Foundations

What Is an SSD? Flash Memory vs. Hard Drives

Why solid-state drives behave so differently from the spinning disks they replaced.

A solid-state drive (SSD) stores your data in NAND flash memory — silicon chips with no moving parts. A traditional hard disk drive (HDD) stores data as magnetic patterns on spinning platters, read by a mechanical arm. That single difference — silicon vs. spinning metal — explains almost everything about how SSDs behave.

HDDs pay a mechanical seek cost for random access; SSDs read any location almost instantly.

What flash memory can and cannot do

Flash memory has one deeply important quirk that shapes the entire design of an SSD: you cannot overwrite data in place. Once a location holds data, you must erase it before you can write to it again — and erase happens in large chunks, not per byte.

A flash cell can be programmed (written) quickly, but to reset it you must erase a whole block of thousands of cells at once. Reads are fast, writes are slower, erases are slowest and coarse-grained.

Analogy. Think of a flash block like a page written in pen, not pencil. You can fill empty lines, but to change a line you must photocopy the lines you want to keep onto a fresh page and shred the old one. That copy-and-shred cycle is the heart of how SSDs work.

Why this matters

Because overwriting is impossible, an SSD constantly plays a shell game: it writes new data to fresh space, marks the old copy as stale (invalid), and later reclaims that stale space by erasing whole blocks. Managing that game well — with minimal extra writes and even wear — is what separates a good SSD from a bad one, and it's exactly what the EyanaSSDSim simulator lets you watch.

In the EyanaSSDSim paper & simulator. The whole simulator exists to make these invisible internal mechanics visible. Everything else in this blog builds on this one fact: flash can't be overwritten in place.

Next: how flash is physically organized into cells, pages, blocks, planes, dies, and channels.

Next →Inside the Chip: Cells, Pages, Blocks, Planes, Dies, Channels