Exchange 2010 RTM only supported Continuous Replication File Mode.
Exchange 2010 SP1 now supports Continuous Replication Block Mode.
Ok so whats the difference?
Continuous Replication Block Mode
A form of continuous replication that replicates blocks of ESE transaction data between the ESE log buffers on the active mailbox database copy to replication log buffers on one or more passive mailbox databases copies. What does this mean? This means Exchange 2010 has replicate log buffers before the transaction log file is closed. This reduces the amount of transactions that are lost should a failover occur. If a transaction log is not played to the passive node in time, data may be lost during a failover. This is what AutoDatabaseMountDial is for, controls if the database will be mounted based on the based on the number of log files missing by the copy being mounted.
Continuous Replication File Mode
A form of continuous replication that replicates closed transaction log files from the active mailbox database copy to one or more passive mailbox database copies. This means the transaction logs need to be closed before they can be shipped. This is one of the reasons transaction log size as reduced from 5MB to 1MB as of Exchange 2007 - to minimize data loss during Exchange failover.
Please note if a log file does not replicate in time when a failover occurs - the hub transport server in the same site has a service a process known as transport dumpster. The transport dumpster resides on every hub transport server by default however it is not used unless a mailbox failover occurs. What the transport dumpster does is hold email that has already been delivered to the active mailbox server for a specified period of time. In the event that a DAG node fails which prevents the most recent logs from being replicated over, the transport dumpster can redeliver this email.
Transport Dumpster was first introduced in Exchange 2007 for CCR (Continuous Cluster Replication). Please see:
So now your asking me why introduce Continuous Replication Block Mode if there is a process called Transport Dumpster that replays emails after a failover? Good question!
The answer is not all exchange mailbox transactions are caught in transport dumpster. Transport dumpster, as it resides on the hub transport will only ever capture SMTP based transactions. There are other internal transactions Exchange generates directly on the mailbox database. These transactions will not reside within Transport Dumpster. Also things like de-duplication software that replaces emails with stubs. These cause transactions to occur which are not registered in Transport Dumpster.
Can I switch between Continuous Replication File Mode and Continuous Replication Block Mode if I'm running Exchange 2010 SP1?
No - The system can automatically switch between file mode and block mode based on the passive copy's ability to keep up with continuous replication.