This solution applies to the Barracuda Backup Service.
The most common cause of failed incremental (also called 'Smart' or 'Log') Exchange backups through the Barracuda Backup Agent for the Barracuda Backup Service is the presence of Circular Logging. You may see an error in your Backup Service UI or Reports that looks like this:
Incremental backups cannot be performed while circular logging is enabled for <Exchange Storage Group>. Disable circular logging to perform incremental backups.
To be able to perform Smart or Log backups of Exchange, you will need to uncheck the Enable Circular Logging setting for the Exchange storage groups on the Exchange server.
Circular Logging is designed to save storage on your Exchange server by preventing the continuous buildup of transaction log files on your drive. This is achieved by creating and writing to only four transaction log files. Once the fourth log is full it cycles back to the first log and overwrites it. This creates a logging cycle, which will go on to overwrite the second, third, and fourth, and then repeating. Exchange does this because it assumes that by the time the fourth log is filled then the first log must have been committed by that point.
When trying to run a Smart or Log backup, the transaction logs are used and saved to be "replayed" when a restore is attempted on that data. If log recycling (Circular Logging) is present, this is not possible. This causes restoring to only be available to the point of the last full backup as the transaction logs are inconsistent in sequence and thus unreliable.
You will need to reboot the Exchange server after making this change. It is also important to note that before attempting another Smart or Log backup, you must perform a full backup. This will ensure that the transaction logs have been reset and are ready for use in a Smart or Log backup.
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