[mrtg] 0 value in "target" - for adding different values

[Grzegorz Mucha]

Hello,

I'm not very advanced in MRTG configuration so please forgive me if the
answer to my question is trival.

I use MRTG to monitor several important parameters on my server,
including traffic, load, memory and temperature. One of things I measure
is CPU load. I use two values for building a graph: current user load
and current system load now, which is expressed in .cfg file like this:

Target[localhost.cpu]:ssCpuRawUser.0&ssCpuRawSystem.0:public at localhost

(both ssCpuRawUser and ssCpuRawSystem are defined in proper MIB in my
system)

But this measuring is not perfect. What I would like to see on my graph is:
- user load
- user load + system load

As far as I understood "Target" argument syntax, I can add several
values to calculate totals, but only in pairs like:

Target[sth]: valueA&valueB + valueC&valueD

and it is impossible to do

Target[sth]: (valueA + valueC)&valueB

am I right?

If it's true, what I could do in my case is to write something like this:

Target[localhost.cpu]:ssCpuRawUser.0&ssCpuRawSystem.0:public at localhost +
ZeroValue&ssCpuRawUser.0:public at localhost

where "ZeroValue" is something that is _always_ equal to 0 (zero). But I
have no idea how to define this arithmetical zero.

The above example is only one of many useful. For instance I would like
to see on one chart the following values:
- free physical memory
- used swap memory
There's now SNMP parameter for used swap, so the only way to have this
is to calculate (total swap - free swap)

Is there any way to do this except for looking for non-significant SNMP
parameter that seems to return 0 every time?

Best regards, Grzegorz

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