Bulletin 78 - EXCHANGE SOFT AND HARD RECOVERY.doc

This Bulletin goes into more details about rules for hard and soft recovery and how they work. Seeing that we depend on soft and hard recovery in many disaster recovery situations, it is good to get more familiar with the subject. ? EXCHANGE SOFT AND HARD RECOVERY ? This is broken down as follows: ? 1. Introduction 2. Soft recovery ??? - Some of the fundamental rules of transaction log file replay ??? - Advanced Soft Recovery Scenarios 3. Hard Recovery 4. Additional reading ? Here it is: ? 1. Introduction ? As used in Microsoft Exchange Server 2003, the word recovery must be distinguished from the word restore. Restore is the act of putting database and log files back into place on a server, and recovery is the act of replaying transaction logs into the restored database. There are two forms of recovery: ??? -?Soft recovery?- A transaction log replay process that occurs when a database is re-mounted after an unexpected stop, or when transaction logs are replayed into an offline file copy backup of a database. ??? -?Hard recovery?- A transaction log replay process that occurs after restoring a database from an online backup. 2. Soft recovery ? In the default soft recovery scenario, an external event unexpectedly stops an Exchange database, but the database and log files remain intact and in place. When the database is mounted again, Exchange reads the checkpoint file and begins to replay the transaction log that is listed as the checkpoint log. If no checkpoint file exists, replay begins with the oldest log file available in the transaction log folder for the storage group. Exchange writes to the database files completed transactions found in the log file that have not already been written and reverses any incomplete transactions. Exchange never begins writing a transaction into the database files until all the operations composing it have been secured to the log files. You do not need to physically undo or back out a transaction in the database if all uncommitte