--- title: Every cheap QLC SSD is secretly two drives, and the fast... description: Cheap QLC NAND SSDs behave like two drives in one: a small, fast pseudo-SLC cache built from unused cells, and the slower native QLC storage underneath. Once... canonical: https://daily.dev/posts/every-cheap-qlc-ssd-is-secretly-two-drives-and-the-fast-one-gets-smaller-the-more-you-fill-it-mkyabwxrg twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Every cheap QLC SSD is secretly two drives, and the fast one gets smaller the more you fill it | daily.dev og:description: Cheap QLC NAND SSDs behave like two drives in one: a small, fast pseudo-SLC cache built from unused cells, and the slower native QLC storage underneath. Once... og:url: https://daily.dev/posts/every-cheap-qlc-ssd-is-secretly-two-drives-and-the-fast-one-gets-smaller-the-more-you-fill-it-mkyabwxrg og:image: https://api.daily.dev/og/posts/mKyAbwXRg.png og:image:alt: Every cheap QLC SSD is secretly two drives, and the fast one gets smaller the more you fill it og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Every cheap QLC SSD is secretly two drives, and the fast one gets smaller the more you fill it **[XDA Developers](https://daily.dev/sources/xda-developers)** · 4 min read · 0 upvotes · 0 comments ## Summary Cheap QLC NAND SSDs behave like two drives in one: a small, fast pseudo-SLC cache built from unused cells, and the slower native QLC storage underneath. Once the cache is exhausted, write speeds can collapse dramatically—TechPowerUp measured an HP FX700 dropping from roughly 5GB/s to 130MB/s after writing 460GB, a speed comparable to a mechanical hard drive. The fast cache shrinks as the drive fills and grows back as files are deleted; Tom's Hardware found the Intel 660p's fast region fell from 140GB to just 12GB on a nearly full 1TB drive. The drop only matters for large sustained writes exceeding the cache; QLC drives remain a good value for read-heavy, rarely-rewritten storage like game libraries or media archives, while TLC is recommended for frequent large file transfers or video editing. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### why does my QLC SSD slow down so much when copying large files QLC SSDs use a small pseudo-SLC cache carved from unused NAND cells to handle fast writes temporarily, then fold that data into slower four-bit QLC storage when idle. Once a single large write exceeds the cache size, speeds collapse; one tested $100 PCIe Gen 4 QLC drive dropped from about 5GB/s to 130MB/s after 460GB was written, comparable to a mechanical hard drive. _daily.dev surfaces storage deep-dives like this for anyone troubleshooting sudden SSD slowdowns during big transfers._ ### how much fast cache does the Intel 660p SSD actually have when it's nearly full On the 1TB Intel 660p, the fast pseudo-SLC cache shrinks from about 140GB when mostly empty down to just 12GB once the drive is nearly full, since the cache is drawn from whatever QLC cells remain unused. Deleting files grows the cache back; filling the drive shrinks it. _Developers picking storage for build servers or workstations can track these tradeoffs via daily.dev before buying._ ### should I buy a QLC or TLC SSD for video editing TLC is the better value for workloads involving constantly moving large files, editing video, or frequently filling a drive to the brim, because QLC drives are built for read-heavy, capacity-first storage where writes are infrequent. QLC remains a good cheap option for a Steam library, media collection, or bulk archive that is rarely rewritten. _daily.dev helps developers weigh hardware tradeoffs like QLC versus TLC before committing to a purchase._ ## Similar posts on daily.dev - [Every SSD has a hidden fast lane, and running out of it is why big file copies crawl](https://daily.dev/posts/every-ssd-has-a-hidden-fast-lane-and-running-out-of-it-is-why-big-file-copies-crawl-7zwebkic3) · XDA Developers · 0 upvotes · 0 comments - [TrueNAS proves that the TLC premium isn't worth it for your next NAS SSD](https://daily.dev/posts/truenas-proves-that-the-tlc-premium-isn-t-worth-it-for-your-next-nas-ssd-r5wgwely4) · XDA Developers · 0 upvotes · 0 comments - [Your SSD is slowing down because it's too full, not too old](https://daily.dev/posts/your-ssd-is-slowing-down-because-it-s-too-full-not-too-old-p6tm11jsa) · XDA Developers · 0 upvotes · 0 comments - [Hard drives on backorder for two years as AI data centers trigger HDD shortage — delays forcing rapid transition to QLC SSDs](https://daily.dev/posts/hard-drives-on-backorder-for-two-years-as-ai-data-centers-trigger-hdd-shortage-delays-forcing-rapi-dfoqyehrw) · Hacker News · 2 upvotes · 0 comments - [PCIe 5.0 SSDs are overkill, and manufacturers finally realized most people don't need them](https://daily.dev/posts/pcie-5-0-ssds-are-overkill-and-manufacturers-finally-realized-most-people-don-t-need-them-7mmmr6e6s) · XDA Developers · 0 upvotes · 0 comments --- Tags: [#storage](https://daily.dev/tags/storage) [View this post on daily.dev](https://daily.dev/posts/every-cheap-qlc-ssd-is-secretly-two-drives-and-the-fast-one-gets-smaller-the-more-you-fill-it-mkyabwxrg) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Every cheap QLC SSD is secretly two drives, and the fast one gets smaller the more you fill it","url":"https://daily.dev/posts/every-cheap-qlc-ssd-is-secretly-two-drives-and-the-fast-one-gets-smaller-the-more-you-fill-it-mkyabwxrg","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/every-cheap-qlc-ssd-is-secretly-two-drives-and-the-fast-one-gets-smaller-the-more-you-fill-it-mkyabwxrg"},"datePublished":"2026-10-03T16:00:33.712Z","dateModified":"2026-10-03T16:01:13.610Z","description":"Cheap QLC NAND SSDs behave like two drives in one: a small, fast pseudo-SLC cache built from unused cells, and the slower native QLC storage underneath. 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