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Cache recalculation on PRTG update or server/service restart.




Our PRTG environment has 12,000 sensors of various kinds running on a Windows Server 2012 R2 VM with 6 cores and 12 GB of memory.

We've just upgraded the PRTG installation from v15.1.15.2022+ to and it's taking quite a long time for all of our sensors to instantiate. Nearly all of them went green initially, then slowly dropped into an unknown state with a counter of 500+ "cache calculation" background tasks pending, and PRTG has been sitting in this state for quite some time now.

My understanding is that this happens because PRTG needs to recalculate its historical data cache every time it is updated or otherwise restarted. However, I'm not really clear on whether this is normal behavior or how long this recalculation is supposed to take for a given environment size. It seems somewhat unlikely that it's normal for PRTG to take literally hours to bring all of its sensors up after every restart - so I assume it's more likely that there's something out of place with our installation. We are already looking at decreasing historical data retention periods and lowering our average sensor polling intervals. Is there anything else we should be looking at?

I've read a few other KB articles on this question, but none of them have provided the kind of answer I'm looking for.


cache restart upgrade

Created on Oct 29, 2015 12:58:12 AM

1 Reply



Hello, we appreciate your inquiry.

Setups above 10,000 sensors are what we consider large installations. We recommend the following system/hardware in such cases:

  • Physical Server, no Cluster, 8 Cores, 16 GB RAM and 2 TB of disk for 365 days of historic data.

The cache recalculation for 12,000 sensors can also take quite a while(many hours). The extra CPU utilization on the core server may lead to unknown sensors, this can particularly affect sensors on the Local Probe. Decreasing historical data retention periods and lowering our average sensor polling intervals are a good way to reduce load and speed up the recalculation.

Please note that the recalculation can't be interrupted nor paused, restarting the core-server will restart it from the beginning. The cache recalculation isn't always required/triggered during updates, only for updates which contain major changes (That's the case for 15.1.15 to 15.4.20).

Best Regards,

Created on Oct 30, 2015 12:43:46 PM by  Luciano Lingnau [Paessler]

Last change on Aug 13, 2019 8:26:26 AM by  Sebastian Kniege [Paessler Support]

Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.