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SSD DRAM, SLC Cache and NAND Types: What the Spec Sheet Hides

Two SSDs with the same capacity and peak speed can behave very differently after a long write. These are the traits behind that gap.

By The PC Journal8 min read

The short answer

The headline speed of an SSD describes short bursts. How it behaves on long writes and mixed work depends on its NAND type (TLC or QLC), whether it has its own DRAM or borrows system memory through Host Memory Buffer, and how large its SLC write cache is. For a boot drive or a game library, a DRAM-less TLC drive is often fine; for sustained heavy writes, look for a drive that states its sustained behaviour, not only its peak.

What DRAM does inside an SSD

An SSD keeps a map that translates the addresses the operating system uses into the physical places on the flash where data really lives. A drive with its own DRAM chip holds that map in fast memory. A drive without DRAM keeps most of it on the flash itself and reads parts of it as needed.

To soften the cost, many NVMe drives without DRAM use Host Memory Buffer, a feature in the NVMe standard that lets the drive borrow a small slice of the computer's system memory for the map. This helps random access on typical desktop use. It also means the drive depends on the host supporting the feature, which is normal on current Windows PCs but is not guaranteed in every environment, such as some older systems or a drive placed in an enclosure.

None of this means a DRAM drive is automatically better or a DRAM-less one automatically worse. Controller design, firmware and the flash also matter. The practical reading is that DRAM matters more for workloads that touch the whole drive heavily at random, such as a busy scratch disk or a database, and less for a boot drive and a game library.

SLC cache and why write speed changes

Most consumer drives write incoming data quickly by temporarily using part of their flash in a faster, single-bit-per-cell mode called an SLC cache. While the cache has room, writes run at the advertised speed. When it fills during a long copy, the drive has to write directly to the slower multi-bit cells and the speed drops.

The size of the cache can depend on how much of the drive is free, so the same drive can write fast when mostly empty and slower when nearly full. Manufacturers state the cache size for some drives and not for others, so the absence of a figure is a reason to look for a review that tested sustained writes.

For everyday use you rarely notice any of this, since normal writes are small and the cache has time to empty in the background. It shows up when you copy a very large amount of data in one go, such as moving a whole game library or a day of high bitrate footage.

TLC versus QLC

The number refers to how many bits each flash cell stores.

TLC (3 bits per cell)QLC (4 bits per cell)
Typical positionMainstream and performance drivesCapacity-focused and budget drives
Sustained writesGenerally holds speed better once cache is fullOften drops further when cache is full
Endurance ratingCommonly higher, so check the TBW for the specific driveCommonly lower, so check the TBW for the specific drive
Good fitBoot drive, game library, mixed workMostly-read libraries, secondary storage

What to read on a product page

NAND type
Look for TLC or QLC in the spec table. If it is not stated, check the maker's datasheet or a review.
DRAM or HMB
Check whether the drive lists a DRAM cache or says DRAM-less, and whether it states HMB support.
Endurance (TBW)
Compare the rated terabytes written for the capacity you are buying, because the rating scales with capacity.
Sustained write
Look for a test that writes a large file to a nearly full drive, since that shows cache exhaustion.

Which traits to prioritise

A DRAM-less TLC drive suits you

Choose this if

  • You mainly boot Windows, launch apps and load games
  • You do not write very large files in one go
  • You want a lower price at the same capacity

Look for DRAM and higher endurance

Choose this if

  • You run a scratch disk, a database or virtual machines
  • You write many gigabytes daily
  • You edit large media projects directly from the drive

A QLC drive can work

Choose this if

  • The drive mostly stores files you read more than you write
  • You want the largest capacity for the price
  • You keep a backup of what matters

Common mistakes

  • Reading peak speed as sustained speed

    Instead: Treat the printed numbers as the best case with an empty cache.

  • Rejecting every DRAM-less drive

    Instead: Judge it by your workload. HMB handles typical desktop use well on supported systems.

  • Filling the drive to the last gigabyte

    Instead: Keep free space, since write caching and wear behaviour both benefit from it.

Where to go next

  1. Best NVMe SSDs for gaming Boot and game drives.
  2. Best 4TB NVMe SSDs Where QLC and TLC choices show up.
  3. Best SSDs for OS For boot drives.

Sources

  1. NVM Express specificationsNVM Express · Reference
  2. Intel Product Specifications (ARK)Intel · Manufacturer documentation

This guide explains a decision process. It does not contain product rankings, benchmark results or hands-on measurements. How we research

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