Using copy-only snapshot backups to affect how differentials are based

When you use both full backups and snapshot backups to protect SQL Server, the previous snapshot backup expires after the next snapshot backup is created. If you require a point in time restore before the latest backup, the differentials are based on a snapshot backup that no longer exists. Alternatively, NetBackup lets you create copy-only backups that are out-of-band so the backup does not reset the differential baseline. Differential backups are then based on the last full backup.

If a failure occurs and is detected immediately, you can restore the last full backup. Then you can replay the necessary transaction logs to achieve recovery. However, if a failure is not detected until after the next full backup, then there are no snapshot backups available to restore. When you use copy-only backups, each differential is instead based on the last full backup that is not copy-only. You can restore the last full backup, restore the latest differential backup, then restore the necessary transaction log backups before the error occurred.

The copy-only attribute appears in the properties for the snapshot backup image. Differential backups are automatically associated with the correct full backup. The SQL Agent recognizes these backups when it selects the recovery set for the full database restore.