[rrd-users] Re: Fwd: rrdtool resets (sorry)

BAARDA, Don don.baarda at baesystems.com
Wed Jan 31 00:03:33 MET 2001


> -----Original Message-----
> From:	Patrick Ammann [SMTP:Patrick.Ammann at nextra.ch]
> Sent:	Tuesday, January 30, 2001 9:24 PM
> To:	rrd-users at list.ee.ethz.ch; Christian Pearce
> Subject:	[rrd-users] Fwd: rrdtool resets (sorry)
> Hi all,
> i have some problems in understanding the way to get the "spikes/wraps"
> out 
> of my graphs.
> i just read a lot in the mailing list and i finally came to the following 
> result.

	Sounds like people are getting confused... "spikes/wraps" are really
very simple, and rrd handles them for you. 

> 1. I could set the "ds[in].min = 0" (as Tobi said). This should cause that
> a negative difference would reslut in a "nan" or "UNKNOWN" value. Did i 
> understand this correctly ?

	Setting min=0 will do nothing for type COUNTER... COUNTER will
always produce values >=0, because it  assumes a counter that has gone
backwards has wrapped, and compensates for it.

> 2. If i change the ds[in].type from "COUNTER" to "Derive" then i should
> get 
> rid off all problems (as discussed in the mail list), right ?

	Using DERIVE with min=0 is a nasty hack... DERIVE is just a COUNTER
that can go backwards. It is implemented as a COUNTER that doesn't check for
wraps. Hence, a DERIVE with min=0 is a COUNTER without automatic
wrap-handling. This can sometimes be useful if your counter wraps at some
strange value (ie, not a 32, or 64 bit counter), though COUNTER with a
reasonable max will usually produce the same result. If your DS is a counter
that should always increase until it wraps around, use COUNTER. Only use
DERIVE if your DS can count backwards.

	So the FINAL WORD on wraps, resets, and spikes is; set reasonable
max and min values!!!!!

	The only other thing to be careful about is ensure that your DS
counter cannot wrap twice in a single heartbeat interval, though typically
you would need a very long heartbeat and/or very fast counter for that to

	If you use counter with reasonable max and min values, wraps will be
automatically handled, and resets will (usually) be marked as "UNKNOWN". You
will never get unreasonable spikes in your graphs.

	So I'll say it again; set reasonable max and min values!!!!!

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