A recent technical analysis conducted by storage provider Scality contrasts the power density of hard disk drives (HDDs) and solid-state drives (SSDs) under various scenarios. 

Contrary to prevailing beliefs, SSDs, despite lacking moving parts, do not universally offer superior energy efficiency over HDDs. Scality's data indicates that, contingent upon workload and drive capacity, HDDs can demonstrate a power density advantage ranging from 19% to 94% over SSDs.

In the study, HDDs—with their integral mechanical components like platters and actuator arms—contend against the presumption of SSDs' lower power consumption attributed to their non-moving composition. Using the Micron 6500 ION 30.72TB QLC SSD and the Seagate Exos X22 22TB 7,200 RPM HDD for comparative analysis, the research emphasizes HDDs' power consumption efficiency, not necessarily their performance.

Using the terabyte per watt (TB/watt) metric for power density, in read-intensive tasks, HDDs have a 19% edge, and in write-intensive tasks, they lead by a significant 94%. While HDDs have a 14% higher power draw at idle, their power consumption during active read and write tasks is lower by 37% and 68%, respectively. Intense workloads reflect similar patterns, with HDDs using 40% and 63% less power during read-intensive and write-intensive tasks, respectively. It's pertinent to mention that these statistics might shift as drive capacities evolve.

For their tests, Scality employed two primary models: one with read-centric tasks composed of 10% idle, 80% reading, and 10% writing, and another focusing on write-intensive tasks with 10% idle, 10% reading, and 80% writing. Drives were operated in power states corresponding to these distributions.

However, Scality emphasizes that power consumption shouldn't dictate the sole criteria for choosing between SSDs and HDDs. The decision should factor in specific workload demands. While SSDs excel in read-heavy and latency-sensitive operations, HDDs are optimal for managing unstructured data workloads.

Scality Study on HDD vs. SSD Power Density: A Technical Perspective