[HOWTO] Use VDEF items

[JasperJ]

Hi,

I've figured out something that I consider fairly cool.

When you graph something like fan speeds, you'll often find that the values of each fan will stay within a very narrow band, but they won't necessarily be terribly close together, so you can't simply use an appropriately set scale to see everything usefully, and the graph will not be terribly informative:

http://www.insaneoc.com/graphs/graph_370.html

To solve this, I decided that I wanted to graph the *difference* between the given data point and the average value of the sensor over time. In other words, how much more or less is it now than it usually is?

Here's the result, and this is how I accomplished it.

http://www.insaneoc.com/graphs/graph_413.html

With some help from a friend, I figured out that what I wanted from RRDtool was VDEF:average_datasource=datasource,AVERAGE. VDEFs are like CDEFs, except they work on the entire dataset, rather than just each value individually. In the case of the VDEF above, it first puts the entire datasource (probably just the portion that coincides with the graph you're making. I think) on the stack, and then AVERAGE, obviously, takes that and returns a single value representing the average of the datasource. Note that VDEFs exist from RRDtool 1.2 onward only.

Now, cacti does not support VDEFs in any direct way, so this was a bit tricky. I figured out that when you make a cacti CDEF, the strings in that CDEF get passed pretty much verbatim to RRDtool, though. So what I ended up doing in order to draw a LINE2 respresenting the difference between the average of the dataset as a whole and the current value was the following:

First, insert one graph item with type COMMENT, and apply the CDEF "Make average, stick in avgCURRENT_DATA_SOURCE". This CDEF consists solely of a single Custom String: CURRENT_DATA_SOURCE VDEF:avgCURRENT_DATA_SOURCE=CURRENT_DATA_SOURCE,AVERAGE

Assuming for the moment that the current data source is named 'a' by cacti, this translates into a piece of RRDtool commandline like:

CDEF:cdefa=a VDEF:avga=avg,AVERAGE

Because the item is a COMMENT, the cdefa that is produced is not used in the rest of the graph definition, but we *have* defined a variable "avga".

The next step is simple: Add a LINE2 and apply the CDEF "graph diff with avgCURRENT_DATA_SOURCE", which consists of CURRENT_DATA_SOURCE,avgCURRENT_DATA_SOURCE,- .

Because cacti uses the same data source identifier for both the COMMENT and LINE2 items, avgCURRENT_DATA_SOURCE will work across the two items.

Other graphs that use this trick:

http://www.insaneoc.com/graphs/graph_414.html
http://www.insaneoc.com/graphs/graph_415.html

[mgb]

Very impessive,

I'm going to try this out on some of my temp graphs i have. This will be more useful than the current graphs we are seeing.

Thanx

[koe]

Hi JasperJ

Thanks for your tips with VDEF.

I'm trying to use the VDEF TOTAL in the graph template to obtain the total data accumulated for a determinated time range ( the area of the graph! ) but I can not do it !

Could you help me? How can I build a graph template with VDEF TOTAL?

Thanks.

[gandalf]

Wow, that's indeed cool. It's like a "rrdtool graph injection" vulnerability
We should fix this security issue asap
But it does not really replace a full VDEF support. This is still to come
Reinhard