[rrd-users] trying to understand the relationship between source data, what's in rrd and what gets plotted

[Mark Seger]

Alex van den Bogaerdt wrote:
> On Fri, Jul 20, 2007 at 12:31:25PM -0400, Mark Seger wrote:
>   
>> more experiments and I'm getting closer...  I think the problem is the 
>> AVERAGE in my DEF statements of the graphing command.  The only problem 
>> is I couldn't find any clear description or examples of how this works.  
>> I did try using LAST (even though I have no idea what it does) and my 
>> plots got better, but I'm still missing data points and I want to see 
>> them all.  Again, I have a step size of 1 second so I'd think everything 
>> should just be there...
>>     
>
> Last time I looked, which is several moons ago, the graphing part
> would average different samples which needed to be "consolidated"
> due to the fact that one was trying to display more rows than there
> were pixel columns available.
>   
Ahh yes, I think I see now.  However, and I simply point this out as an 
observation, it's never good to throw away or combine data points as you 
might lose something really important.  I don't know how gnuplot does it 
but I've never see it lose anything.  Perhaps when it sees multiple data 
points it just picks the maximum value.  hey - I just tried that and it 
worked!!! 

This may be obvious to everyone else but it sure wasn't to me.  I think 
the documentation could use some beefing up in this place as well as 
some examples.  At the very least I'd put in an example that shows a 
series that contains data with a lot of values <100 and a single point 
of 1000.  Then explain why you never see the spike! I'll bet a lot of 
people would be shocked.  I also wonder how many system managers are 
missing valuable data because it's simply getting dropped out off.

-mark
> (I wrote consolidated surrounded by quotation marks because it isn't
> really consolidating what's happening)
>
> In other words: unless your graph is 50k pixels wide, you will have
> to select which 400 out of 50k rates you would like to see, or you
> will have to deal with the problem in a different way. For example:
>
> If you setup a MAX and MIN RRA, and you carefully craft their parameters,
> you could do something like this:
>
> * Consolidate 60 rates (1 second each) into one (of 60 seconds).
>   This means setting up an RRA with steps-per-row 60.
> * Display 400 x 60 seconds on a graph (or adjust the graph width,
>   together with the amount of CDPs to plot).
> * Do this using (you fill in the blanks):
>     DEF:MyValMin=my.rrd:minrra:...
>     DEF:MyValMax=my.rrd:maxrra:...
>     CDEF:delta=MyValMax,MyValMin,-
>     AREA:MyValMin
>     AREA:delta#FF0000:values:STACK
>   (That first area does not plot anything, and it is not supposed to.
>   The second area displays a line from min to max.)
> * Do the same for 3600 steps per row, and 400x3600 seconds per graph
>
> and so on.  Of course you can adjust the numbers to your liking.
>
> HTH
>