In part 1 of this series, I discussed how to connect to the Exchange database cache and the importance of transaction logs in database transactions. In this next part, you'll learn how transaction logging and recovery work in Exchange.
The following blog post is a trimmed-down excerpt from the e-book "Exchange Server Troubleshooting Companion."
Transaction Logging and Recovery in Exchange
Of course, there is another reason that Exchange writes transactions to the log files before being committed to the database (.edb) file. By quickly committing transactions to disk (via transaction logs), it means that the transactions exist in two locations; memory and disk. If a failure occurs that causes the Information Store to terminate unexpectedly, the most recent transactions are still available and can be replayed into the database from the transaction logs after the database is mounted and to bring the database up-to-date.
For a long time, it was recommended to place your Exchange database file (.edb) onto different spindles than your transaction logs. This is still the recommendation when only one copy of a database exists (non-DAG protected). In fact, this is not for performance reasons but to assist recovery in the event of a storage failure. Say you lost your database drive, leaving you only with the transaction logs. Technically, if you still had every transaction log present since the database was first created, you could use ESEUTIL to generate a new database and commit every transaction from the logs into it. However, this is not usually the case. People usually resort to an Exchange-Aware backup, which leads us to why an Exchange-Aware backup truncates log files. When a Full Exchange-Aware backup is performed against a database, the .edb file is copied to the backup location, as well as any transaction logs present for the database. With these files, the database can be restored and the database can be brought into a Clean Shutdown state, meaning all transactions in the logs have been committed to the .edb file and the database can be mounted. As the backup completes, it sends a command to the ESE database engine stating that any transaction logs older than a certain point can be truncated (deleted). These logs are no longer required on the Exchange server because we now possess a copy of them in the backup set.