During a normal backup operation, whether it be a streaming backup (Windows 2003/Exchange 2007 and earlier) or a VSS backup, all database pages are checked for the checksum value. If the calculated checksum value isn’t the same as the checksum stored in the database page an error is raised.
But when using an Exchange 2007 CCR environment and you create a backup of the passive copy of the Storage Group, the active copy of the database is never checksummed. Also, when you create a differential backup of the Exchange data using DPM, a block level backup is created an the database is never checksummed.
You can enable checksumming in these scenario’s so that the active copy of an Exchange 2007 Database is checked as well. This is part of the Online Maintenance in Exchange 2007 and can be enabled by entering a registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
To enable the database checksumming, add the following key:
REG_DWORD: Online Maintenance Checksum
Value: 1 (value of 0 will disable it)
It is also possible to enable page zeroing in these scenarios by adding the following key:
REG_DWORD: Zero Database Pages During Checksum
Value: 1 (value of 0 will disable it)
This is also recorded in the event log. ID 717 is logged when checksumming is started and ID 721 is logged when checksumming has ended. ID 718 is logged when page zeroing is started and ID 722 is logged when page zeroing has ended.
Using Database Checksumming you'll always know the status of your databases, even if your (active) database is not part of a backup cycle!