[hobbit] Negative spikes in rrd graphs

[Buchan Milne]

On Tuesday 10 March 2009 11:53:12 Iain M Conochie wrote:
> 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?

This is determined by the actual DS definition the RRD tool library uses, 
which for the NCV collector is set by the NCV_<testname> environment variable, 
which is typically set in hobbitserver.cfg. Show us what you have for 
NCV_mysql in hobbitserver.cfg, and please read the sections on DERIVE and 
COUNTER in the rrdcreate man page.

Regards,
Buchan