[mrtg-developers] Mathematical operations on OIDs

[nisha]

Hi Simon,

Thanks for the response. But my graph needs to have only the positive values
and not negative values. 
The negative values are caused because the oid3 - (oid4 * 10) calculation
returns a negative value.
In my case when oid3 - (oid4 * 10) is a negative value (not always though),
how can i handle them in mrtg?

And another query is what will happen if the 4 counters (oid1 to oid4)
overflow?
For example, the oid4 counter resets at a slower rate than the oid3 counter.
There are chances of getting a spike on the graph when oid3 value is very
high and oid4 value is negligible and 
oid3 - (oid4 * 10) is approx equal to oid3.
how can we handle this scenario?


Regards,
Nisha



Westlake, Simon wrote:
> 
> Nisha,
> 
> For reference, this is the script I use in order to tune my files (I graph
> cable modem signal levels and I have to tune all the files to allow them
> to display negative dB values)
> 
> for RRD in `ls /var/www/mrtg/*/*-down.rrd` ; do rrdtool tune $RRD -i
> ds0:-60 -a ds0:60 blah ; done
> 
> You will obviously have to modify the values to match your setup..
> 
> HTH
> 
> --
> Simon Westlake
> 
> 
> -----Original Message-----
> From: mrtg-developers-bounces at lists.oetiker.ch
> [mailto:mrtg-developers-bounces at lists.oetiker.ch] On Behalf Of S Shipway
> Sent: Wednesday, October 21, 2009 5:29 PM
> To: mrtg-developers at lists.oetiker.ch
> Subject: Re: [mrtg-developers] Mathematical operations on OIDs
> 
> 
> MRTG is able to handle negative values, provided that:
> a)       you are using RRDTool as your backend, not native mode, and
> b)       You manually set the Minimum for the DS appropriately within the
> RRD file using RRDTune (MRTG always sets the minimum to 0).
> 
> Also, unless this is a gauge, you might need to use 'derive' rather than
> 'counter' in order to cope with negative slopes.
> 
> Steve
> 
> ________________________________
> From: nisha [via MRTG Mailinglists]
> [mailto:ml-node+3864251-1100011265 at n2.nabble.com]
> Sent: Wednesday, 21 October 2009 11:04 p.m.
> To: Steve Shipway
> Subject: RE: [mrtg-developers] Mathematical operations on OIDs
> 
> Hi Steve,
> 
> This solution worked. But now i have a problem when the calcualtion
> results in a negative value.
> 
> Lets say [oid1 - (oid2 * 5)] gives a positive value and [oid3 - (oid4 *
> 10)] gives a negative value then the rrds are not able to update these
> values and hence my graph shows Nan
> 
> Have you come across something like this before? Can mrtg can handle this?
> 
> Regards,
> Nisha
> 
> S Shipway wrote:
> This is not possible as you state it; Target line calculations can only be
> done on an OID pair.  However you might achieve it like this:
> 
> Target[targetname]: oid1&oid3:comm. at host - ( oid2&oid4:comm at host * 5 + (
> pseudoZero&oid4:comm. at host * 5 ) )
> 
> Note that the spaces around the brackets and arithmetic signs are
> essential.  'pseudoZero' is a magic MRTG virtualOID that always resolves
> to zero.
> 
> Check the documentation on complex calculated Target lines online at
> www.mrtg.org<http://www.mrtg.org/> or get a copy of the book from
> www.steveshipway.org/book<http://www.steveshipway.org/book>
> 
> Your example below is using oid4x10 in one place but oid4x8 in another but
> I guess that's just a typo.
> 
> Steve
> 
> ________________________________
> From: nisha (via Nabble) [mailto:ml-user+230579-585948030 at n2.nabble.com]
> Sent: Thursday, 27 August 2009 11:02 p.m.
> To: Steve Shipway
> Subject: [mrtg-developers] Mathematical operations on OIDs
> 
> Hi all,
> 
> I want to perform mathematical operations on OIDs in the target line. For
> example, there are 4 oids as shown below:
>         traffic packet
> In oid1 oid2
> Out oid3 oid4
> 
> 
> Now i want to apply the following formula
>         traffic in  = oid1 - (oid2 * 5)
>         traffic out = oid3 - (oid4 * 10)
> 
> My target line in router cfg file is like this:
> Target[targetname]: oid1&oid3:comm at hostname - (oid2 * 5)&(oid4 *
> 8):comm at hostname
> 
> Is this possible in MRTG? Can the router cfg file handle oids with
> mathematical operations?
> 
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