Negative spikes in rrd graphs

[Iain M Conochie <iain (at) shihad.org>]

Good morning Xymonners!

 I have an issue I was hoping someone could help me with. I have 
created an NCV definition for a mysql script to graphs slow queries and 
also queries per second. The definition is here:

[mysql]
       TITLE MySQL Queries
       YAXIS Amount
       DEF:questions=mysql.rrd:questions:AVERAGE
       DEF:slow=mysql.rrd:slowqueries:AVERAGE
       LINE2:questions#0000FF:Queries
       GPRINT:questions:LAST: \: %5.1lf (cur)
       GPRINT:questions:MAX: \: %5.1lf (max)
       GPRINT:questions:MIN: \: %5.1lf (min)
       GPRINT:questions:AVERAGE: \: %5.1lf (avg)\n
       LINE2:slow#00FF00:Slow Queries
       GPRINT:slow:LAST: \: %5.1lf (cur)
       GPRINT:slow:MAX: \: %5.1lf (max)
       GPRINT:slow:MIN: \: %5.1lf (min)
       GPRINT:slow:AVERAGE: \: %5.1lf (avg)\n


This is all well and good. Here is a shortened output of the script:

threads: 4
questions: 681188
slow queries: 2

Now Questions is ALWAYS increasing; so i felt that the average statement would be the correct one. However, if i restart the mysql service or even the box and Questions drops to 0 then bad things happen to the graphs. The spike downwards to large negative numbers. Now rrd is doing what it should be doing i.e. giving me an average of the range of numbers. Is there another definition I can use to not have this effect?


Cheers

Iain