Well, the Backblaze blog post only talks about the raw spindle costs, and strongly implies that 4TB drives were the lowest cost option and ignores the existence

Well, the Backblaze blog post only talks about the raw spindle costs, and strongly implies that 4TB drives were the lowest cost option and ignores the existence of any disks larger than 8TB. Which is true only if you are talking about consumer drives and only concerned about raw spindle costs. Otherwise, it's a highly misleading post. The electricity cost and the fractional share of the disk tray and server to attach the HDD are mostly constant no matter whether you use a 4TB, 8TB, 10TB, or 14TB disk. So if you are only worried about renting fewer cabinets and paying less for electricity, then if 8TB drives make sense, 14TB drives would make even more sense, no? Of course, you have to worry about more than just bytes; you also have to worry about IOPS. So it starts being about whether you can use nearly all of the IOPS in every drive. (This is why having a "hot pool" and a "cold pool" of disks really don't make that much sense. You'll leaving money on the table by wasting the potential usable IOPS in your cold pool. Instead you can get a much more TCO positive solution by mixing your hot and cold data so you case use most of the IOPS and most of the bytes for all of your drives.) But if you think SSD's are so "cheap", why not use 14TB drives plus SSD's? Perhaps because using SSD's for storage (as opposed to cache) really isn't that cheap as people think. Sure, lots of IOPS, but unless the bytes you need to read are the right ones to be stored in the SSD, the cost per byte for SSD's is quite astronomical, even taking electricity, server attachment costs, etc., into account. As a result, I believe using large numbers of spinning drives will make sense for a lot longer than you think.