Using copy-only snapshot backups to affect how differentials are based
SQL Server records the history of successful database backups in the msdb system database. It uses this history in to decide how to base differential backups. In particular, SQL Server creates differential database backups as cumulative with respect to the last full database backup that it has recorded in the msdb. This action allows for a quick recovery in case a failure has been detected after the last full database backup.
Assume that full backups are created every day at midnight; differentials are created every day at 6AM, noon, and 6PM; and transaction log backups are created every two hours. If a failure occurs at 7:50 P.M. on Tuesday, then a point in time recovery could be achieved with a restore of: the full database from Tuesday at 12AM, followed by the differential at 6PM on Tuesday, and finally the transaction log at 8PM (choose "to 7:50 P.M.").
However, with Instant Recovery backups, you may not retain the daily full backup after the next full backup is created. If you require a point in time restore before the latest backup, the differentials are based on the backups that no longer exist. The alternative is to recover based on the last full backup that was retained. And you have to use a potentially long sequence of transaction log images.
To resolve this issue, NetBackup lets you create the SQL Server Snapshot Client backups that are not recorded in the msdb. To create these backups, NetBackup uses the copy-only backup feature, which allows the backups to be created as out-of-band.