Hi Ryan, Thanks for that very clear explanation. I suppose 24 hours is a long time with a wide variation in the peaks and troffs. As you say I am going to experience an over estimation every time I miss troff data or an under estimation if I miss the peaks.
Using the TOTAL function across a period that uses a 24 hr consolidation function isn't that useful anyway so I will drop it off my graphs. Regards, Chris On 29 December 2011 05:00, Ryan Kubica <[email protected]> wrote: > > A RRA (aside from the initial step RRA) is a consolidation function. You've > created your RRD with some consolidation period (24 hour) and an xff which > is a factor of pdp's (step datapoints) that will be used to 'create' the > average (or total) in the RRA. > > So ... if you have set xff to .5 then half of your datapoints used to create > the RRA can be missing and rrdtool will 'pretend' the rest would have been > there -- based on your xff value. > > for a 5 minute RRA and total based on 1 minute data and an xff of .5: > > 1 min values: 3 3 4 5 6 = 4.2 = total 1,260 > 1 min and null: 3 3 null 5 6 = 4.25 ((3+3+5+6)/4) = total 1,275 > 1 min and 2 nulls: 3 3 null null 6 = 4 ((3+3+6)/3) = total 1,200 > > now the fun part: > > 1 min and null > from a low datapoint: null null 4 5 6 = 5 = total 1,500 > > > Based on the above example, you can see that any data missing in your > 'troff' (low traffic part of the day) will skew your daily Total high - but > ONLY for your 24 RRA ... because you're 24 hour RRA is doing this xff and > then the Total function is operating off of the value and multiplying back > out throughout the day. It's doing exactly what you told it to do. > > The real solution is to not use a 24 hour RRA, ignore the first 24hour set > of data, or never miss data. :-) I'd side on the don't use 24 hour RRA's; > they are a waste in many ways and obviously have inherent calculation error > in them when they don't have all data available. > > -Ryan _______________________________________________ rrd-users mailing list [email protected] https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
