Are HDDs greener than SSDs?

Here’s a wrinkle in corporate environmental efforts. Futurum Research is backing a recently published academic paper that suggests hard disk drives (HDD) could be greener than solid state drives (SSD) when taking into consideration the manufacturing process.

The paper in question is called “The Dirty Secret of SSDs: Embodied Carbon” and was published last year by Swamit Tannu, computer science professor at the University of Wisconsin-Madison, and Prashant J. Nair, an assistant computer science professor at the University of British Columbia.

The paper states that the biggest carbon emissions happen at the time of manufacture, with production of SSDs generating much more carbon than disk drives of equal capacity.

The study claims manufacturing a gigabyte of flash emits 0.16 Kg of CO2 and is a significant fraction of the total carbon emission in the system. HDDs, by comparison, have an average manufacturing emission of 0.02 Kg-CO2e/GB.

The authors estimate that manufacturing storage devices has resulted in 20 million metric tonnes of CO2 emissions in 2021 alone, but they did not specify the breakdown among SSDs and HDDs.

The paper noted SSDs “have about 8x higher embodied cost compared to Hard Disk Drives (HDDs) with an identical capacity.”

The reason is that hard drive manufacturing is far less reliant on semiconductors, which are the real carbon producers. To be sure, a hard drive does have a chipset on it for the controller, but by and large, it is a mechanical device. An SSD is all chips and PCB (printed circuit board).

“The takeaway here is that it’s not as cut and dry as ‘SSDs are more sustainable than HDDs because they use less energy,’ which is what a lot of vendors promote,” Lewis told me by email. “A lot of the discussion around sustainability gets focused only on the emissions caused by operations, and ignore other sustainability impacts, such as emissions incurred during manufacturing and along the value chain.”

The Tannu and Nair study did look at the operational power consumption of both forms of hard disks and found that there is indeed a significant difference. It assumes average HDD power consumption to be 4.2W, while a comparable SSD consumes just 1.3W power.

It found that over a five-year lifespan, a 1TB hard disk will consume 184 kW of power while a 1TB SSD will consume just 57 kW.

The difference in lifespan is a key factor that Lewis believes was missed in the initial study. The refresh cycles of HDDs and SSDs are very different. Hard drives are typically replaced after four to five years, while an SSD can run for up to 10 years. At least one SSD vendor, VAST Data, is offering a 10-year warranty for its drives. No hard disk maker offers a 10-year warranty for any of their mechanical drives.

“While the report does acknowledge that SSDs contribute significantly less emissions while in use due to energy efficiency, it suggests that this difference might not overcome the environmental cost of manufacturing when evaluating the emissions over the total lifespan,” Lewis wrote.

This doesn’t mean that IT teams looking to improve sustainability should only purchase HDDs, said Lewis. It means that there needs to be a more comprehensive evaluation of emissions across the entire lifecycle, including manufacturing, energy efficiency, lifespan of the device, and potential to reduce physical footprint.

Lewis believes that the high initial carbon costs of making the SSD will be offset by the much longer life cycle.

“My estimations, using the calculations in the research study but changing the SSD to last 10 years, made it very close,” Lewis said. “The calculation for the HDD was still slightly lower, but when considering other factors, such as the increasing density of SSD devices that might allow for an overall reduced footprint in the data center, or a reduction in e-waste, it’s likely that the SSD comes out ahead.”