--- description: "SSD glossary: NVMe, PCIe Gen5 and Gen6, TLC vs QLC, DWPD, EDSFF, pSLC cache, CXL, and every term on a modern spec sheet." title: SSD Glossary image: https://www.storagereview.com/wp-content/uploads/2025/05/StorageReviewSquare.webp --- [![Storage Review]()](https://storagereview.com/) --- [Facebook](https://www.facebook.com/@storagereview) [X (Twitter)](https://twitter.twitter.com/storagereview) [Instagram](https://www.instagram.com/storagereview/) [LinkedIn](https://www.linkedin.com/company/storagereview-com) [YouTube](https://www.youtube.com/user/storagereview?sub_confirmation=1) [Email](mailto:info@storagereview.com) [Spotify](https://open.spotify.com/show/1y6VnznABhHeOSMOmbDTz0) [Reddit](https://www.reddit.com/r/StorageReview/) [Discord](https://discord.gg/TwMHb4azdC) [RSS Feed](https://www.storagereview.com/rss.xml) [TikTok](https://www.tiktok.com/@storagereview) [Facebook](https://www.facebook.com/@storagereview) [X (Twitter)](https://twitter.com/storagereview) [Instagram](https://www.instagram.com/storagereview/) [LinkedIn](https://www.linkedin.com/company/storagereview-com) [YouTube](https://www.youtube.com/user/storagereview?sub_confirmation=1) [Email](mailto:info@storagereview.com) [Spotify](https://open.spotify.com/show/1y6VnznABhHeOSMOmbDTz0) [Reddit](https://www.reddit.com/r/StorageReview/) [Discord](https://discord.gg/TwMHb4azdC) [RSS Feed](https://www.storagereview.com/rss.xml) [TikTok](https://www.tiktok.com/@storagereview) [![StorageReview.com](https://www.storagereview.com/wp-content/uploads/2026/07/Storage-Review-2x.png)](https://www.storagereview.com) ≡ Menu - [Home](https://www.storagereview.com/) - [Storage Reviews](https://www.storagereview.com/review) - [Consumer Reviews](https://www.storagereview.com/consumer) - [Enterprise Reviews](https://www.storagereview.com/enterprise) - [Ubiquiti Reviews](https://www.storagereview.com/best/ubiquiti-reviews) - [SR Merch](https://store.storagereview.com) - [Leaderboards](https://www.storagereview.com/best) - [Best Storage Arrays](https://www.storagereview.com/best/storage-arrays) - [Best Enterprise SSDs](https://www.storagereview.com/best/enterprise-ssds) - [Best Servers](https://www.storagereview.com/best/servers) - [Best Desktops for Local AI](https://www.storagereview.com/best/desktops-local-ai) - [Best Laptops for Local AI](https://www.storagereview.com/best/laptops-local-ai) - [Best Local LLM Tools](https://www.storagereview.com/best/local-llm-tools) - [Agentic AI Hardware](https://www.storagereview.com/best/agentic-ai-hardware) - [Best SSDs & Hard Drives](https://www.storagereview.com/best_drives) - [Best Portable SSDs](https://www.storagereview.com/best/portable-ssds) - [Best Business Laptops](https://www.storagereview.com/best/business-laptops) - [Best Mobile Workstations](https://www.storagereview.com/best/mobile-workstations) - [Best Desktop Workstations](https://www.storagereview.com/best/desktop-workstations) - [Best Mini PCs](https://www.storagereview.com/best/mini-pcs) - [Best Laptop Battery Life](https://www.storagereview.com/best/laptop-battery-life) - [Storage Reference Guide](https://www.storagereview.com/storage-reference-guide) - [About SR](https://www.storagereview.com/about-storagereview) - [StorageReview.com Sweepstakes Rules and Regulations](https://www.storagereview.com/storagereview-com-sweepstakes-rules-and-regulations) Search [Home](https://www.storagereview.com/) » SSD Glossary # SSD Glossary The vocabulary of solid-state storage, defined the way the terms are actually used in 2026. Where a definition is best understood in practice, we link the review or coverage where the concept does real work. New to SSDs entirely? Start with the [SSD Reference Guide](https://www.storagereview.com/ssd-reference-guide). ## Interfaces and form factors **NVMe:** The protocol built for flash, replacing SATA’s disk-era command model with massively parallel queues over PCIe. Every performance SSD today is NVMe. **PCIe Gen4 / Gen5 / Gen6:** The bus generations that set an SSD’s speed ceiling; each doubles bandwidth. Gen4 tops out near 7.5GB/s, Gen5 near 15GB/s, and Gen6 enterprise drives are [in mass production at 28GB/s and beyond](https://www.storagereview.com/news/samsung-pm1763-pcie-gen6-ssd-enters-mass-production-with-28-4-gb-s-reads). **M.2:** The gumstick form factor of consumer SSDs, most commonly 2280 (22mm x 80mm), connecting over PCIe. **U.2 / U.3:** 2.5-inch datacenter form factors, now yielding to EDSFF in new platforms. **EDSFF (E1.S / E3.S / E3.L):** The enterprise form factor family designed for flash rather than inherited from disks. E3.S has