Why RAM
never wears out,
but SSDs do
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The full narration of the video.
Two parts inside a computer look a lot alike: on the left, a stick of memory; on the right, a solid-state drive. Both are small boards covered in little black chips. Yet you can write to memory all your life and never wear it out. Write enough to a solid-state drive, and it does wear out.
So what's the difference? Let's follow one electron as it stores a bit in each. Memory first. Every single bit in memory lives in a tiny bucket. A bucket full of electrons means one. An empty bucket means zero.
The bucket has a door. The door opens, and electrons walk in. The door closes, and the bit is stored. The electrons go through the door and never touch a wall. So no matter how often the door swings, the bucket doesn't wear. The catch: the bucket leaks a little. So every sixty-four milliseconds, memory tops up every bucket again.
Cut the power and nobody tops them up, so the buckets drain. That's why memory forgets everything when you switch off. A solid-state drive needs the opposite: it has to remember for years with the power off. So its bucket has no door. It's sealed on every side by a very thin wall. That wall is more than ten thousand times thinner than a hair.
Electrons can't get out, so the bit stays put. But with no door, they can't get in either. The solution? Apply a high voltage and force the electrons straight through the wall. It's a rough push. Every squeeze leaves a few tiny scars in the wall, and some scars trap electrons.
To rewrite it, a reverse voltage drags the electrons back out. That's another hit. The scars pile up, the wall starts to leak, and more electrons get stuck. Finally the drive can't tell zero from one, and that cell is done. How many squeezes can a cell take? One that stores a single bit: about a hundred thousand.
Today's drives save money by packing three bits into each cell, which means eight levels. With the marks that close together, a little damage causes misreads, so they last only one to three thousand cycles. So the drive is clever. It spreads writes evenly across every cell, and keeps spare cells hidden to swap in when one fails. A typical good drive comes with a promise: fill a third of it every single day for five years, and it'll be fine.
In the end, memory has a door, so it never wears out, and never remembers. The drive has only a wall, so it remembers, and it wears out. One surprise: an old drive with worn-out walls, unplugged in a drawer, slowly leaks its electrons. The industry standard only asks a worn-out drive to hold its data for one year unplugged in a thirty-degree room. So don't keep your precious photos on just one old drive.
Got another strange computer question? Leave it in the comments.
Two boards that look alike
Both are small boards covered in little black chips.
Yet you can write to memory all your life and never wear it out, while a solid-state drive does wear out if you write enough. Both chips are made of silicon. The difference hides somewhere much smaller: the tiny cell that holds one bit. Let's follow one electron as it stores a bit in each.
A bucket with a door
A bucket full of electrons means one. An empty one means zero.
The door opens and the electrons walk in; the door closes and the bit is stored. They go through the door and never touch a wall, so however often the door swings, the bucket doesn't wear out.
The catch:
memory's bucket leaks
So every 64 milliseconds, memory tops up every bucket again. That's over a dozen times a second, and you never notice.
Cut the power and nobody tops them up, so the buckets drain. That's why memory forgets everything when you switch off, and why it's only the computer's scratch paper.
Its bucket has no door,
sealed on every side
A drive has to remember for years with the power off.
So its bucket is wrapped in a very thin wall, just a few nanometers thick, more than ten thousand times thinner than a hair. The electrons can't get out, so the bit stays put. But with no door, they can't get in either.
Forcing electrons
through the wall
A high voltage forces the electrons straight through the wall. To rewrite the cell, a reverse voltage drags them back out again.
It's a rough push. Every squeeze leaves a few tiny scars in the wall, and some scars trap electrons (the red dot inside the wall). The scars pile up, the wall starts to leak, and more and more electrons get stuck.
The more a cell holds,
the faster it wears
A drive reads a cell by checking how full of electrons it is. A cell that stores one bit has just two wide levels and can take about a hundred thousand squeezes.
To be cheaper, today's drives pack three bits into each cell, which means eight levels. With the marks that close, a little damage causes misreads, so a typical cell lasts only one to three thousand cycles. Once the drive can't tell zero from one, that cell is done.
So the drive
learned two tricks
Trick one: spread the writes evenly so no cell gets squeezed all the time. Trick two: keep spare cells hidden, and quietly swap one in when a cell fails.
A typical good drive comes with a promise: fill a third of it every day for five years, and it'll be fine.
In the end:
one has a door, one has only a wall
Memory has a door, so it never wears out, and never remembers.
The drive has only a wall, so it remembers, and it wears out.
in a drawer
slowly forgets
Don't keep precious photos on just one old drive.
