This option examines the number of expired in the performance monitor data tables. An expired record is defined as a record which is marked as overwritable, but has yet to be overwritten by a new record. Typically, the percentage of expired records to total records is small (less than 10%); if a large configuration change is made (some examples are below), the ratio of expired records may be different. In practice, we recommend this ratio not exceed 25% for any of the performance monitor tables (see below). You can purge all expired records for a given performance monitor table from the Table Maintenance property page in the Database Tools dialog. For more information, see the Database Tools - Database Maintenance dialog help.
Example changes:
Performance monitor data tables:
This part of the WhatsUp Health Threshold checks these specific database tables when looking for expired records.