UK How QLC flash stores data
A QLC SSD is a solid-state drive built on quad-level cell NAND flash, which stores four bits in each memory cell. A flash cell holds a charge, and the drive reads the cell by measuring its voltage. With one bit per cell the controller only has to tell two voltage levels apart. With four bits it has to tell 16 levels apart, inside the same voltage window.
Packing more bits per cell means more gigabytes per chip and a lower cost per terabyte, which is why QLC appears in high-capacity consumer drives and in read-intensive data centre SSDs. The trade-off is margin. The 16 levels sit close together, so a small drift in charge, from wear, heat or time, is enough to cause a read error, and the controller has to work harder to correct it.
SLC, MLC, TLC and QLC compared
Each step from SLC to QLC adds one bit per cell, doubles the number of voltage states and cuts the number of program/erase (P/E) cycles a cell survives. The figures below are typical for current 3D NAND and vary by manufacturer and process.
| Flash type | Bits per cell | Voltage states | Typical P/E cycles | Typical use |
|---|---|---|---|---|
| SLC (single-level cell) | 1 | 2 | Around 50,000 to 100,000 | Write-intensive industrial and military storage |
| pSLC (TLC run as SLC) | 1 (on TLC silicon) | 2 | Around 20,000 to 30,000 | Industrial logging, boot and recording drives |
| MLC (multi-level cell) | 2 | 4 | Around 3,000 to 10,000 | Older industrial and enterprise SSDs |
| TLC (triple-level cell) | 3 | 8 | Around 1,000 to 3,000 | Most current client, enterprise and industrial SSDs |
| QLC (quad-level cell) | 4 | 16 | Around 500 to 1,500 | High-capacity, read-heavy storage |
Penta-level cell (PLC) flash, with five bits and 32 states, is in development, and follows the same pattern: more capacity, less endurance.
QLC vs TLC in practice
TLC is the mainstream choice today, and QLC is its higher-capacity alternative. The differences that show up in use are these.
-
//
Endurance. A QLC drive is usually rated for fewer terabytes written (TBW) than a TLC drive of the same size. Endurance is often quoted as drive writes per day (DWPD): TBW is roughly capacity × DWPD × 365 × warranty years, so a 1 TB drive at 0.3 DWPD over five years is rated for around 550 TB.
-
//
Sustained write speed. QLC drives write first to a fast cache area run as SLC. Once a long write fills it, speed drops to the native QLC rate, which on client drives can be slower than a hard drive.
-
//
Read speed. Reads are close to TLC, so QLC performs well for boot, applications and data that is written once and read many times.
-
//
Data retention. With narrower voltage margins, worn QLC cells hold data for less time when powered off, especially when stored warm.
-
//
Cost and capacity. QLC gives the most capacity per pound and per watt, which is why it is used for large read-mostly data sets.
Is QLC right for industrial systems?
For most industrial and embedded systems, the answer is usually no for the main drive. Industrial workloads tend to write constantly and in small blocks: logs, sensor data, databases and video. They also run in hot cabinets and vehicles, where heat speeds up charge loss and wear. That combination favours pSLC or industrial TLC flash, rated across the industrial temperature range and backed by a fixed bill of materials.
Video is the clearest example. Industrial NVRs record around the clock, so their drives see a full write load every day, and recording drives should be chosen on TBW first and capacity second. Boards built for vehicles and defence, such as our rugged single board computers, often take SLC or pSLC flash modules for the same reason.
QLC does have a place at the edge. Model files, maps, media libraries and reference data sets are written once and read many times, and a QLC data drive in edge AI servers can hold them at lower cost, alongside a higher-endurance drive for the operating system and logs.
How to tell if an SSD uses QLC
Drive makers do not always print the flash type on the label, so check the datasheet. Four clues point to QLC. The flash is listed as QLC, 4-bit or four bits per cell. The TBW rating is low for the capacity, often well under half the figure for a TLC drive of the same size. The datasheet quotes write speed only for the cache, or gives a much lower sustained write figure. And the drive is a high-capacity model sold at an unusually low cost per terabyte.
Industrial SSDs make this easier. Their datasheets state the flash type, the TBW or DWPD rating, the temperature range and the data retention, and the bill of materials does not change without notice. If any of those are missing, ask before the drive goes into a system that has to run for years.
Quick answers on QLC SSDs
QLC SSDs: common questions
What does QLC stand for?
Quad-level cell. It means each NAND flash cell stores four bits of data, using 16 distinct voltage levels.
Is TLC better than QLC?
For endurance and sustained writes, yes. TLC typically survives two to three times as many write cycles and holds its write speed for longer. QLC wins on capacity per pound, and reads at much the same speed.
Which is best: SLC, MLC, TLC or QLC?
It depends on the write load. SLC and pSLC last longest and suit write-heavy industrial use. TLC is the balanced default. QLC suits large, read-mostly storage.
Is QLC NAND cheaper than TLC?
Per terabyte, yes. Storing a third more bits in the same cells gives more capacity from each wafer, so QLC drives cost less per terabyte at the same size.
How long does a QLC SSD last?
Years, at light write loads. Divide the drive’s TBW rating by the data you write each day to estimate its life. A desktop writing a few gigabytes a day will rarely wear one out, while a recording system can.
We choose the flash type for every drive we fit as standard, matching SLC, pSLC, TLC or QLC to the write load, temperature and service life of the system before we image and validate it.