COMPUTER TECHNOLOGY
Section 3
Long-Term Storage
Everything that keeps your files when the power goes off — the drives inside the machine, the ones you carry, the server down the corridor, and the data center on the other side of the world.
What storage is
Slides 2–4
The drives themselves
Slides 5–8
Comparing them
Slides 9–11
Where else files live
Slides 12–15
Keeping it
Slides 16–17
Storage
The purpose of storage in a computer is to hold data or information and get that data to the CPU as quickly as possible when it is needed.
Computers use several kinds of storage: drives that are located inside the computer, and flash drives, DVDs or compact disks (CDs) that are used externally.
the three kinds of drive, shown to scale
It has to be permanent
Storage keeps its contents when the power goes off. That is the one thing separating it from RAM, which empties the moment you shut down.
It has to be fast enough
The processor works far quicker than any drive. Most of the design of storage is about not making the CPU wait.
It does not have to be here
A file can sit on this machine, on a server in the building, or in a data center a thousand miles away.
Why a Computer Needs Several Kinds
No single kind of storage is fast, large and cheap all at once. Fast storage is expensive and small; cheap storage is large and slow. So a computer uses several layers at the same time and shifts data between them.
Where it sits | Roughly how fast | What it holds |
RAM | 50,000 MB/s | Whatever is open right now. Wiped at shutdown. |
NVMe SSD | 7,000 MB/s | The operating system, programs, and the work in progress. |
SATA SSD | 550 MB/s | A second drive, or the main drive in an older machine. |
Hard drive | 150 MB/s | Films, photo libraries, music, backups — bulk that can wait. |
Network or cloud | Depends on the connection | Files shared between devices, and the copy kept somewhere else. |
Fast is expensive
RAM costs many times what an SSD does per gigabyte, and an SSD several times what a hard drive does. Nobody can afford a machine made only of the fast stuff.
Each layer feeds the one above
Work moves up when it is needed and down when it is not. Starting a program is it climbing from the drive into RAM.
Why all of them at once
A modern PC runs RAM for now, an SSD for today, and a hard drive or the cloud for everything it might want later.
Memory
Your computer uses two types of memory: primary memory and secondary memory.
Primary memory is the main memory (RAM) where the operating system resides.
Secondary memory is the storage that keeps your files: an SSD or hard drive inside the computer, and CDs or flash drives outside it. Secondary storage cannot be directly accessed by the CPU.
Turn the computer off and primary memory is wiped, so the operating system has to be loaded in again every time. That is why it lives on secondary storage.
RAM — the primary memory
Primary
Fast, and built to talk straight to the processor. Also temporary: cut the power and it is empty.
Secondary
Slower, but it keeps everything. The processor cannot reach it directly, so data is copied into RAM first.
What booting is
Starting a computer is mostly this copy: the operating system moves from the drive into RAM before anything else can happen.
Solid State Drives
An SSD is a type of mass storage device similar to a hard disk drive (HDD). Unlike hard drives, SSDs do not have any moving parts (which is why they are called solid state drives). Instead of storing data on magnetic platters, SSDs store data using flash memory.
an M.2 SSD — no moving parts at all
What "solid state" means
Nothing moves. No motor, no spinning platter, no arm swinging across a surface — only chips and the current running through them.
How a cell holds a bit
Each cell traps a charge behind an insulating layer. Charge or no charge is the on and off, and it stays put with the power gone.
And tougher
Drop a running hard drive and the head can scrape the platter. An SSD has nothing to knock out of place.
Hard Disks Drives (HDD)
Inside the hard disk drive case you will find circular disks made from aluminum alloy or glass, coated with a magnetic layer.
On the disks, there are many tracks or cylinders.
Within the hard drive, an electronic reading/writing device called the head passes back and forth over the cylinders, reading information from the disk or writing information to it.
Hard drives spin at 5400 to 7200 rpm (Revolutions Per Minute) - that means that in one minute, the hard drive spins 5400 to 7200 times!
a hard drive with the lid removed
Platters
The disks are called platters. A drive usually holds several, stacked with a tiny gap between them and spinning together.
Tracks
Each surface is divided into thousands of concentric rings. The same ring across every platter is called a cylinder.
