Comparisons · Internal SSDs (NAND type and cache design)

TLC vs QLC and DRAM vs DRAM-less (HMB): reading post-SLC-cache write speed off the datasheet

Two drives can both claim the same "up to X MB/s sequential write" and still differ by an order of magnitude once you write tens of gigabytes without stopping. Samsung's FAQ states that its SSDs use Intelligent Turbo Write, which "improves performance by increasing write speeds through internal SLC buffering", that this "is not directly used in multi-bit cells such as TLC or QLC but writes to the SLC Buffering area instead", and that when the drive is idle the data moves to the multi-bit cells and frees the buffer. The catalogue number is therefore the figure while that buffer is working. Two SATA drives make the gap concrete: according to Samsung's datasheets, the TLC-based 870 EVO sustains 530MB/s after the cache at 1TB and above (300MB/s at 250GB and 500GB), while the QLC-based 870 QVO drops to 80MB/s at 1TB and 160MB/s at 2TB and above. Endurance and warranty differ too: 600TB and five years for the 1TB 870 EVO, 360TB and three years for the 1TB 870 QVO. This guide ties NAND type and cache design (on-board DRAM or HMB) to the specific rows and footnotes that state them.

Published 2026-09-19 · Updated 2026-09-19 · FaultNote editorial policy

TLC vs QLC and DRAM vs DRAM-less (HMB): reading post-SLC-cache write speed off the datasheet overview: 1. TLC and QLC are bits per cell — and the labelling varies, 2. The headline write speed is the figure while the SLC cache lasts, 3. DRAM versus DRAM-less (HMB) is a separate axis from NAND type, 4. Which rows to read, and how to decide by use — An overview of this guide’s steps and checks, not a screenshot of the app.

Who this guide is for and what to prepare

People who write tens of gigabytes at a time — video exports, game installs, first backups — and want to know whether a cheap high-capacity drive is enough
People who see "DRAM-less" or "HMB" on a spec sheet and cannot tell whether it will be noticeable
People who want to know which rows and footnotes of a datasheet predict the slowdown they will hit after purchase

What you need

The manufacturer's datasheet PDF for each candidate — the version that includes the footnotes, not the summary on the product page
Your own write pattern: how many gigabytes in one go, how often, and whether the drive is the boot drive or a data drive
What it will connect to (SATA or NVMe, and the slot generation) and which Windows version you run, because HMB depends on the in-box driver

1. TLC and QLC are bits per cell — and the labelling varies

NAND flash is named after how many bits each memory cell stores. KIOXIA's BiCS FLASH infographic describes its TLC as "3-bit-per-cell" and its QLC as "4-bit-per-cell, quadruple-level-cell", and records that the company first introduced 4-bit-per-cell technology (QLC BiCS FLASH) in June 2017 and reached 1.33 terabits on a single chip in July 2018. More bits per cell means more capacity in the same area, but also more voltage levels each cell has to distinguish.

There is a labelling trap here. Samsung writes its NAND as "V-NAND 3bit MLC" and "4bit MLC V-NAND". The NAND Flash Memory row of the 870 EVO datasheet reads "Samsung V-NAND 3bit MLC" and the 870 QVO reads "Samsung 4bit MLC V-NAND". Both strings contain "MLC", but the first is what the industry calls TLC and the second is QLC. Read the bit count in that row rather than relying on the model suffix.

The bit count shows up in endurance and warranty as well as speed. For two drives on the same interface with the same MKX controller, Samsung publishes the TBW (total bytes written) and warranty periods in the table below. That a warranty ends at whichever comes first — the years or the TBW — is covered in this site's separate guide to SSD endurance.

Scroll horizontally to see the full table →

1. TLC and QLC are bits per cell — and the labelling varies
Item	870 EVO (TLC)	870 QVO (QLC)
NAND row on the datasheet	Samsung V-NAND 3bit MLC	Samsung 4bit MLC V-NAND
Common name	TLC (KIOXIA: 3-bit-per-cell)	QLC (KIOXIA: 4-bit-per-cell)
TBW (1TB / 2TB)	600TB / 1,200TB	360TB / 720TB
Warranty	5 years (or the TBW, whichever comes first)	3 years (or the TBW, whichever comes first)
Capacities offered	250GB to 4TB	1TB to 8TB

2. The headline write speed is the figure while the SLC cache lasts

Samsung's SSD FAQ describes Intelligent Turbo Write as a technology that "improves performance by increasing write speeds through internal SLC buffering", adds that it "is not directly used in multi-bit cells such as TLC or QLC but writes to the SLC Buffering area instead", and says that when the SSD is not writing or reading "the data moves to the multi-bit cell, freeing up space in the SLC Buffering area". Writing one bit per cell avoids distinguishing fine voltage levels and is therefore fast, but that area is finite and a long continuous write exhausts it.

