Flash is <i>not</i> getting cheaper faster than HDD&#x27;s, once you control for write endurance.   One of the big messages in the paper is that IOPS matter.  If you only have a petabyte or two of data, you can use a brute force solution such as flash or 15k drives.   But at Cloud scale (we used to say Google scale, but there other companies like Amazon and Microsoft and Facebook that run at these scales now) you just can&#x27;t afford that much flash.<p>So the trick is to use every last IOPS that you can out of each and every disk, and that means provisioning and spreading out your data across an entire fleet of disks.  The alternative, where you silo your storage in disk pools, and where some disks or disk pools have IOPS that are unused --- wasted, and where you make up for that by using flash for your hot and warm workloads is just a much more expensive way to do things.<p>It&#x27;s for that reason that SMR drives aren&#x27;t all that interesting.  Sure, you get 20% more capacity, but at the cost of burning most of your IOPS for GC.  Or if you move all of your SMR-friendly data to the SMR drives, that&#x27;s cold data, and it means that you&#x27;ve wasted all of the IOPS that those SMR drives are capable of.  Which means the rest of your disk fleet is now much hotter, and you might have to buy more CMR spindles to cover the cost --- at which point the cost advantage of SMR drives disappears.<p>(I helped to author the section about SMR drives, and why we are interested in hybrid SMR&#x2F;CMR drives as a solution to this problem; read the paper for more details.)