Glossary
NAND flash
NAND flash is non-volatile memory that stores data as electrical charge trapped in cells and keeps it with the power off. It is the medium inside every SSD. Data is written in pages but erased only in much larger blocks, and that mismatch drives both the internal housekeeping of a drive and the wear of its cells.
Cells, pages and blocks
A NAND cell holds a level of charge, and the level encodes bits. Cells are grouped into pages, the smallest unit that can be written, and pages into blocks, the smallest unit that can be erased. A block holds hundreds of pages. Current chips stack cells vertically in well over a hundred layers, a design known as 3D NAND, which raises density without shrinking each cell further.
Bits per cell set density and wear
| Type | Bits per cell | Charge levels | Typical role |
|---|---|---|---|
| SLC | 1 | 2 | Fastest and most durable; now mostly a write buffer inside other drives |
| MLC | 2 | 4 | Largely replaced in enterprise drives |
| TLC | 3 | 8 | The mainstream enterprise type, balanced for mixed workloads |
| QLC | 4 | 16 | Highest density and lowest cost per terabyte, with slower writes and lower endurance |
Each added bit doubles the number of charge levels a cell has to keep apart, which narrows the margin between them. Narrow margins make writes slower and leave less tolerance for the damage each erase does. The four-bit type is covered under QLC flash.
Why a drive writes more than the host sends
A page cannot be overwritten. When data changes, the new version goes to an empty page and the old one is marked stale. Over time blocks fill with a mix of live and stale pages, and to reclaim space the drive copies the live pages elsewhere and erases the whole block. This background copying is garbage collection, and the extra writes it produces are called write amplification.
For example, if a host writes 1 TB and garbage collection copies another 2 TB internally, the cells absorb 3 TB of writes, three times what the host sent. Every one of those writes counts against the life of the cells.
Write patterns decide the outcome
Large sequential writes fill blocks cleanly, so little live data needs moving later. Small random overwrites scatter stale pages across many blocks, and the drive spends effort relocating data that never changed. Two identical drives with the same host write volume can therefore wear at very different rates, a contrast that runs through sequential vs random I/O more generally. The ratings that capture the limit are explained under flash storage endurance.
NAND differs from NOR flash, which reads individual bytes quickly and suits firmware storage. NAND gives that up for much higher density and lower cost, which is why drives are built from it. Its charge also leaks slowly when unpowered, faster on worn cells and in heat, so flash is a poor medium for leaving data on a shelf for years.