[rrd-users] VDEF MINIMUM

[Oliver]

Hi Alex,

Thank you for your detailed reply which has certainly helped me understand 
RRA calculations a bit better.

However I am only using one RRA (I should have pointed this out in my first 
post):

filename = "noise.rrd"
rrd_version = "0003"
step = 30
last_update = 1313098265
ds[noise].type = "GAUGE"
ds[noise].minimal_heartbeat = 60
ds[noise].min = NaN
ds[noise].max = NaN
ds[noise].last_ds = "9.4"
ds[noise].value = 5.6364270600e+01
ds[noise].unknown_sec = 0
rra[0].cf = "AVERAGE"
rra[0].rows = 5760
rra[0].cur_row = 4995
rra[0].pdp_per_row = 1
rra[0].xff = 5.0000000000e-01
rra[0].cdp_prep[0].value = NaN
rra[0].cdp_prep[0].unknown_datapoints = 0

Does this influence your previous reply?

Thanks,
Oliver.

-----Original Message----- 
From: Alex van den Bogaerdt
Sent: Tuesday, August 09, 2011 2:00 PM
To: rrd-users at lists.oetiker.ch
Subject: Re: [rrd-users] VDEF MINIMUM

You are looking at averages. Your two tests use different RRAs. For
instance: one RRA consolidates 1 PDP into 1 CDP, the other consolidates 3
PDPs into one CDP. Look at 10,20,30,40,50,60. One RRA has all 6 values,
the other RRA has average(10,20,30) equals 20, average(40,50,60) equals
50.

minimum(10,20,30,40,50,60) equals 10. minimum(20,50) equals 20. Your test
would return 10 and 20.

It should work much better if you look at minimums of minimums:

The other RRA would contain min(10,20,30) equals 10, min(40,50,60) equals
40. Then min(10,20,30,40,50,60) and min(10,40) both return 10.

There can be two places where consolidation takes place. One is when data
is copied into RRAs, the other is when rrdtool graph needs data in a
resolution which is not available. If you take both into account, you
should be able to get predictable results.

HTH
Alex