[rrd-users] Re: rrdtool create

Alex van den Bogaerdt alex at ergens.op.het.net
Thu Dec 9 00:00:38 MET 2004

On Wed, Dec 08, 2004 at 11:45:54PM +0100, Georges Toth wrote:
> > Counter, or derive.
> i would prefer not to recreate everything :-)

It is possible to change the datasource type of an existing
database.  This should probably not be done untill you have
a better understanding of how rrdtool works.

> > You didn't set a proper maximum, or your counter increases to _almost_ the
> > full value in one month.
> i didn't set a maximum, that's correct.

It may be true that you didn't set it, but it's not correct :)

> the data it plotted... some hours before and some hours after... it contained 
> some PETA values etc...
> very strange

No, it's not.

Current counter value = 9,000,000,000
300 seconds later the counter value is 100.

(the numbers are just example numbers, I can't deduce this)

What has happened?  RRDtool is programmed to deal with counter wraps.
Your counter seems to have increased from 9,000,000,000 and wrapped
at the maximum number to 100.  The increase is 2^64 (maximum number,
plus 1) -9,000,000,000 (starting point) +100 (end point).

This happened in 300 seconds, so thats about 61,500,000,000,000,000
per second.  You programmed your RRA to behave like this.

Would you have set a sane maximum rate, as instructed by the documentation,
this rate would have been ignored and you'd have ended up with unknown
data.  Since you really don't know the rate before and after the reset,
that's not unreasonable.

> > #1 Just before the counter gets reset, update your database. One
> >    second later, update the database with "U" (for unknown).
> >    Another second later (preferably exactly when the counter is reset)
> >    update the database with zero.
> >    This way you have the most accurate results
> hmmm...
> > #2 Use DERIVE as your datasource type.  When the counter is reset, RRDtool
> >    will see this as a negative rate.  Tell RRDtool it should not accept
> >    negative rates (minimum value set to zero)
> that's probably the best way to go...

Only if you're willing to ignore "This way you have the most accurate results".

> only wondering now if i can keep my old data... hope so :-/ ... ?

That depends on what you use now, how well you read the documentation and,
if needed, how well your backup-restore procedures work.

"rrdtool tune" can be used to alter the data source type and to change
the minimum allowed rate.

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