Storage tiering and lifecycle

Design the storage-tiering and lifecycle system for a media archive holding 50PB of assets (raw camera footage, masters, derivatives) with wildly varying access patterns: some assets are hammered, most are touched once and then cold for years, and a few cold ones occasionally need to come back fast for a re-edit. Storage cost is the dominant concern, but a retrieval that takes hours when an editor needs it now is a product failure. Design the tiers, the placement/migration policy, and how you handle retrieval-latency expectations.