How Do the Embodied Carbon Dioxide Equivalents of Flash Compare to HDDs? [Part 1]

There are academic papers that have been published in the last couple of years comparing the embodied carbon dioxide equivalent (CO2e) of hard disk drives (HDDs) and solid-state disks (SSDs). One example is the University of Wisconsin paper "The Dirty Secret of SSDs: Embodied Carbon," published in October 2023. In that paper, researchers compared the embodied CO2e of consumer-level HDDs and SSDs manufactured in 2017 on a single device to single device basis. If you take its comparisons at face value, you'd walk away believing that SSDs are up to 8x worse than HDDs when it comes to manufacturing (aka non-use phase) emissions and that building storage systems from flash is much worse for the environment than building them with HDDs.

While that may have been true a few years ago, today, published data from HDD and SSD manufacturers shows that the embodied CO2e of SSDs has decreased much faster than it has for HDDs, and that a current comparison has the manufacturing emissions associated with SSDs at less than 2x that of HDDs.

Getting to an Updated Embodied CO2e Baseline

In comparing HDDs to SSDs, the University of Wisconsin study showed 20kg CO2e/TB for the HDD and 160kg CO2e/TB for the SSD, an 8x difference! Using publicly released data from SSD drive vendors for enterprise drives manufactured in 2021, I compared an 18TB Seagate EXOS X20 HDD with a CO2e/TB of 1.2kg with a 15TB Seagate Nytro 3332 SSD with a CO2e/TB of 2.91kg. Instead of an 8x difference, the SSD had a CO2e/TB of only 2.4x that of the comparable HDD.

If we look at industry trends since 2017, there's no doubt that SSD manufacturing emissions per PB have been rapidly decreasing (due to both manufacturing process improvements and increased renewable energy use). In fact, the embodied CO2e/PB of SSDs is decreasing faster than the CO2e/PB of HDDs. The table in Figure 1 below was taken from the fiscal year 2022 sustainability report of a leading manufacturer of HDDs and (at the time) SSDs and shows the average greenhouse gas (GHG) emissions intensity ratio across all the vendor's manufactured HDDs and SSDs for successive years.

Figure 1: A GHG emissions intensity ratio comparison of HDDs and SSDs in FY22 published by a leading drive vendor.

The GHG emissions intensity ratios for HDDs and SSDs per PB sold in 2020 showed SSDs at 2.5x higher (4.3/1.7), but it's clear to see that each subsequent year emissions intensity of SSDs was decreasing by much more than HDDs. By 2022, the data shows that the SSD GHG emissions intensity ratio for SSDs per PB had dropped by 49% to 2.2, whereas the HDD GHG emissions intensity ratio had dropped by only 29% to 1.2, showing that SSDs were only 1.83x higher (2.2/1.2) in GHG emissions intensity that year.

Keep in mind that enterprise SSD densities are increasing faster than enterprise HDD densities, a trend that is very likely to continue well into the future. With 30.72TB SSDs already generally available, 61.44TB SSDs are likely to become generally available within the next one to two years. The largest HDDs in general availability are in the 20TB-24TB range, with devices up to 36TB likely to become more widely available within the next one to two years. As device density increases, the CO2e/TB of SSDs is expected to shrink faster than that of HDDs.

If the decreasing GHG emissions intensity ratio trend for SSD continues alongside the accelerating SSD density advantage over HDDs, it's clear that the SSD GHG emissions intensity ratio will soon achieve parity with HDDs.

Conclusion

Recent public statistics on the embodied CO2e of HDDs and SSDs indicate the two drive technologies are much closer than what has been provided in the past. In a device-to-device comparison, there is no disputing HDDs enjoy a slim but ever-narrowing advantage over SSDs in embodied emissions. Does this mean that SSD-based enterprise storage systems are worse for the environment than HDD-based ones? We'll look into that in Part 2 of this blog.