SLC, MLC, TLC and QLC NAND Explained: SSD Endurance, TBW, DWPD and How Long an SSD Really Lasts

Every SSD has a finite number of writes, and the type of NAND flash inside decides most of it. Yet spec sheets compress that story into a single TBW figure, and buyers are left guessing whether a cheaper QLC drive will survive a point-of-sale terminal, a surveillance recorder or a busy office PC. This guide explains how NAND types differ, how to read endurance ratings, and how to calculate whether a drive will outlast the device it goes into.

How NAND cells store data

NAND flash stores data as electrical charge in memory cells. The more bits each cell stores, the more voltage levels the controller has to distinguish, and the more each program/erase (P/E) cycle wears the cell down. Storing more bits per cell makes flash cheaper and denser, but slower to write and less durable.

The cycle counts are typical ranges, not guarantees, and they vary by manufacturer and generation. Modern 3D NAND stacks cells vertically in over a hundred layers. That allows larger cells than old planar flash, so today's 3D TLC usually lasts longer than planar MLC did a decade ago.

Watch out for marketing labels. Some manufacturers have described TLC as "3-bit MLC", and "MLC" on a cheap drive does not always mean 2 bits per cell. For volume purchases, ask for the NAND type in writing.

TBW and DWPD: reading endurance ratings

Manufacturers do not publish P/E cycles for consumer drives. They publish two endurance figures that cover the warranty period instead:

• TBW (terabytes written) is the total volume of data the host can write before the warranty ends, regardless of years.
• DWPD (drive writes per day) is how many times the full capacity can be written every day during the warranty period. It is common for data-centre drives.

Example: a 1TB TLC drive rated for 600 TBW with a 5-year warranty works out to 600 ÷ (1 × 365 × 5) ≈ 0.33 DWPD. That means about 330 GB written every day for five years. Typical consumer ratings scale with capacity:

Larger drives have more cells to share the writes, so endurance rises roughly in proportion to capacity. Choosing the next capacity up is often the cheapest way to gain endurance.

How long will an SSD last in your device?

The answer depends on how much data the device writes, not on the drive alone. Divide the TBW rating by the daily write volume:

For most PCs and business terminals, endurance is not the limiting factor. The drive will be retired for capacity or age long before it wears out. It becomes critical for continuous recording, logging, caching and write-heavy industrial systems.

One more factor is write amplification. Small random writes force the controller to move and rewrite more data internally than the host sends, so a logging device writing small records wears NAND faster than a recorder writing large sequential video files of the same total size. Spare area (over-provisioning) reduces write amplification. A drive sold as 480GB or 960GB usually reserves more spare flash than a 500GB or 1TB model built on the same NAND.

SLC cache: why fast drives slow down

Most TLC and QLC SSDs temporarily write incoming data in SLC mode, storing one bit per cell for speed, then fold it into TLC or QLC when the drive is idle. This SLC cache is why a drive can quote high write speeds.

• When a long transfer fills the cache, write speed drops to the native speed of the NAND. On a TLC drive that may still be several hundred MB/s or more. On some QLC drives it can fall to around 100 MB/s or lower.
• Dynamic caches shrink as the drive fills, so a nearly full QLC drive has much less burst performance.
• For video capture, disk imaging or factory preloading, check sustained write speed, not just the peak figure.

pSLC: SLC endurance at a lower cost

True SLC NAND is expensive. Many industrial SSDs, eMMC and memory cards instead run TLC or MLC flash in pseudo-SLC (pSLC) mode, storing only one bit per cell. Capacity drops to about one third of the TLC capacity, but endurance often rises to tens of thousands of P/E cycles and write behaviour becomes far more consistent. pSLC is a common choice for PLCs, medical devices, vehicle systems and gateways that write small amounts of data constantly for many years.

DRAM, HMB and controller design

Endurance is not only about NAND. The controller tracks where every block of data lives using a mapping table.

• DRAM SSDs keep this table in a dedicated DRAM chip, which helps sustained random writes and heavy multitasking.
• DRAM-less NVMe SSDs use Host Memory Buffer (HMB) to borrow a small slice of system RAM. They perform well for everyday use and cost less.
• DRAM-less SATA SSDs cannot use HMB. They are fine for light workloads but weaker under heavy random writes.
• Power-loss protection (capacitors that flush data when power is cut) matters in industrial devices that may lose power abruptly.

Data retention: SSDs are not archives

Flash cells slowly leak charge when unpowered. The JEDEC endurance standard (JESD218) expects a client SSD at its full rated endurance to retain data for at least one year powered off at 30°C, and an enterprise SSD for three months at 40°C. New drives with little wear retain data much longer, but heat and heavy wear shorten retention. For long-term cold storage, keep multiple copies and power drives on periodically, or use media designed for archiving.

How to check SSD health

SSDs report wear through S.M.A.R.T. data, readable with tools such as CrystalDiskInfo on Windows or smartctl on Linux.

• Percentage Used (NVMe): the share of rated endurance consumed. It can exceed 100% while the drive still works.
• Data Units Written or Total Host Writes: actual data written, which you can compare with TBW.
• Available Spare: remaining spare blocks. A falling value is an early warning sign.
• Media errors and reallocated blocks: any steady increase means it is time to plan a replacement.

Which NAND type should you choose?

When you request a quotation, ask for the NAND type, DRAM or HMB design and TBW for each capacity, so that drives are compared like for like. UNITRENTH supplies SATA, mSATA and NVMe SSDs for PCs, terminals and embedded systems. To pick the right form factor, read our NVMe vs SATA guide.

Frequently asked questions