metadata

Every rank writes its own shard, a metadata file maps every chunk to a global tensor, and a loader intersects the old layout with the new. The bytes per rank for a 405B, the manifest that makes the write atomic, the metadata that makes resharding possible, and the host-memory staging that keeps the GPUs running.

One gives you POSIX semantics and hundreds of gigabytes per second at a price per terabyte that hurts at petabyte scale. The other is cheap and durable and answers a first byte in tens of milliseconds. The cost arithmetic that makes most clusters run both, and the metadata limit that decides how you shard.