[mrtg] mrtg in vmware

[Steve Shipway]

You have to be very careful about virtualising an MRTG server because of clock skew.

If the MRTG server is calculating any rates (IE, if it have a Target which is NOT type 'gauge') then it is liable to clock skew.  This is when the virtual system clock lags behind and jumps forward erratically due to the virtualisation scheduling of the server.  You do not normally notice this because the lag is when the virtual server is inactive; however it can cause inaccuracies when calculating rates of change, particularly if the virtual container is heavily loaded (high guest 'ready time' in VMware), or if the polling interval is short.  It will result in the graphs becoming more 'spiky' then in reality.

In addition, if you are monitoring CPU or Memory usage of a virtual machine via MRTG, then the values you get will be completely misleading.  The only way to get real values is from the virtual container itself - eg, with VMware, from the VirtualCentre API.  I have a MRTG plugin for VMWare to achieve this.

Finally, a heavily-loaded MRTG server will have high disk IO, making it a bad candidate for virtualisation as this affects performance.

I'd really recommend you don't virtualise a MRTG server, but if you have to, then make sure the ready time is very low, you use 5min polling, and don't use MRTG 'native mode'.

If you're planning on attending LISA09, I'm going to try and give a talk on 'Virtualisation and MRTG' via Skype at the MRTG BoF session.

Steve


________________________________
From: mrtg-bounces at lists.oetiker.ch [mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of Ramika Lawson
Sent: Thursday, 1 October 2009 11:20 a.m.
To: mrtg at lists.oetiker.ch
Subject: [mrtg] mrtg in vmware

Hi Steve,
How can MRTG be virtualized?

Regards,

Ramika Lawson
Senior Network Engineer - ITS

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