We sometimes get people asking about the difference between VSS Full Backup and VSS Copy Backup in Windows Server 2008.
Basically, for certain applications (such as Exchange), VSS Copy Backup preserves logs and VSS Full Backup truncate logs. Yes, we know, this explanation is not very enlightening per se…
The point is that some applications create log files. For example, Exchange server creates a transaction log file. If you are using only Windows Server Backup to protect the applications, you can use any option to backup the data. However, if you are using a third party backup application, you should not use VSS Full Backup because it will delete the logs at the end of the backup. If the third party backup application relies on those logs to take incremental backups, it would take full backups all the time – a less than optimal situation…
So, when you do a VSS full backup, you create backup of all the files – but after that, the backup application may truncate logs on the file system.
On the other hand, when you do a VSS copy backup, all files are backed up and you preserve the all the applications files including log files on the live system. That means, you do preserve information on which files where changed, deleted, etc, and this will not affect the sequence of incremental and differential that might happen independent of the copy backup. Therefore, a copy backup cannot be used as a starting point for incremental/differential backups/restores.
Just a final technical note: The backup type (full, copy, incremental) can be specified by a VSS-based backup application at the beginning of the backup session, using the IVssBackupComponents::SetBackupState. In response to that, any application that implements a VSS writer can choose to truncate the logs in the OnBackupComplete VSS event. This is one of the last events that a VSS-based backup application (such as DPM) sends to all affected writer at the end of the backup session.
