Just a quick note on an issue we've seen a time or two here in DPM product support. Backup administrators who have scheduled System Center Data Protection Manager (DPM) backups for Microsoft Exchange databases might run into backup failures if circular logging is turned on for those Exchange databases. In such cases, DPM incremental backups fails with VSS error 0x800423f4 and a retry of the backup job does not resolve the problem. This is expected behavior because any backup application will not be able to perform reliable incremental backups for the simple reason that the log gets automatically overwritten once the Exchange application reaches the end of the log. If the Exchange administrator follows the recommended guideline of turning off the circular logging flag on the Exchange databases then backups can be reliable. Here is a good blog post from the Exchange feature team that describes this issue : http://msexchangeteam.com/archive/2010/08/18/455857.aspx.
Hope this helps,
J.C. Hornbeck | System Center Knowledge Engineer
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