrote:
> ...
> Ken, if a derived metric appears in a pmlogger config then can we just
> ensure the operands are logged
> rather than logging the derived metric itself? ...
pmlogger _could_ (but does not currently) know that the metric is 
derived ... but it has no way (nor does any other PMAPI client) to 
determine the operand metrics involved in the expression defining the 
derived metric.
It is less intrusion to fix pmlogger than to add API support to return 
metrics used to define a derived metric.
If the derived metric definition is unchanged, then replay would produce 
the same results as live.
If the derived metric definition was subject to change, you have no 
choice but to log the operands, not the derived metric.