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SK hynix’s 321-layer NAND has moved beyond a demo: the company began mass production of a 1Tb TLC version in 2024, then announced mass production of a separate 2Tb QLC version in 2025. The taller stack can support higher-capacity NAND devices and denser storage designs, but 321 layers does not mean an SSD has 321 times the capacity—and the company’s product comparisons are not independent SSD benchmarks.
From demo to production: what SK hynix announced
The “demos” in the original headline refers to an earlier stage. At Flash Memory Summit in August 2023, SK hynix displayed 321-layer NAND samples and said it was targeting mass production in the first half of 2025. The company announced mass production of its 321-layer 1Tb TLC NAND in November 2024, with customer supply planned from the first half of 2025. In August 2025 it announced mass production of a distinct 321-layer 2Tb QLC NAND product. In April 2026, SK hynix reported beginning supply of a 321-layer QLC cSSD solution for AI PCs. The 2023 sample announcement, the 2024 TLC announcement, the 2025 QLC announcement and the 2026 cSSD supply announcement describe those milestones.
These are milestones for NAND components and a storage solution, not proof that a particular retail SSD using the announced NAND is available to consumers.
What “321-layer 4D NAND” means
NAND is nonvolatile flash memory used in SSDs and smartphone storage. The layer count refers to the vertical stack of cell layers inside the memory device. It is not a direct multiplier for an SSD’s capacity, speed, endurance or price: a finished drive’s specifications also depend on how its NAND is configured and combined with other components.
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SK hynix calls its technology 4D NAND. Its 2023 announcement described the 321-layer part as fifth-generation 4D NAND. The company attributed the stack to a “Three Plugs” process: after three plug-processing stages, it electrically connects the plugs, using low-stress material and automatic plug-alignment correction. A plug is a vertical hole through the layers used to form cells. This process description comes from SK hynix, rather than an independent assessment.
The 1Tb TLC and 2Tb QLC versions are different products
The 2024 and 2025 announcements refer to distinct cell types and capacities, so their performance figures should not be read as a direct head-to-head comparison. TLC (triple-level cell) stores three bits per cell; QLC (quadruple-level cell) stores four. SK hynix announced a 1Tb 321-layer TLC device in 2024 and a 2Tb 321-layer QLC device in 2025.
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| Announced NAND | Capacity and cell type | SK hynix’s comparison | Stated applications |
|---|---|---|---|
| 321-layer TLC, announced in 2024 | 1Tb; TLC | Compared with the previous 238-layer generation: productivity 59% higher; data-transfer speed 12% higher; reading performance 13% higher; data-reading power efficiency more than 10% higher. | SSDs for AI data centers and on-device AI, among other emerging AI uses. |
| 321-layer QLC, announced in 2025 | 2Tb; QLC | Compared with prior QLC products: data-transfer speed doubled; write performance up to 56% higher; read performance 18% higher; write power efficiency more than 23% higher. SK hynix described 2Tb as double the capacity of existing solutions. | PC SSDs, enterprise data-center SSDs and UFS smartphone storage. |
Every performance, productivity and power figure in this table is a company-reported comparison in the relevant SK hynix announcement, not an independent test. The baselines differ: the TLC figures refer to the previous 238-layer generation, while the QLC figures refer to prior QLC products. SK hynix’s “double the capacity” wording is its own comparison with existing solutions; it should not be treated as a universal comparison across all NAND products.
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For its 2Tb QLC device, SK hynix said it increased the number of planes from four to six. The company describes planes as independent units that enable more parallel processing and improve simultaneous read performance. Six planes are a design feature of this announced QLC product, not a general property of every 321-layer NAND device.
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SK hynix also said its 32DP method enables 32 NAND dies per package. The company presents this as a way to help achieve twice the integration density for ultra-high-capacity AI-server eSSDs. That is a stated design aim, not a measured capacity increase for every finished drive.
What this could mean for AI storage and SSD capacity
Higher-capacity NAND devices can give manufacturers more options for building dense storage products. SK hynix positioned the 1Tb TLC NAND for AI-related uses that need low power and high performance, including AI data-center SSDs and on-device AI. For its 2Tb QLC NAND, the company named PC SSDs, enterprise SSDs for data centers and UFS smartphone storage. Its April 2026 announcement adds reported supply of a QLC cSSD solution for AI PCs.
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Those intended uses do not establish that every AI PC, data-center SSD or phone will use this NAND. Nor do the announcements show how a specific finished SSD performs or what capacity, endurance, price or retail availability it will have. The documented figures describe NAND products and SK hynix’s own comparisons—not independent consumer-drive tests or a one-to-one translation from layer count to finished-drive capacity.
What the announcements do—and do not—show
- Established by SK hynix: 321-layer sample demonstrations in 2023, mass production of 1Tb TLC in 2024 and 2Tb QLC in 2025, and reported supply of a 321-layer QLC cSSD solution for AI PCs in April 2026.
- Useful for understanding the technology: the TLC and QLC variants have different capacities, cell types, comparison baselines and stated applications; the QLC announcement also specifies six planes and 32 dies per package using 32DP.
- Not established by these announcements: independent validation of the claimed performance improvements, a named customer, or consumer retail availability of a particular SSD incorporating the announced NAND.
SK hynix frames these launches as part of its ambition to become a “full-stack AI memory provider.” That is the company’s strategic positioning, not an independent conclusion about its market standing.
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