The Dirty Secret of SSDs: Embodied Carbon

Overview

This short paper by Swamit Tannu and Prashant J. Nair was published in ACM SIGEnergy Energy Informatics Review in 2023 (volume 3, issue 3, pp. 4-9). The record rests on the text read via arXiv:2207.10793; the ACM version is not under an open licence and cannot be redistributed. The paper estimates the embodied carbon of flash storage from 94 vendor life-cycle assessment (LCA) reports for systems containing SSDs, and of hard drives from 24 HDD LCA reports.

The authors define a Storage Embodied Factor (SEF), in kgCO2e per GB. The average SEF for SSDs is 0.16 kgCO2e per GB, derived from 94 LCA reports from eight vendors and validated with four-fold cross-validation. SSDs have about 8x higher embodied cost than HDDs of the same capacity; the HDD embodied cost, from 24 reports covering 512GB-6TB, is at least an order of magnitude lower than the SSD cost.

The paper then compares 1TB SSD and HDD life-cycle emissions over 5 and 10 years. At 20% active and 80% idle, with an average of 4.2 W for the HDD and 1.3 W for the SSD, 5-year emissions are about 99.6 kgCO2e for the HDD (20 kg embodied) and about 184 kgCO2e for the SSD (160 kg embodied). Over 10 years, with replacement at 5 years, they are 199 and 369.2 kgCO2e. An example desktop (4-core CPU, 8 GB DRAM, 1TB HDD, 512GB SSD) shows 473 kgCO2 embodied and 278 kgCO2 operational over 5 years; the SSD is the largest embodied component.

Proposed mitigations include choosing the storage medium with embodied carbon in mind, extending SSD life (wear levelling, SLC/MLC-aware data placement, stronger ECC) and reusing worn MLC flash as lower-capacity SLC. Flash cells endure only 10,000 to 100,000 write cycles. Server emissions are concentrated in the operational phase, whereas about 80% of an iPhone's emissions come from manufacturing. As a limit, the estimates depend on vendor LCA reports and may vary for specific products.

Key points

1. The average Storage Embodied Factor for SSDs is 0.16 kgCO2e per GB, derived from 94 LCA reports from eight vendors and validated with four-fold cross-validation.
2. SSDs have about 8x higher embodied cost than HDDs of the same capacity; HDD embodied cost, from 24 reports (512GB-6TB), is at least an order of magnitude lower.
3. 1TB drive at 20% active / 80% idle: 5-year emissions about 99.6 kgCO2e for the HDD (20 kg embodied) and about 184 kgCO2e for the SSD (160 kg embodied); over 10 years with replacement at 5 years, 199 and 369.2 kgCO2e.
4. Example desktop (4-core CPU, 8 GB DRAM, 1TB HDD, 512GB SSD): 473 kgCO2 embodied and 278 kgCO2 operational over 5 years; the SSD is the largest embodied component.
5. Flash cells endure only 10,000 to 100,000 write cycles; wear levelling, SLC/MLC-aware placement, stronger ECC and reuse of worn MLC as SLC are proposed.
6. Server emissions are concentrated in the operational phase, whereas about 80% of an iPhone's emissions come from manufacturing.

Using it in design

• In the storage catalogue, store embodied carbon per drive SKU: a default of 0.16 kgCO2e/GB for SSD (Tannu and Nair SEF) and a much lower HDD default.
• Compute rack and storage-tier embodied carbon from installed capacity.
• Provide a storage-tier calculator that adds embodied and operational CO2e over the planned life (default 5 years with replacement), so designers can see when HDD has the lower total.
• Expose drive service life and endurance as fields so that longer-life options lower annualised embodied carbon.

Who should read it

Designers and sustainability analysts planning storage tiers and calculating embodied carbon.

Publication details

Authors

Swamit Tannu; Prashant J. Nair

Publisher

ACM SIGEnergy Energy Informatics Review (Association for Computing Machinery (ACM))

Year

2023

Document number

arXiv:2207.10793

Volume

3

Issue

3

Pages

4-9

Document language

English

Source type

Scientific

Access

Free to read

DOI

10.1145/3630614.3630616

Licence

arXiv distribution permission — reuse conditions remain with the rights holder

Citation