Silo is defined as:
Silo Storage enables you to store deduplicated backups on secondary storage devices. The ability to store deduplicated data on secondary storage reduces the storage requirements and facilitates longer retention periods. The efficient use of storage space enables storing large volumes of backup data, which reduces the cost of backup storage and long term data retention.
Silo Storage also enables effective use of the primary disk storage by managing the disk space and periodically moving the data to the Silo Storage. This efficient disk management reduces the cost of storage.
Silo Storage provides the ability to store deduplicated data in tapes. This is performed as follows:
- Backup data from clients is first deduplicated in a primary disk.
- The deduplicated data, which includes the deduplication store and the data (called as Silos), are then copied to tape by running a Silo backup job.
- During a restore operation, if the data is available in disk it is restored from the disk. If the data is not available, then the necessary tapes are accessed to bring back the data to the disk, and then restored to the client.
To directly answer your question; If the primary is siloed it will only be on tape for the extent of the retention. Now if you Silo'ed your dash copy that can be your extended rentention silo copy.
In a tier scenario, this makes sense as your Fast primary disk is DASH'ed to slower secondary disk then silo'ed to tape.