[mrtg] OID Math

[Robert Gahl]

I've read much of the documentation on doing math on the OIDs but I can't 
seem to come up with the proper syntax for doing it properly.

We are able to query JVM (Java Virtual Machine) Total Memory and Free 
Memory (OIDs 1.3.6.1.4.1.2021.51.101.4 and 1.3.6.1.4.1.2021.51.101.3) from 
some SNMP code we wrote, and I can query these and get data from them.

The problem I'm facing is that I want to display Total Memory and Used 
Memory, so I would like to have one graph be Total Memory and the other 
graph be Total Memory minus Free memory (i.e., Used Memory). I've tried the 
following definition in MRTG:

Target[mem]: 
1.3.6.1.4.1.2021.51.101.4:public at 192.168.15.85&1.3.6.1.4.1.2021.51.101.4:public at 192.168.15.85 
- 1.3.6.1.4.1.2021.51.101.3:public at 192.168.15.85:

but it doesn't work. Any suggestions on what I can do or how to reorganize 
this to make it work? I'm lost on what the exact syntax should be.

Thanks.

===
Bob Gahl Bicycle (Ryan Vanguard) Mobile ||     @
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"If you're trying to be politically correct you're like a chameleon
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somebody?" Robin Williams

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