Hello Mai,
Usually DMC deletes the closed alerts which are 30 days older.
There are a few situations where DMC does not delete the alert:
1.) If the alert still has the open status
2.) If the monitored database has been deleted
If the alerts belong to an old database, you can run the script to deleted old data as described in DMC manual at:
https://www.ibm.com/docs/en/db2-data-mgr-console/3.1.x?topic=support-cleaning-up-monitor-data-deleted-connections
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Cintia Ogura
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