When the Data Collectors are not connected or display a status of Unknown, this could be a result of a bad upgrade of the DC or it could be other components of Performance Management not working.
Here are some troubleshooting steps that could help reconnect your Data Collectors.
Release: CAPM 20.2
Release: CAPM 21.2
Confirm Status of Data Collector/s
systemctl status dcmd
systemctl status activemq
Stop and Restart your Data Collector/s
systemctl stop dcmd
systemctl stop activemq
systemctl start dcmd (this should spawn activemq automatically)
Stop DC, move data and Restart your Data Collector/s:
systemctl stop DCMD
systemctl stop activemq
ps -ef | grep java
kill -9 <PID> (if Needed)
cd /opt/IMDataCollector/apache-karaf-*/
mv data data.old
systemctl start dcmd (this should spawn activemq automatically)
Stop/Restart your Data Collector/s and Aggregator:
Stop DC: systemctl stop dcmd
systemctl stop activemq
Stop DA: systemctl stop dadaemon
systemctl stop activemq
Start DA: systemctl start dadaemon (this should spawn activemq automatically)
Start DC: systemctl start dcmd (this should spawn activemq automatically)
In Performance Center UI go to Admin/Data Sources/data sources and Re-sync the DA (note: re-syncing can take up to 5 min)