Measured per gigabyte, which seems a little bit silly for a lot of applications (especially server applications). For example, a modern SSD can do as many rando

Measured per gigabyte, which seems a little bit silly for a lot of applications (especially server applications). For example, a modern SSD can do as many random 4kB read IOs in a second as the average hard drive can do in a week! So if you're IO bound, then the cost is much much lower. Throughput is closer, but there's a nearly 10x and increasing gap there too with modern NVMe devices. If you're going to worry about SSD carbon footprint in servers, you need to include those factors too. May server applications are scaled on IO or throughput and not on storage, and its much harder to solve IO and throughput problems with tiering than it is to solve storage problems.