Hard Disk Drive
Spinning mirrors and a flying needle: the cheapest way to keep a lot of data.
How big?About the size of a paperback; the platters spin 120 times a second.
Real: What it looks like.
Keys: arrows rotate · + / − zoom · 0 reset · 1–4 views · S signal · T tour · L labels · Space spin. Models are stylised and built from code: proportions are honest, details are simplified.
What it is
Data is stored as tiny magnetic patches on glass or aluminium platters. A read/write head flies a few nanometres above the surface on a cushion of air (or helium), closer than a fingerprint is thick.
To read something, the arm swings to the right track and waits for the spot to spin underneath. That wait is why hard drives are slow at random access but fine for long, steady reads.
Why AI needs it
Data lakes hold petabytes of raw text, images and video, and keeping all of it on flash would cost a fortune. Big object-storage clusters are mostly hard drives, with copies spread so a dead drive loses nothing.
Every labeled part
- 1
Platters
Mirror-smooth discs coated in magnetic material. Signal mode spins them.
- 2
Spindle motor
Spins the platters, typically 7,200 times a minute.
- 3
Read/write heads
One per platter surface, flying nanometres above it.
- 4
Actuator arm
Swings the heads across the tracks in a few thousandths of a second.
- 5
Voice-coil magnet
A strong magnet and coil move the arm, the same principle as a loudspeaker.
- 6
Controller board
Underneath. Turns magnetic wiggles into bits.
- 7
Data + power
SATA or SAS connection to the server.
Try it · concept lab
Spinning vs. solid state
Compare random reads with one long read.
Hard drive
≈ 100–200 reads/s
The arm has to swing and wait for the spot to spin round, every time.
SSD
≈ 500,000+ reads/s
No moving parts: any cell, any time, many at once.
Animation is slowed way down. Typical figures; drives vary.
Big idea: Moving parts make random access slow. Hard drives are for big, steady, cheap storage; SSDs are for fast access.
Try it · concept lab
Break some drives
Tap drives to kill them. Which files survive?
Tap drives to break them.
Each file lives on 3 drives. Any 2 can die.
Big idea: At data-lake scale drives fail every day. Spreading copies, or clever math pieces, means nothing is lost.
Swap it: other ways to do the same job
SSD
Hundreds of times faster at random reads, no moving parts, still several times the cost per terabyte.
Tape
Even cheaper and lasts decades on a shelf, but you wait minutes to fetch something.
Cloud object storage
Rent it instead of buying it. Underneath, it's racks of these drives.
Talk about it
- Q1
A hard drive is a spinning disk with a tiny arm floating above it. What other spinning things store sound or pictures?
- Q2
Big storage systems keep extra copies so one broken drive loses nothing. How many copies of your favorite photo would you keep?
For grown-ups: there are no right answers here. Ask a question, then ask "why do you think that?" The reasons matter more than the answer.
Printable question sheet (PDF)