Snapshot high-speed backup requires as many backup volumes as the number of specified backup generations. Accordingly, snapshot backup is disabled if:
All the specified number of generations have been backed up, and
There is no new volume that can be used as a backup volume.
Performing snapshot high-speed backup simultaneously with the same transaction volume specified performs two backups in parallel.
However, performing snapshot high-speed backup simultaneously more often than the number of preservation generations cancels the backup operations starting from the oldest one. In other words, you cannot perform more simultaneous backups than the specified number of generations.