You're referring to https://en.m.wikipedia.org/wiki/Hierarchical_storage_managem.... AFAIK, most systems work at the file level, but if you address space is large enough, you could implement this as a virtual memory system that swaps RAM to disk, disk to slower disk, slower disk to tape, tapes to tapes stored offline, etc.

I don't know what Amazon uses, either, but I expect Glacier data to be pushed out to some layer that doesn't need power while idle at all, heavily checksummed and also replicated across the globe. The relatively heavy prices tag for accessing it probably is there to keep contention on some relatively thin pipe low. For example, if they use blue ray, they may have only a few readers online for every 100 or 1000 customers, both to keep prices low and to differentiate Glacier more from their other products.