This knowledge base article provides steps of one approach which may resolve a, "Database Not Responding" (DNR) message on the Global Scan page. When this happens, check the uptime.log file for messages similar to the following line, but note that the numbers differ from the example.
011-03-29 11:40:48,873 ERROR (ProtocolHandler:79) -In command entities=168,169,171,175,176,177,179|commandType=query|commandQueryName=GET_GLOBAL_SCAN_ENTITIES
This type of message likely indicates that the Global Scan page is attempting to display recent performance data from an element that was removed or deleted from Uptime Infrastructure Monitor. To verify this condition, run the following queries to see if there are entries in the entity_latest_data_sample table that do not correspond to an existing element in the entity table:
select host_id from entity_latest_data_sample where host_id not in (select entity_id from entity);
For each ID that is listed from the above query, run the following two queries, replacing <ID> with the ID number. The first query should return an empty set but the second should have one or more results.
select entity_id from entity where entity_id=<ID>;
select * from entity_latest_data_sample where host_id=<ID>;
To remove the erroneous performance data that is causing this issue, run the following delete statement.
delete from entity_latest_data_sample where host_id=<ID>;
Once the delete statement runs for each of the IDs from the first query, view the Global Scan page to verify that the issue is resolved.