The head
It floats a fraction of the width of a hair above the surface, on a cushion of air made by the spinning.
How Information Is Stored
Do you remember how the binary code uses on and off commands to represent information? Every kind of storage records everything that way. The only difference is what gets switched.
A hard drive uses electricity and magnetism. The head creates a pattern of magnetized and non-magnetized areas on the disk's surface: magnetized is on, non-magnetized is off.
An SSD traps an electrical charge inside a memory cell. Charged is on, empty is off, and nothing has to move.
An optical disc has pits burned into its surface. A laser reads pit or flat as off and on.
a hard drive head — one of the three ways
Why it stays put
None of the three needs power to hold its state. That is the whole difference from RAM, which empties at shutdown.
Why the speeds differ
A hard drive must swing a head into place and wait for the disk to come round. An SSD reaches any cell straight away.
Why they fail differently
A head can strike a platter. Flash cells wear out after many rewrites. A disc scratches. Same job, different weaknesses.
How SSDs Changed Computers
For about fifty years the slowest part of a computer was the drive. Processors got faster every year, memory got faster, and the machine still sat there while a spinning disk found the next piece of a file.
SSDs removed that wait, and the effect was larger than any processor upgrade of the same period. A computer that starts in seconds rather than minutes is the same machine with a different drive in it.
the part that did it
The old bottleneck
A hard drive had to physically move a head and wait for the disk to come round. Everything else in the computer waited with it.
What it enabled
Laptops with no moving parts, phones and tablets, machines that wake instantly instead of booting, and drives that survive being carried around.
The best upgrade
Putting an SSD in an old computer makes more difference than any other single change. It is the standard advice for a machine that feels slow.
How Fast Is Each One?
Not all drives are equally quick, and the gap is far wider than most people expect. The three kinds sold today sit on a ladder, and where a drive sits on it does more for how fast a computer feels than almost anything else.
largest and slowest on the left, smallest and fastest on the right
NVMe SSD
The fastest, and what new machines ship with. A bare stick that clips onto the motherboard and talks straight to the processor. Around 7,000 MB per second.
SATA SSD
Still solid state and still quick, but held back by the older SATA connection it plugs into. Around 550 MB per second — roughly a tenth of NVMe.
Hard drive
Spinning platters and a moving head. Around 150 MB per second, but far cheaper per terabyte, which is why bulk storage and backups still use them.
Which One For What?
Most desktops built for gaming or creative work use both. The SSD holds the operating system, the programs and whatever is being played or edited right now, because those are the things where waiting is annoying.
The hard drive holds everything that just needs to sit somewhere: finished video, photo libraries, music, and the backup of the SSD. None of that is any better for being read quickly.
a hard drive, a SATA SSD and an NVMe drive
Put on the SSD
Windows, your programs, and the game or project you are working on now. Anything you wait for while it loads.
Put on the hard drive
Films, photo libraries, music, old coursework, backups. Files you open occasionally and never race through.
Cost is the reason
A terabyte of hard drive costs a fraction of a terabyte of SSD. Buying both is cheaper than buying enough SSD for everything.
Comparison of storage space
How much a drive of each size will actually hold.
Drive size | Music | Video | Photos | Movies |
320 GB | 5,400 hrs | 320 hrs / 13 days | 100,000 | 161 hrs |
500 GB | 8,500 hrs | 500 hrs / 20 days | 155,000 | 250 hrs |
750 GB | 13,100 hrs | 770 hrs / 30 days | 240,000 | 384 hrs |
1 TB | 17,000 hrs | 1,000 hrs / 40 days | 310,000 | 500 hrs |
2 TB | 34,000 hrs | 2,000 hrs / 80 days | 620,000 | 1,000 hrs |
1 TB (Terabyte) = 1000 GB (Gigabytes) = 1 million MB (Megabytes) = 1000 million KB (Kilobytes)
The ladder
1000 kilobytes make a megabyte, 1000 megabytes make a gigabyte, and 1000 gigabytes make a terabyte. Each step is a thousand times the last.
Read across a row
A 1 TB drive holds either 17,000 hours of music or 500 hours of film — not both. Every file competes for the same space.
