A Differential backup of a Microsoft Exchange Server 2010 database saves changes to the database that have occurred since the last Full or Incremental backup. When the database files and log files are restored by the system, they can be recovered to the state they were in at the last Differential backup.
The data stored in a Differential backup includes only the transaction log files up to the current checkpoint. Differential backups do not delete or change the log files, nor do they change the database headers. Using a Differential backup to recover a database requires the restoration of only two data sets: the last Full backup, and then the most recent Differential backup.
The disadvantage of using Differential backups is that the Differential backups duplicate the backed up data in each backup until a Full backup is performed. If there are many Differential backups taken between Full backups, the storage space required can greatly exceed that required by the same number of Incremental backups. Exchange does not allow a Differential backup to occur when there has been no previous Full or Incremental backup to establish the starting point for Differential backups.
Since backups taken by Store Writer from the active database locations and the backups taken by Replication Writer from the copy database locations both use VSS technology, these backup can be used interchangeably. A Full backup taken from the copy location can be followed by a Differential backup from the active location and vice versa. One basic restriction to keep in mind in these scenarios is the last backup state is maintained in the active database’s header, and the changes to the database header is written to transaction logs, replicated and replayed at the copy database location just like all other transaction logs in DAG deployments. Interoperability of backups and restores between Store Writer and Replication Writer enables backup applications to provide the functionality to run all backups exclusively on a specific DAG node irrespective of whether the node is active or passive as well as to run backups exclusively from the passive node or exclusively from the active node.
When the VSS_BACKUP_TYPE enumeration in the Volume Shadow Copy Service is set to VSS_BT_DIFFERENTIAL, the following components will be included in a Differential backup:
A database with the logical path Microsoft Exchange Server\Microsoft Information Store\<Server Name>\<Database GUID>
A log file with the logical path Microsoft Exchange Server\Microsoft Information Store\<Server Name>\<Database GUID>
Similarly, if the Differential backup is taken from the DAG copy locations using the Replication Writer, above components will be included in the Differential backup with the exception that in the logical file paths the \<Server Name> will be pre-pended by \Replica to designate the actual files coming from DAG copy location.