[rrd-users] Problem explaining RRD values

[Panos Kassianidis]

Hello everybody. I would like the expert's help on the following problem.
I have set up an RRD with the following configuration :

filename = "TEL1000_0.rrd"
rrd_version = "0001"
step = 1
last_update = 1139483171
ds[TEL1000_0].type = "GAUGE"
ds[TEL1000_0].minimal_heartbeat = 150
ds[TEL1000_0].min = NaN
ds[TEL1000_0].max = NaN
ds[TEL1000_0].last_ds = "UNKN"
ds[TEL1000_0].value = -0.0000000000e+00
ds[TEL1000_0].unknown_sec = 0
rra[0].cf = "AVERAGE"
rra[0].rows = 70080
rra[0].pdp_per_row = 15
rra[0].xff = 5.0000000000e-01
rra[0].cdp_prep[0].value = -1.5028035000e+01
rra[0].cdp_prep[0].unknown_datapoints = 0
rra[1].cf = "AVERAGE"
rra[1].rows = 70080
rra[1].pdp_per_row = 30
rra[1].xff = 5.0000000000e-01
rra[1].cdp_prep[0].value = -1.5028035000e+01
rra[1].cdp_prep[0].unknown_datapoints = 0
rra[2].cf = "AVERAGE"
rra[2].rows = 52560
rra[2].pdp_per_row = 60
rra[2].xff = 5.0000000000e-01
rra[2].cdp_prep[0].value = -1.5028035000e+01
rra[2].cdp_prep[0].unknown_datapoints = 0


As you can see I have set the step to 1 sec and I am keeping RRA's for 
15,30 and 6 0 seconds. I provide values to the RRD with manually 
configured intervals of 15 seconds. For example this is a set of 
values that I provide:


timedate: 1139482244  value: -0.611518263817
timedate: 1139482260  value: -2.77656555176
timedate: 1139482275  value: -1.96811783314
timedate: 1139482290  value: 2.1477060318
timedate: 1139482305  value: 1.08122718334
timedate: 1139482320  value: -2.4864552021
timedate: 1139482336  value: -1.34511160851
timedate: 1139482351  value: -0.433948069811
timedate: 1139482366  value: -1.17775833607
timedate: 1139482381  value: 0.130312412977
timedate: 1139482397  value: 2.14147639275
timedate: 1139482412  value: 1.20305621624
timedate: 1139482427  value: 1.62215995789
timedate: 1139482442  value: -2.02085781097
timedate: 1139482458  value: -3.43694186211



When I  fetch the values from the RRD I get the following:

1139482260:  -2.7765660000e+00
1139482275:  -1.9681180000e+00
1139482290:  2.1477060000e+00
1139482305:  1.0812270000e+00
1139482320:  -2.4864550000e+00
1139482335:  -1.3451120000e+00
1139482350:  -4.9469226667e-01
1139482365:  -1.1281706667e+00
1139482380:  4.3107333333e-02
1139482395:  2.0073984000e+00
1139482410:  1.3281786667e+00
1139482425:  1.5662794667e+00
1139482440:  -1.5351222667e+00
1139482455:  -3.2481308000e+00

Ok I understand that RRD changes the timestamps so as to fit data in 
its 15sec intervals.
But why does it also change the values that I give it? If you notice 
many values( from timestamp 1139482351 on) are not the same but differ 
somehow and this difference doesn't have anything to do with rounding.
Is it trying to average something? Because I believe it shouldn't!!!
Since the finest resolution RRA is every 15secs and I provide data 
spaced 15 secs apart it doesn't have data to average does it?


			Thank you very much in advance

					Panos

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