[mrtg] Re: How many OID can I read with one mrtg system

[Matthew Petach]

--- Rich Adamson <radamson at routers.com> wrote:
> > do have anybody experience with a mrtg on a windows system which measure
> > about 200 OID at the same time?
> > How many power (cpu, ram and so on) should have this windows system?
> > Any hints to do this?
> > 
> > In the past a measured about max. 50 different values on one PC wiht
> > windows and mrtg.
> 
> If all 200 OID's are in the same polling app, the cpu isn't going to
> make much difference. The issue will be the round-trip time required
> to poll a remote device and obtain a response. The polling happens
> sequentially, one oid/device after another.
> 
> If the polling cycle is every five minutes and all oid's/devices are
> within the same polling app, and "each" oid requires a 100 millisecond
> poll-response time, then the best one can do is something around
> 25,000 oid's (assuming I didn't screw up the math).

I think your math might be off by a bit...
100ms poll-response means you can poll 10 OIDs per second; a five
minute cycle is 300 seconds, which means you max out at about 3,000
OIDs per box if you're doing them all linearly.
I've found that I can run about 4 groups of devices in parallel on
a single box before the disk I/O becomes a bottleneck, which gives
about 12,000 total OIDs per 5 minute cycle; each interface needs 2
OIDs, plus every router gets its uptime info polled, so you're well
below 5,000 interfaces on a reasonable system every 5 minutes.

> The real key is knowing exactly what your poll-response times are,
> and what else is running on that same box that consumes processor
> cycles.

Actually, I've found that disk I/O is my main bottleneck.  The actual
work of sending and receiving the queries is relatively trivial compared 
to updating the on-disk records; obviously, using RRD instead of log
format makes a large difference here.

Matt

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