Why drives fill up
Video is the reason. One minute of 4K footage can be several hundred megabytes, so a phone fills a terabyte faster than you would think.
Storage You Can Carry
Some storage is meant to be unplugged and taken with you, and for most people that now means a USB stick. A flash drive holds files on the same kind of flash memory an SSD uses, in a case small enough for a pocket.
Memory cards do the same job in cameras, drones and handheld consoles, and an external drive is simply an ordinary drive in a case with a USB lead.
a flash drive going into a USB-A port
Flash drives
The usual way to move a file between machines now. Cheap, tiny, easy to lose, and a poor place to keep the only copy of anything.
Memory cards
SD in cameras, microSD in phones, drones and dash cams. The same flash memory again, in a smaller package.
External drives
A full-size SSD or hard drive in a case with a USB lead. Slower than inside the machine, but the usual way to keep a backup.
Where Discs Are Still Used
Nobody carries files around on a disc any more, and no laptop sold today has a drive to read one. Discs did not disappear though — they moved to the places where a physical copy is still worth having.
A laser reads pits pressed into the surface. Pit or no pit is the on and off, exactly as magnetism is on a hard drive.
an optical drive in a laptop — a rare sight now
Console games
PlayStation and Xbox still sell games on disc. A disc can be lent, resold or kept, which a download cannot.
Films
DVD and Blu-ray still beat streaming on picture quality, and a disc keeps working when a service drops the film or closes.
The ROM in CD-ROM
Read Only Memory — the same term met in Section 2. Pressed once at the factory, readable forever, impossible to change.
Storage on a Network
Not every drive is inside the computer you are using. A drive can sit on the network instead, and any machine on that network can reach it.
At school your work saves to the H: drive. Those files live on a school server, so you can sign in to any computer in the building and find them waiting.
A NAS — Network Attached Storage — is the same idea at home: a small box holding one or more drives, plugged into the router rather than into a computer.
a NAS with its drive bays open — it plugs into the router, not a computer
One copy, many devices
A phone, a laptop and a games console can all reach the same files, so there is no need to copy anything between them.
It keeps working
A NAS often holds several drives that mirror each other. If one fails the files are still there on the others.
It needs the network
No network, no files. That is the trade for not carrying them, and it is why the H: drive is unreachable from home.
Storage in the Cloud
Cloud storage means the files sit on someone else's computers and you reach them over the internet. Google Drive, OneDrive, iCloud and Dropbox all work this way.
The cloud is not vague or magical. It is a building full of racked drives — a data center — owned by a company, kept cool and powered around the clock.
a data center — the cloud is other people's drives
Reachable anywhere
Any device that can sign in gets the same files. Lose the laptop and the work is still there.
Someone else's rules
The company decides the price, the space and how long deleted files are kept. Accounts can be locked or closed.
Not automatically private
Files sit on hardware you do not control. That matters more for some things than others, and is worth thinking about.
What Actually Kills a Drive
Drives do not usually fail because someone did something dramatic to them. They fail because they run hot, get knocked about, lose power at the wrong moment, or simply get old.
None of that is rare, and none of it gives any warning. Which raises the question the next slide answers: what happens to your work when it does?
Heat and age
Both kinds wear out. A hard drive’s motor and bearings wear mechanically; an SSD’s memory cells can only be rewritten so many times. Heat brings both forward.
Knocks and power cuts
A running hard drive knocked hard can have its head strike the platter. Losing power part way through a write can leave a file half-saved on either kind.
Not magnets
A hard drive is sealed in metal and already contains stronger magnets than any you would carry. An SSD stores charge, so magnetism does nothing to it at all.
Backup
A backup is a second copy of your files kept somewhere else, so that losing one copy does not lose the work.
Drives fail without warning, laptops are dropped and stolen, and files get deleted by accident. None of that is unusual — the only question is whether there is another copy when it happens.
an external drive — the simplest backup there is
Three, two, one
Keep three copies of anything that matters, on two different kinds of storage, with one of them somewhere else entirely.
Sync is not backup
A synced folder copies your mistakes too. Delete a file on the laptop and it disappears from the cloud as well.
Test it
A backup nobody has ever restored from is a guess. Open a file from it occasionally and check it actually opens.