become the default slot in AI and storage servers; see the [KIOXIA CD9P-R](https://www.storagereview.com/review/kioxia-cd9p-r-review-read-intensive-gen5-up-to-61-44tb) for a current E3.S drive in the lab. **SATA:** The legacy interface, capped near 550MB/s. Relevant only for old systems and some boot use. ## NAND and media **NAND flash:** The non-volatile memory inside every SSD, organized in pages and blocks, written in pages and erased in blocks. **SLC / MLC / TLC / QLC:** One, two, three, or four bits per cell. More bits mean more capacity per dollar with lower endurance and speed. TLC is the performance mainstream; QLC owns the capacity tier, up to [245TB in one drive](https://www.storagereview.com/review/micron-6600-ion-245tb-ssd-review-a-quarter-petabyte-per-drive-bay). **pSLC (pseudo-SLC) cache:** A slice of TLC/QLC flash run in one-bit mode as a fast write buffer. Burst speeds come from this cache; sustained speeds are what remain when it fills, which is why our reviews test past it. **3D NAND / layer count:** Stacking cells vertically; modern flash exceeds 200 and now 300+ layers, with [375-layer wafer-bonded NAND announced](https://www.storagereview.com/news/sk-hynix-at-fms-2026-16-high-hbm4-wafer-bonded-375-layer-nand-and-a-tiered-memory-pitch). **HBF (High Bandwidth Flash):** An emerging stacked-flash class positioned between HBM and SSDs for AI memory tiers; it received [its first open spec in 2026](https://www.storagereview.com/news/high-bandwidth-flash-gets-its-first-open-spec-512gb-stacks-and-up-to-3-0tb-s). ## Performance and endurance **IOPS:** Input/output operations per second, the measure of small-block random performance. Block size matters when comparing: enterprise SSD IOPS are conventionally quoted at 4K, so treat figures quoted at other block sizes carefully. **Latency:** Time to complete a single operation; the metric that most affects how fast a system feels. Flash wins over disk here by orders of magnitude. **Sequential vs random:** Large contiguous transfers vs scattered small ones. Marketing leads with sequential; real workloads are usually mixed. **DWPD:** Drive writes per day, the enterprise endurance rating over the warranty period. Mixed-use drives like the [Micron 9550 MAX](https://www.storagereview.com/review/micron-9550-max-review-balanced-performance-for-ai-db-and-analytics) carry higher DWPD than read-intensive models. **TBW:** Terabytes written, the consumer endurance rating. **Wear leveling:** Controller logic spreading writes evenly across flash so no block wears out early. **Garbage collection / TRIM:** Background reclamation of deleted-data blocks, and the host command that tells the drive which blocks are free. **Overprovisioning:** Flash reserved beyond stated capacity for endurance and performance consistency. ## Architecture and features **Controller:** The processor managing flash, the host interface, caching, and error correction. The single biggest differentiator between drives; see the [SanDisk WD\_BLACK SN8100](https://www.storagereview.com/review/sandisk-wd_black-sn8100-review-elite-performance-in-a-gen5-ssd) for what controller tuning delivers. **DRAM / DRAM-less / HMB:** Onboard memory for the address map, or Host Memory Buffer borrowing system RAM in value drives. **Power-loss protection (PLP):** Capacitors that flush in-flight data on power failure; standard in enterprise, absent in consumer drives. **NVMe namespaces:** Logical subdivisions of one physical drive, used in enterprise deployments. **ZNS / FDP:** Zoned Namespaces and Flexible Data Placement, host-guided data placement schemes that reduce write amplification in hyperscale deployments. ## Emerging and adjacent **CXL:** A coherent interconnect over PCIe for memory expansion, pooling, and tiering; [CXL 4.0 IP now reaches 128 GT/s](https://www.storagereview.com/news/synopsys-cxl-4-0-ip-hits-128-gt-s-claims-3-6x-kv-cache-offload-over-ssds). **GPU-initiated storage:** Letting GPUs issue reads directly to NVMe drives rather than routing through the CPU, formalized in [NVIDIA’s SCADA model](https://www.storagereview.com/news/nvidia-scada-puts-storage-control-on-the-gpu-as-cufile-goes-open-source). **KV cache offload:** Moving LLM inference context from GPU memory to flash tiers, one of the workloads reshaping enterprise SSD requirements. **HAMR:** Heat-assisted magnetic recording, hard drive technology relevant here as the capacity floor SSDs compete against; covered in our [Exos M 30TB review](https://www.storagereview.com/review/seagate-exos-m-30tb-review-hamr-ing-storage-density). 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