Scenario: A mailbox database or mailbox database replica was created on a server and you receive the following warning:
WARNING: Please restart the Microsoft Exchange Information Store service on serverservername after adding new mailbox databases. Alternatively, you can restart the server
Reason: The information store service in Exchange 2013 decides how much memory it will allocate for each database during the information store service startup. Therefore if you add databases after this is service is started, the information store does not have a predetermined amount of memory to manage the database.
Note: The database is still functional even if you do not restart the service, but restarting the serice is still recommended.
Note: In earlier versions of Exchange, the information store service tried utilizing as much memory as it could at the information store service startup.