[rrd-users] Re: rrdtool fetch with computed resolution

[Simon Hobson]

Sven Ubik wrote:

>I am not sure about your question. Are you asking why I want to fetch data
>with arbitrary time resolutions?
>
>It is interesting to see dynamics of network characteristics in different
>timescales. RRD file can store statistics in only a limited set of time
>resolutions, which are fixed and cannot be changed later. Why not to
>compute statistics over different time resolutions than those directly
>stored in RRD files?

Lets get this clear, as I read it you want to extract data at a later 
date which has a different resolution to what you have stored ?

The only way to do this is to store data covering the whole period 
you may want to chart over, and at the best resolution that you will 
ever want - you can't extract information later that you don't have 
stored. That's likely to be a huge store, and also not what rrd is 
about.

You can make rrd do that by simply telling it to store a lot of 
samples at high resolution - it will make a big rrd and take a lot of 
processing to use. rrd is designed to allow you to automatically 
consolidate data so that you can (typically) have high resolution 
over a short time, and low resolution data over a long time, 
balancing flexibility with efficiency of storage and computation.

For example, I have just set up a system that keeps network traffic 
(average & max, in and out, 255 sets) with 5 minute resolution up to 
2 days, 1/2hr up to 14 days, ... down to 12hour samples over 2 years. 
The rrd file is already 28M and the grapher needs anything up to 800M 
of memory to draw the graphs. I hate to think what the storage and 
processing requirements would be for 2 years of data at 5 minute 
samples ! Actually, neglecting overhead (not inconsiderable in 
itself) it's something in the order of 200MB times whatever the size 
of the float values is (ie a 4 byte value would require over 800MB of 
storage).

Because we're never likely to be interested in the value over a 5 
minute sample 8 months ago, were happy to trade off the resolution 
for less storage and more importantly less processing, to produce 
graphs that will give us an overview. rrd makes that almost trivial 
to do.

Simon

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