Just to be a contrarian, I took the pro-tape position. After all, tape is cheap, and tape on the shelf doesn't need power or even data center-grade air conditioning.
But as I made my arguments, I realized that the problem was that we were asking one back-end storage system to solve all our problems. What we really need is more intelligence in the archive management software. Our archiving, or content management, system should be able to store its index on a disk platform and store its data across multiple tiers of storage, based on the data's retrieval urgency.
The archive software should have simple processes for retiring a storage pool or media. A simple command should start the process of migrating the data from on old disk array to a new one, or from 400 LTO-3 to 100 LTO-5 tapes so you can retire your LTO-5 drives and upgrade to LTO-7s. A message when the migration is complete can then tell you it's OK to turn off your old storage.
The archive software can, as some do, also be responsible for data reduction and tracking the multiple copies of your archival data that long-term retention requires. It doesn't matter how reliable a storage system you use if the building it is in burns to the ground. Your data needs to be stored in multiple locations, and the archive system should keep track of where those copies are--even if it relies on the storage back end to get it from point A to point B.
Before you spend a lot of time and money on the storage part of your archive system, think about how you'll get the data there and whether some problems aren't better solved at that layer.
