a solid-state drive
/ SSD /
A hard disk is like a record player with moving parts; an SSD is more like a giant grid of light switches that hold their position even when unplugged. There is nothing spinning, no arm, no needle — just flash memory chips that store bits as trapped electric charge in tiny cells. To find any piece of data the SSD does not walk anywhere; it simply addresses the right chip and reads it electronically, the way RAM does, only this memory keeps its contents when the power is off.
Concretely, an SSD stores data in flash cells organised into pages (commonly 4 to 16 KB), the unit it can read or program (write), and pages are grouped into much larger erase blocks (hundreds of pages). Flash has a peculiar, asymmetric rule that shapes everything: you can read any page fast, you can program a page only if it is already erased, and you cannot erase a single page — you can only erase a whole block at once, which is slow. So you cannot overwrite data in place; to change a page you must write the new version to a fresh, already-erased page and mark the old one stale, later erasing whole blocks in the background. Managing all this hidden complexity is the flash translation layer, which together with wear leveling, garbage collection, and the TRIM command keeps the drive fast and long-lived.
Why it matters for the OS: an SSD changes the rules a disk taught us. There is no seek time and no rotational latency, so random access is nearly as fast as sequential — which means the classic disk-scheduling algorithms (built to minimise arm movement) buy almost nothing. The new concerns are different: flash cells wear out after a limited number of erase cycles, the erase-before-write rule causes write amplification (writing more than you asked), and the OS should tell the drive which blocks are free (TRIM) so cleanup is efficient. SSDs are vastly faster and more rugged than disks, but they are not just faster disks — they fail differently and are managed differently.
Updating one 4 KB page on an SSD does not overwrite it. The drive writes the new data to a free, pre-erased page elsewhere, redirects that logical block to the new page, and marks the old page stale; later, when a whole block is mostly stale, garbage collection erases it to reclaim space.
No in-place overwrite: write elsewhere, mark stale, erase later.
An SSD is not simply a faster hard disk. It has no seek time, wears out with writes, cannot overwrite in place, and is managed by its own translation layer — treating it exactly like a disk wastes its strengths and ignores its real limits.