The short version
An SSD stores your files as tiny amounts of electric charge trapped in flash memory chips. There are no moving parts, so it can find any file almost instantly, and unlike RAM it keeps the data with the power off. Three things decide how good one is:
- LinkSATA, or NVMe over PCIe 3.0, 4.0 or 5.0. This sets the top speed.
- FlashHow many bits each cell stores (TLC or QLC). This affects speed, lifespan and price.
- ChipThe controller chip that manages it all, plus its cache. This decides how well the drive holds up when busy or nearly full.
Why an SSD feels so much faster than a hard drive
Typical timings, slowed down 100 timesA hard drive stores data on a spinning disk and reads it with an arm that has to swing to the right track. An SSD just sends an electrical signal to the right chip. When a PC boots or opens an app, it reads thousands of small files scattered all over the drive, which is the worst case for a hard drive.
Inside a flash cell: SLC, MLC, TLC and QLC
Endurance figures are rough typical valuesEach flash cell traps electrons in an insulated layer, and the drive reads how much charge is there. Storing more bits per cell means splitting the same charge range into more levels. That packs in more data for less money, but the levels get narrower, so reading and writing them takes more care and the cell wears out sooner.
When the charge drifts into the wrong level, the controller's error correction fixes the bit. That's routine on every SSD; it only becomes a problem when a cell is badly worn. Drive warranties express lifespan as TBW (terabytes written). A good 1TB TLC drive is often rated around 600 TBW, far more than most people write in the warranty period.
Pages, blocks and why SSDs need empty space
Shrunk to 3 blocks of 8 pages so you can follow itFlash has an awkward rule: data is written in small pages, but erasing only works on whole blocks of many pages, and a page has to be erased before it can be written again. The SSD's controller hides this from your PC. Step through what it does.
The SSD's caches: SLC cache and DRAM
- SLC cache. TLC and QLC drives write incoming data into part of their flash as if it were SLC (1 bit per cell), which is much faster, then rewrite it more densely later. A big copy that overflows this cache slows down sharply, a lot more on QLC drives than TLC ones. Its size ranges from a few gigabytes to a few hundred, depending on the drive, and it shrinks as the drive fills up.
- DRAM or HMB. The controller keeps a map of where every page lives. Faster drives hold that map in their own DRAM chip; many good budget NVMe drives borrow a slice of your PC's RAM instead (Host Memory Buffer). Cheap SATA drives without either slow down most under heavy use.
NVMe vs SATA: the connection sets the top speed
Typical top sequential speeds of fast drivesSATA was designed for hard drives and tops out around 550 MB/s. NVMe drives connect over PCIe, the same kind of link your graphics card uses, normally with 4 lanes, and each PCIe generation doubles the speed per lane. NVMe can also handle thousands of requests at once, where SATA handles one queue of 32.
Today, 2TB: SATA from …, NVMe PCIe 4.0 from …, PCIe 5.0 from ….
What this means when you buy
- Any SSD beats a hard drive for your Windows drive and apps. Keep hard drives for bulk storage like video archives and backups.
- A PCIe 4.0 NVMe drive with TLC flash is the sweet spot for most PCs. PCIe 5.0 drives cost more and run hotter, and mainly help with huge file copies and video work.
- QLC is fine for a game library, since games are written once and read many times. Avoid it for drives that take lots of big writes.
- Leave 10–20% free so garbage collection and the SLC cache have room to work.
- Check the slot. Make sure your motherboard has a free M.2 slot, and which PCIe generation it supports. A PCIe 5.0 drive in a PCIe 4.0 slot just runs at 4.0 speed.