[rrd-users] Re: graphing different time periods & RRA usage

Alex van den Bogaerdt alex at ergens.op.het.net
Mon Apr 26 08:49:51 MEST 2004

On Sun, Apr 25, 2004 at 08:04:02PM -0700, Jesse Driskill wrote:

> rrdtool create f502_cur_conns.rrd -s 30 \
> DS:f502curconns:GAUGE:120:0:U \
> RRA:AVERAGE:0.1:1:60 \
> RRA:AVERAGE:0.2:2:60 \
> RRA:AVERAGE:0.5:20:8640 \
> RRA:AVERAGE:0.5:7200:7 \
> RRA:AVERAGE:0.1:86400:60
> My goeal is to recodr th following averages: (I may back these off in the future, but for now this provides the granularity I think I need). 
> # every 30 seconds per 30 minutes
> # 1 minute average per hour
> # 10 minute average per day
> # 1 hour average per week
> # 12 hour average per previous 30 days
>    My question is, how does rrd know which value to use when generating the graphs? If I view a one week graph, say last week, does it automatically use the correct PDP's or do I have to tell it to use the correct ones? Or what if I generate a graph that is nine days vs a nice clean amount... how does it handle thi mix of datapoints? 

RRDtool will try to match the amount of data you're asking for as good
as possible.  If there is a tie, the RRA with the smallest amount of
time per interval wins.

If you're asking for nine days, there's only one RRA that can satisfy
your request so that one (the last) will be chosen.

> # 1 hour average per week
> -s 30
> RRA:AVERAGE:0.5:7200:7

Each CDP is 7200 PDPs of 30 seconds each, which is one hour.  You have
just 7 of those CDPs. This is 7 hours, not 7 days.

Suppose it was indeed 7 days, and suppose you were asking for the time
interval of "--end now --start end-7d", "now" will most likely NOT be
an exact multiple of one hour.  The last fetched interval is therefore
the current hour. For instance: now==08:47, the interval of 08:00-09:00
is included.  The start of the data is "now-7d" so this will be last
week 08:47.  The interval 08:00-09:00 is included again.  The fetched
interval is thus 08:00 last week to 09:00 this week which is MORE than
7 days.  Again, there's only one RRA that can satisfy your request.

>    If this is answered in a map page or tutorial, please just point me to it & I'll find the answers, otherwise I appreciate any help you could provide.

I think you should be able to find it in the mail archives.  In any
case, you could have a look at the source of the program; this also
explains what's happening.

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