The datasheet footnotes say so explicitly. Footnote 3 of the 870 EVO states that sequential write measurements are based on Intelligent TurboWrite technology and that "the sequential performance measurements after Intelligent TurboWrite region are 300MB/s (250/500GB), or 530MB/s (1TB/2TB/4TB)". Footnote 3 of the 870 QVO uses the same construction: "Performances after Intelligent Turbowrite are 80 MB/s (1TB), 160 MB/s (2/4/8TB)". Both drives list 530MB/s sequential write in the main table, close to the SATA ceiling. The difference only appears after the cache is gone.

Footnotes 4 and 5 of the 870 QVO give post-cache random figures too: at QD1, 5.0K IOPS for reads and 22K IOPS (1TB) or 34K IOPS (2/4/8TB) for writes. The main table lists 35,000 IOPS for QD1 random write, so a 1TB drive past its cache is at roughly two thirds of the headline number. All of these are Samsung's own figures measured on its own test setup (sequential with CrystalDiskMark 5.0.2 and random with IOmeter 1.1.0 for the 870 EVO); this site did not measure any drive.

Smaller capacities are penalised as well. The 870 EVO holds 530MB/s after the cache at 1TB and above but falls to 300MB/s at 250GB and 500GB. The 870 QVO is 80MB/s at 1TB and 160MB/s above it. "Buy small because it is cheap" and "buy a big QLC drive because it is cheap" both surface in long writes, in different ways.

Scroll horizontally to see the full table →

2. The headline write speed is the figure while the SLC cache lasts
Measurement condition (Samsung datasheets)	870 EVO (TLC)	870 QVO (QLC)
Main-table sequential write (cache active)	530MB/s (all capacities)	530MB/s (all capacities)
Sequential write after the cache (1TB)	530MB/s	80MB/s
Sequential write after the cache (2TB and above)	530MB/s	160MB/s
Sequential write after the cache (250GB / 500GB)	300MB/s	Not applicable (range starts at 1TB)
QD1 random write after the cache	Not stated on the datasheet	22K IOPS (1TB) / 34K IOPS (2/4/8TB)
How speed recovers	Samsung's FAQ says data moves from the SLC buffer to the multi-bit cells while the drive is idle	Same (idle time is required)

3. DRAM versus DRAM-less (HMB) is a separate axis from NAND type

"DRAM-less" is a different question from TLC versus QLC. An SSD controller keeps a table mapping logical addresses to physical pages. A DRAM-equipped drive holds that working area in DRAM on its own board; a DRAM-less drive borrows part of the host PC's main memory through the Host Memory Buffer (HMB). Microsoft's developer documentation defines the feature as one "that provides a mechanism for the host to allocate a portion of host memory for the controller to use exclusively".

Even within Samsung's NVMe range the two designs coexist. The 990 PRO datasheet lists a "DRAM Cache Memory" row of 1GB LPDDR4 at 1TB, 2GB at 2TB and 4GB at 4TB. The 990 EVO Plus datasheet has "Cache Memory: HMB (Host Memory Buffer)" in the same place and no on-board DRAM. Both use V-NAND TLC, so this is a cache difference rather than a NAND difference, and DRAM-less does not automatically mean bargain-basement.

HMB needs host support. Microsoft's NVMe feature table — which indicates support in the in-box StorNVMe driver on Windows 10 version 1903 and later — lists Host Memory Buffer as supported. The flip side is that host memory and the driver are now part of the equation, so an old OS or an unusual environment does not carry the same assumptions as a DRAM-equipped drive. HMB also consumes host RAM, which is worth a look on a machine with little memory and several drives.

Note also that footnote 4 of the 990 EVO Plus datasheet says write performance "was measured with Intelligent TurboWrite technology being activated" and that "Intelligent TurboWrite operates only within a specific data transfer size" — without stating how large that size is or what the speed becomes afterwards. Datasheets that publish a post-cache number, as the SATA 870 series does, are the exception. That is the main reason a slowdown is hard to predict before buying an NVMe drive.

Scroll horizontally to see the full table →

3. DRAM versus DRAM-less (HMB) is a separate axis from NAND type
Item	990 PRO (with DRAM)	990 EVO Plus (HMB)
Cache row on the datasheet	DRAM Cache Memory: 1GB / 2GB / 4GB LPDDR4 by capacity	Cache Memory: HMB (Host Memory Buffer)
NAND	Samsung V-NAND TLC	Samsung V-NAND TLC
Interface	PCIe 4.0 x4, NVMe 2.0	PCIe 4.0 x4 / 5.0 x2, NVMe 2.0
Headline sequential write	6,900MB/s	6,300MB/s
Speed after the cache	No figure on the datasheet (the footnote only says TurboWrite was active)	Also no figure
Host-side condition	Does not use host memory	Borrows host RAM; the in-box Windows driver supports Host Memory Buffer

4. Which rows to read, and how to decide by use

There are only four places to look on a datasheet: NAND Flash Memory (3-bit or 4-bit), DRAM Cache Memory or Cache Memory (LPDDR4 capacity or HMB), the footnote attached to sequential write (was the cache active, and is a post-cache number given), and TBW with the warranty period. A product page headline normally omits the footnote and the cache row entirely, so open the PDF.

