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Comment by guidedlight | Hacker News Reader

I remember trying to explain to my grandparents how many individually addressable bits were in a 128MB SD card. To them the number was impossibly high.

I wish they were stay alive today, to see a 8TB MicroSD card. ;-(

I wonder what the limit is.

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> I wonder what the limit is.

It has a name: The Bekenstein bound

https://en.wikipedia.org/wiki/Bekenstein_bound

(...granted, that's if you don't care about access-times)

Somewhat off-topic, but it blows me away every time it's brought up that the amount of information you can store in a volume scales by the surface area covering that volume and not the volume itself.

Even wilder is that the amount of matter you can fit in there to store that information scales by the radius.

That is indeed surprising. Is there ELI5?

(I think it's the radius not the area?)

ELI5:

Information can only be stored on energy (mass/light etc.), and the limit for how much energy you can get in a certain volume before it becomes a black hole happens to be proportional to the radius.

ChatGPT says it's about 2.24*10^20 exabytes for an microSD card

Possibly from here? https://electronics.stackexchange.com/questions/14055/how-ca...

I can't remember exactly when I had that conversation with my grandmother, probably around 15 years ago.

My dad asked me to pick up a USB stick, "the cheapest will do" he said, so I got a 1 GB one for £5 or so.

My grandmother was there when I handed it over, she asked how many books could it store.

"About 600 the size of the bible", I said. In retrospect think I'd underestimated the size of the bible by a factor of 3, but almost any compression would fix that easily.

Still, she was flabbergasted.

I think the same category is now 32 GB? I have more than I need, so I'm not buying them anymore.

> individually addressable bits

I don't think any bits are individually addressable in Flash memory.

What do you mean?

Digital storage is kind of pointless if we cannot know exactly which bit came from what logical location.

ie: it's not like analog audio tape, where we get some signal, but don't really know its exact position.

I believe it's that you can't read that small an amount at once; AIUI most memory will only let you read to the byte at most and then you'd have to do some bit-fiddling to pull out individual bits, and I vaguely recall that flash is often only read/write-able in like 4k blocks at a time, so if you want to know the first bit on the device what you actually do is copy the first 4kB into RAM then grab the first byte of the result and AND it with 1<<64 to isolate just the first bit

Because we don't request specific bits from Storage, we address by block, which returns 4k of bits (or whatever the formatted block size is), and then process the data within that block.

Well I was a little wrong, modern NAND supports page, word, and bit level addressing. Older NOR flash only support page and word level addressing. But to access that 1 bit, the flash controller is still reading multiple bits.

With all flash, a cell is the smallest addressable unit. Bits are stored in cells and depending on the flash type it can store anywhere from 1 to 5 bits per cell. So with SLC flash a cell is analogous to a bit, but for the multi-cell flash types a cell might mean 2, 3, 4 or 5 bits.

Flash cells are organized into strings, pages, blocks, planes, and then the flash die itself. Reading and writing of NAND flash occurs at the page level (4-16 KiB per page) but erasing occurs at the block level. So in the case of NAND, the controller cannot read/write a single bit of data.

So while you can specify that only 1 bit be changed in flash storage, there's an entire abstraction layer between you and the flash chip that hides the fact that a lot of data gets moved around to support a single bit being changed.

One limit is mentioned in the article: "The SD Ultra Capacity (SDUC) standard has been around since 2018 and theoretically allows for cards with up to 128TB of storage capacity."

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