Steve Shipway wrote: >I would advise you to store a and b separately. > >You can always use a CDEF in your rrdtool >function to generate a new value c=a-b and then >average or max this, if you so choose.
If you need max(a-b) then you need to store a-b. Lets say the house is 20 all the time, and outside varies from freezing (0) to balmy (20) during a day - a bit extreme I know (at least for the UK), but it makes the point. We'll have step=300, and consolidate down to various periods - including 24 hours per CDP. Early morning, outside (o) reaches minimum of 0, inside (i) is 20, i-o=20. Late afternoon, O=20 and i-o=0. So during the day, max(i=o)=20. Now, we only store i and o, with average and maximum consolidations. Over a 24 hour period, max(i)=20, and max(o)=20, so max(i)-max(o)=0. We need to have stored (i-o), and consolidated it with max function - then we'll find that max(i-o)=20. Same if we want min(i-o). -- Simon Hobson Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed author Gladys Hobson. Novels - poetry - short stories - ideal as Christmas stocking fillers. Some available as e-books. _______________________________________________ rrd-users mailing list rrd-users@lists.oetiker.ch https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
