[mrtg] Re: MRTG Sizing Issues?

Rich Adamson radamson at routers.com
Wed Apr 7 10:52:53 MEST 2004

> Do anybody knows if there are any limitations on
> number of devices/interfaces mrtg can work with?

The upper limit on number of devices is either limited
by cpu or poll-response delay.
> As far as I know, mrtg is a single process perl
> script.
> Therefore, I guess I can run multi-instance of mrtg so
> as to take advantge of multi-cpu to catch up the 5-min
> polling when there are large number of
> devices/interfaces.

Others have run multiple instances before, no problem.
> However, I am not 100% sure if this works and if there
> be any other limitations in large scale deployment.

Two issues: a) cycles consumed by polling, and, b) cycles
consumed building the graphics from the polled data.

The default mrtg configuration is essentially a single-
threaded app whose poll completion time is highly dependent
on the round-trip poll-response time (including propagation
delays, equipment response time, etc). If all devices to
be polled responded in exactly 100 milliseconds (WAN), the
max would be roughly 3,000 devices. However if all
responded in 10 milliseconds (LAN), the max would be
about 30,000 devices. (Obviously, lots of assumptions
in that basic calculation.)

Changing the mrtg config so as to run two independent
polling processes would almost double those numbers.

So, the bottom line is that throwing a multi-cpu motherboard
and/or high speed disk at mrtg isn't going to have much 
impact if the poll-response times are relatively high.

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