Understanding DRAM Cache in SSDs: Why it Matters and How to Choose the Right One

Linus Tech Tips

Summary:

SSDs differ significantly in performance and reliability due to the presence or absence of a DRAM cache chip. This cache is crucial as it stores the Flash Translation Layer (FTL), a map that helps the SSD efficiently locate and manage data on its NAND flash chips. Without DRAM, performance can degrade, especially when the drive is nearly full or under heavy OS load.

Many manufacturers deliberately omit DRAM from some SSD models and fail to disclose this vital information in product specifications, making it difficult for consumers to make informed choices. While alternatives like Host Memory Buffer (HMB) using system RAM exist for NVMe drives, and storing FTL directly on NAND is possible, dedicated DRAM remains the superior solution. A DRAM-equipped SSD is highly recommended for operating system drives to ensure optimal responsiveness during background tasks and can benefit direct storage gaming. Consumers are urged to support transparent manufacturers like Samsung, Kingston, and Acer Predator, who clearly state DRAM presence, and to demand better disclosure from all brands.

Intro [0:00]

• Storage components are often overlooked by consumers, who frequently select SSDs based on capacity and price alone.
• However, the choice of SSD can profoundly influence gaming performance and daily computing responsiveness.
• A critical differentiating factor is the presence of a DRAM cache chip, a feature many manufacturers intentionally omit and obscure.
  • The omission of DRAM cache significantly impacts an SSD's performance and long-term reliability.
• This video aims to clarify the role of DRAM cache, explain its importance, and highlight which manufacturers are transparent about its inclusion.

What's the difference? [1:45]

• Comparing two Western Digital (WD) SSDs reveals a common deceptive marketing practice:
  • The WD Black SN750 (released 2019) included a DRAM cache and had a higher MSRP.
  • The WD Black SN750 SE (released 2021) omits the DRAM chip and is sold at a lower MSRP.
• The "SE" designation misleadingly implies an upgraded "second edition" or "special edition" model.
• Although the SN750 SE utilizes the newer PCIe Gen 4 interface, it barely leverages the increased bandwidth.
  • The removal of the DRAM cache effectively makes it a performance downgrade despite the "next-gen" marketing.
• Product specifications from many manufacturers often prioritize marketing language over transparent technical details, making crucial differences like DRAM cache difficult to ascertain.

  

  The WD Black SN750 (left) was released in 2019 with DRAM, while the SN750 SE (right) was released in 2021 without DRAM, despite its name implying an upgrade.

Why does it matter? [2:44]

• An SSD functions as an intricate, self-contained computer within your PC.
  • It comprises an energy-efficient System-on-Chip (SOC) controller.
    • The SOC controller contains a CPU, firmware, and its own memory.
  • It interfaces with the PC and with the NAND flash chips that store user data.
• The Role of DRAM Cache in SSDs:
  • Stores the Flash Translation Layer (FTL), which serves as a vital map or lookup table.
    • The FTL tells the operating system the exact physical location of all data on the NAND flash chips.
  • Performance Benefits:
    • Significantly improves performance by enabling faster data lookup.
    • Performance benefits are particularly pronounced as the SSD approaches full capacity.
  • Reliability and Organization:
    • Helps the SSD controller efficiently organize data and manage NAND flash.
    • Aids in deciding which blocks of NAND are best to erase and where to write new data.
    • Optimizes wear leveling, extending the lifespan of the NAND flash.

      

      An SSD board showing the NAND Flash Chips and the SOC Controller, which manages data operations.

      

      A diagram illustrating the placement of NAND Flash Chips and the DRAM module on an SSD.

It wasn't always this way [4:16]

• In the past, the presence of a DRAM cache was a key selling point, prominently featured in product specifications for SSDs.
• Today, this crucial information is often absent from product pages and technical documentation, intentionally obscured by manufacturers.
• Examples of Manufacturer Opacity:
  • Western Digital (WD): Exhibits highly inconsistent and confusing branding across its product lines (Black, Blue, Green).
    • The high-end SN850X includes DRAM, but mid-range models like the SN7100 and SN770 do not.
    • The original SN750 had DRAM, but its "SE" (Special Edition) successor removed it.
    • Even within the same WD Blue SATA model, a 2TB version might feature DRAM while smaller capacities lack it.
  • Other prominent brands, including SanDisk, Seagate, MSI, Corsair, Crucial, and NetApp, have also faced criticism for failing to clearly disclose whether their drives use a DRAM cache.

HMB isn't useless [5:30]

• While a dedicated DRAM cache is optimal, DRAM-less SSDs are not necessarily useless or poor-performing for all applications.
• Alternative Cache Solutions:
  • Host Memory Buffer (HMB):
    • This feature, primarily for NVMe SSDs, leverages a small portion of the system's main RAM (host memory) to store the FTL map.
    • While not as fast or capacious as onboard DRAM, HMB allows DRAM-less NVMe drives to achieve decent performance.
    • The PCIe interface's high bandwidth is sufficient to facilitate reasonable performance with HMB.
  • FTL on NAND: In some cases, the FTL map can be stored directly on the NAND flash chips themselves.
    • This is generally the slowest and least efficient caching method available.
• Despite the existence of these workarounds, a dedicated DRAM chip on the SSD remains the superior solution for peak performance and efficiency.

Your OS storage should use DRAM [6:13]

• Recommendation for Operating System Drives: A DRAM cache is highly recommended for any SSD intended as the primary operating system drive.
  • It ensures greater system responsiveness and fluidity.
  • This benefit is particularly noticeable during intensive background operations such as:
    • System updates.
    • Virus scans.
    • Large file transfers or multitasking.
  • The performance advantages of DRAM cache become more critical as the drive ages and fills up, preventing noticeable slowdowns.
• Gaming with Direct Storage: For newer games that utilize direct storage (streaming game assets directly from the SSD to the GPU), a DRAM-cached drive can provide an additional performance advantage.
• The Problem of Information Access: Manufacturers' lack of transparency often forces consumers to:
  • Consult third-party resources (e.g., TechPowerUp) to confirm DRAM presence.
  • Physically remove heat spreaders or stickers from drives, potentially voiding warranties, to inspect for a DRAM chip.

    

    TechPowerUp website, which provides detailed technical specifications including DRAM cache information for SSDs, highlighted by a green box.

What can we do about it? [6:44]

• Consumer Empowerment:
  • Vote with your wallet: Actively choose to purchase SSDs from manufacturers who provide clear and comprehensive product specifications, including details about DRAM cache.
  • Demand transparency: Provide feedback to manufacturers regarding the difficulty of finding crucial technical information like DRAM presence, emphasizing how it negatively impacts the shopping experience.
• Manufacturers with Better Transparency:
  • Samsung: Commendably, Samsung explicitly states whether their SSDs feature DRAM directly on their product pages (e.g., the 990 PRO series).
  • SanDisk: Has shown progress by beginning to specify DRAM cache details in the datasheets for their newer Optimus lines.
  • Kingston and Acer Predator: Both brands are noted for clearly listing whether their SSD products include a DRAM chip.
• As new generations of SSDs, such as Gen 5 drives, continue to enter the market, it is crucial for consumers to prioritize and support brands that value transparency and provide complete information.

  

  Logos of Samsung, Kingston, and Acer Predator, highlighted as manufacturers who are transparent about DRAM cache in their SSDs.