From the use side, the deciding factor is how much you write in one go. If writes finish in a few gigabytes and the drive then sits idle — OS and applications, documents, browsing — the post-cache figure barely surfaces. If you write tens or hundreds of gigabytes at once — exporting video, installing or migrating a large game library, creating a first full backup — the post-cache figure is the elapsed time. Writing 100GB at the 1TB 870 QVO's 80MB/s works out at roughly 21 minutes; at 530MB/s it is roughly 3 minutes.

Free space matters too. Samsung's FAQ says that "SSDs that have gone through Over Provisioning have improved performance and longer lifespans when there is an unallocated area" and that the size of that area can be set in Magician. Simply not filling the drive to the brim works in the same direction.

If a drive you already own slows down partway through a big copy, the cause may be thermal throttling rather than the cache. This site has a separate troubleshooting article that separates the two on a running system. Predicting before purchase is this guide; separating causes afterwards is that one.

Scroll horizontally to see the full table →

4. Which rows to read, and how to decide by use
How you use it	Specification that decides it	What to check
OS, applications, documents (a few GB per write)	The post-cache figure barely surfaces	Prioritise capacity and warranty (TBW). DRAM-less (HMB) can meet the requirement
Video exports and large game installs (tens to hundreds of GB at once)	Post-cache sequential write is the elapsed time	Look for a datasheet that publishes the post-cache number. Within one series the larger capacity is better (870 QVO: 80MB/s at 1TB, 160MB/s at 2TB and above)
Write-once archive use (mostly read afterwards)	Endurance and warranty length	Compare TBW and years (1TB 870 EVO: 600TB and 5 years; 870 QVO: 360TB and 3 years)
PC with little memory, or several drives	HMB borrows host RAM	Check whether the Cache Memory row says HMB or on-board DRAM, and check Microsoft's driver support table
Considering a small-capacity model	Smaller capacities can be slower after the cache	The 870 EVO is 300MB/s at 250GB and 500GB but 530MB/s at 1TB and above. Read the per-capacity footnote
Planning to fill the drive	Headroom in free space	Samsung's FAQ ties an unallocated area to better performance and lifespan. Leave capacity spare

Limitations and requirements

Every speed, TBW and warranty figure here is a manufacturer's published number. Samsung's datasheets state that 870 EVO sequential performance was measured with CrystalDiskMark 5.0.2 and random performance with IOmeter 1.1.0, and that the 990 EVO Plus was measured on a Ryzen 9 7950X system; different conditions give different numbers. This site did not measure any drive.
Few datasheets publish a post-cache figure at all. The 870 EVO and 870 QVO numbers here apply to those series and cannot be transferred to other makers' QLC drives or to NVMe models. Where no figure is published, treat the post-cache speed as unknown before purchase.
This article does not cover prices, stock or rankings. The KIOXIA infographic's most recent entry is March 2021, so its generations and layer counts are not current products; what is cited from it is the bits-per-cell definition. The specifications are those published by each manufacturer as of 19 September 2026.

Frequently asked questions

Should I avoid QLC SSDs?

It depends on how you write. On Samsung's datasheets the QLC 870 QVO lists the same 530MB/s sequential write as the TLC 870 EVO in the main table; the difference appears after the SLC cache is exhausted, where it is 80MB/s at 1TB and 160MB/s at 2TB and above, against 530MB/s for the 870 EVO at 1TB and above. If your writes finish in a few gigabytes this rarely surfaces; if you write tens of gigabytes at a time it is elapsed time. Endurance and warranty differ too (at 1TB, 360TB and three years against 600TB and five years), so the gap compounds for write-heavy use.

Will a DRAM-less (HMB) SSD feel slower?

DRAM-less is a separate axis from NAND type, and it does not map one-to-one onto "slow". Within Samsung's NVMe range the 990 PRO has DRAM Cache Memory (1 to 4GB LPDDR4 by capacity) and the 990 EVO Plus uses HMB, while both use V-NAND TLC. Because HMB borrows host memory, host support is a precondition: Microsoft's NVMe feature table, which indicates support in the in-box StorNVMe driver on Windows 10 version 1903 and later, lists Host Memory Buffer as supported. Check the assumption on machines with little memory or where the in-box driver is not in use.

Where is the speed after the cache actually stated?

In the footnote attached to sequential write in the manufacturer's datasheet PDF. Samsung's 870 EVO and 870 QVO both state the figure "after Intelligent TurboWrite". The 990 PRO and 990 EVO Plus footnotes say that measurements were taken with TurboWrite activated and that it "operates only within a specific data transfer size", but give neither the size of that range nor the speed afterwards. When no figure is published, treat the post-cache speed as unknown and leave headroom if you write large amounts at once.

Sources and verification date

Sources checked: 2026-09-19. These sources support the specifications, procedures or prices discussed here. Check each source for applicable conditions and current information.

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