----Original Message---- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Sunil Modi (IT) Sent: Friday, October 08, 2004 8:41 AM To: [email protected] Subject: [rrd-users] RRD eating values
> I generated some backlogged data from January of 2004 into an RRD. I created > the RRD one month at a time (tedious but RRD had problems taking all the data > in one large update file). I had run a test case of the first month to see > how the data would look and it looked just fine when compared to excel > graphs. > When I finished making the RRD and putting in all 9 months of data, I > generated some graphs, and saw that the graphs are different and the peak > points are different. The single Month Graph had a peak of 100, and the 9 > Month graph had a peak of 60. What happened to the peak of 100? Should I > make separate RRD's for each month to be more accurate? This is expected. If you are using a somewhat standard RRD definition that emulates MRTG behavior, the amount of data that you are averaging to determine your graph point increases the further back in time you go. For example, you may keep 50 hours of 5 minute averages for your daily view which would basically be every data point you entered with minimal averaging. Next, you could have 12 days of 30 minute averages for your weekly view. As you can see, you've just gone from data points every 5 minutes to data points every 30 minutes, averaged out. You lose some granularity with that average but probably not much unless your data points change dramatically every 5 minutes. Next, you may have 50 days of 2 hour averages for your monthly view. Now your 5 minute averages are themselves averaged to one point for every 2 hours. You may begin to see a more significant loss in the granularity. Finally, for the yearly view you may have 600 days of 1 data point per day. This is where you'll see the highest loss in granularity. You're taking a full days worth of 5 minute averages and averaging those to just 1 data point. If this were a traffic graph you'd be averaging both your high day traffic and your low night traffic into one value. While it accurately reflects the average value for the traffic over 24 hours, it doesn't really reflect your highest or lowest values. That is a simplification of what actually happens but the end result is pretty much the same. Because the RRD doesn't typically store every 5 minute average for all time (unless you've defined your RRD to do that), the averaging of the data over time results in the perception of less accurate data, even though it's doing exactly what it's been designed and told to do. Creating separate RRD's for each month _may_ work but it sure seems to me to be a bad way to go about it. You should look at your RRD definition and craft it such that it suits your needs better. -- Marc -- Unsubscribe mailto:[EMAIL PROTECTED] Help mailto:[EMAIL PROTECTED] Archive http://www.ee.ethz.ch/~slist/rrd-users WebAdmin http://www.ee.ethz.ch/~slist/lsg2.cgi
