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How can I avoid losing sensor data when restarting PRTG?



We have found that we very often lose sensor data when restarting the PRTG Server and Probe services. For instance, we recently noticed that PRTG was performing a little sluggishly. We restarted the services and found that around 12 hours of sensor data just disappeared on ALL sensors. I was working in it just before restarting and I'm fairly certain the data was there before. We've had this happen to us a number of times now and would like to find out why so we can avoid losing valuable data. Has anyone else experienced this? Any suggestions or solutions?

lost prtg sensor-data sensor-history

Created on May 28, 2010 8:02:24 PM

Last change on May 31, 2010 1:46:53 PM by  Daniel Zobel [Product Manager]

3 Replies

Accepted Answer



The data is probably there—you only need to recalculate it.

For a faster startup of the software during shut down of the services data is written to a cache file. During startup that cache file is loaded to present the graphs.

If writing the cache file does not work properly for some reason the existing "old" cache file is used and that does not show the most recent data, of course. But the monitoring data is constantly written to the database and from there it can be recalculated.

Please try the following:

  • Stop both PRTG services (core and probe).
  • Head to your data folder (can be discerned from the PRTG Server Administrator tool, under the "Core Server" tab)
  • Delete the "PRTG Graph Data Cache.dat" file
  • Restart the PRTG services
  • Log in to the PRTG web interface
  • There, go to Setup | System Status and check the background tasks. It should state that it is calculating historic data.


  • Refresh the System Status page from time to time (is the number being reduced?)
  • Let it run until it is done and then take a look at the historic data of your sensors—it should now be available.

Created on May 31, 2010 11:14:06 AM by  Aurelio Lombardi [Paessler Support]

Last change on May 31, 2010 1:46:00 PM by  Daniel Zobel [Product Manager]



We have had the exact same thing happen. Is it the case that we need to go through this procedure each time we restart PRTG? In addition, since we have NEVER followed this process, are we going to have duplicates of sensors that we re-created? Would any changes be overwritten?

Created on Jul 1, 2014 2:42:55 PM




recalculating the Graph Data Cache can be initiated from within the web interface since version 14 of PRTG

Setup -> System Administration -> Administrative Tool

select to

Recalculate PRTG Graph Data Cache

the graph data cache is used during startup to be able to display sensor graphs faster.

No data or configuration is touched!!

Usually when you shutdown PRTG the PRTG Graph Data Cache is written to disk.

sometimes this can take a little longer.

If you restart your server e.g. due to updates PRTG is sometimes not given enough time by the system to write the cache to disk. This will leave you with an old cache file, and that one will be used at the next start of PRTG to display the graphs.

But since it is an old cache file it will leave you with the gaps.

Recalculating the cache data from the data in the monitoring database will solve this issue.

Created on Jul 2, 2014 7:21:16 AM by  Aurelio Lombardi [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.