Good post by Brian Seltzer (thank you)
The move away from tape backups towards disk-based backups has been going on for a while now. Storing backups on disk generally means faster backups and faster restores. However, disk isn’t as cheap as tape, and storing many terabytes or even petabytes of data on disk can lead to sprawling storage systems. To reduce the cost and physical size of backup disk, backup storage is commonly deduplicated. This can have a pretty dramatic impact on the size of backed up data, especially if your retention period include multiple full backups.
Read on here