Explained
How a ssd works.
How an SSD works
A solid-state drive stores data in flash memory, with no moving parts. That's why it can find any file almost instantly, where a hard disk has to wait for a spinning platter and a moving head. The drive in this model is a generic M.2 2280 card, 22 millimetres wide and 80 millimetres long, of the kind used in most laptops and many desktops. It talks to the computer over PCI Express using the NVMe protocol.
On the board are three main kinds of chip. The NAND flash chips hold the data. The controller is a small processor that runs the drive. A DRAM chip, when there is one, holds the map that tells the controller where every block of data really lives. Power circuits and the gold edge connector complete the board.
Storing bits as trapped charge
Flash memory was invented by Fujio Masuoka at Toshiba in the 1980s. Each cell is a transistor with an extra layer that can hold electric charge, which stays put even with the power off. Reading the cell means measuring how much charge is there. A TLC cell distinguishes eight charge levels, so it stores three bits; QLC stores four bits using sixteen levels, which packs in more data but wears faster and is slower to write.
To fit more cells, modern flash stacks them in many layers inside each chip, like a skyscraper rather than a car park. Flash also has an unusual rule: data can be written in small pages but only erased in much larger blocks, so the controller is always reorganising data in the background.
Wear levelling and the controller
Every flash cell wears out after a certain number of writes. If the computer kept rewriting the same file in the same place, those cells would fail early. So the controller never writes in place. It writes new data to the least-worn free block, updates its map and later erases the old block. Spreading writes evenly across the whole drive is called wear levelling, and it is why SSDs last for years of normal use.
The controller also corrects bit errors, retires bad blocks and keeps spare capacity in reserve. Open Play and use Write to save photos and copy a movie: each block lights up as it is written, and the wear range stays narrow because the least-worn block is always chosen next.
Parts glossary
- Heat spreader label
- A thin copper-backed label spreads heat from the chips. Some drives use a full metal heatsink.
- NAND flash
- Flash chips store data in cells that trap electric charge. Many layers of cells are stacked vertically inside each chip. Fun fact: A TLC flash cell stores 3 bits by holding one of 8 different charge levels.
- Controller
- The controller is a small processor that translates the computer's requests into flash operations, corrects errors and manages wear.
- DRAM cache
- A DRAM chip holds the map of where every piece of data actually lives in flash. Some cheaper drives skip it and use host memory instead.
- Power circuits
- Regulators convert the supply voltage into the levels the flash and controller need.
- M.2 edge connector
- Gold fingers plug into the M.2 slot. The notch, or key, stops the wrong type of card going in.
- Circuit board
- An M.2 2280 board is 22 millimetres wide and 80 millimetres long. Fun fact: The numbers in M.2 2280 are the board's width and length in millimetres.
