The database storage used for backup does not have SSD storage. You require double the storage for the backup. For example, if the primary database uses 1TB storage and a secondary, replication, database uses another 1TB, then each of these databases require 2TB for backup. The storage pool must be at least 6TB: 1TB for the primary database, 1TB for the secondary database and 4TB for the primary and secondary backup (2TB for each backup).