[rrd-users] Re: Problems with multi-DS RRD that gets asynchronous-DS updates
rader at teak.wiscnet.net
Tue Sep 21 19:51:38 MEST 1999
> From: Alex van den Bogaerdt
> One more thing: You can allow for some unknown values when building
> the RRA. Look at this:
> rrdtool create filename [--start|-b start time]
> [--step|-s step]
> [DS:ds-name:DST:heartbeat:min:max] [RRA:CF:xff:steps:rows]
> You will probably have set xff to 0.5 which means that half of the
> datapoints may be absent while still producing an other-than-unknown
> entry in the RRA.
> When sampling every 300 seconds and having an RRA which stores one
> datapoint per row, there is either 0% or 100% unknown so this won't
> change. However, in the "weekly" graph this unknown value should not
> be there anymore.
Correct: only my daily or sorter graphs have the "async updates
cause graph dropout problem".
So it seems the longer (weekly and greater) RRAs "tolerate"
the U values.
As a work-around, I suspect I could gather data every five minutes,
have the daily RRA have 15 (ten?) minute steps and a .5 xff. But that
causes a lot (imho, too much) of loss of granularity of my data.
I can't just set xff to 1! =;) Is there a way to define a daily
RRA at a five minute polling interval that will allow for async DSs
updates and make correct graphs with a five minute granularity?
I suspect the answer is no. I did #define DEBUG in rrd_update.c
and got a vague understanding of "rrdtool update". Frankly, I think
there's an inherient flaw in the RRD data structure: "last_update"
is kept for the entire RRD only, and yet "last_update" is different
for each DS. I will post the gory details about this theory to the
developers list in a few minutes.
For now, I'm just fudging my data collection times such that all DSes
get updated at the same time.
But note that this is really a kludge: if snmpget for input happens
quickly and the snmpget for output takes 60 seconds, then the data
for input would be skewed by a full 20%!
(For my RRDtool interface, this is not a big issue because my data
gathering operations time out after N (currently set to 5) seconds
by using "alarm" and the ALRM signal in perl.)
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