500 gig per second, if we don't get a flat

Publication date: 17 May 2010
Originally published, in a smaller version, 2009 in Atomic: Maximum Power Computing
Last modified 03-Dec-2011.


There's an old expression: "Never underestimate the bandwidth of a station wagon full of backup tapes."

Actually, that's not the oldest version of it. The original probably had the station wagon was full of 250-kilobyte floppies, or mainframe tape reels, or something. As I write this, the Wikipedia article for "Sneakernet" includes a rather magnificent estimate of the bandwidth of a Boeing 747 full of Blu-Ray discs, and a discussion of the numerous other versions of this calculation. (Jeff Atwood had a look at it a few years ago, too.)

The other day I wondered, because Atomic magazine for some reason pays me to wonder things like this, how much bandwidth a station wagon full of memory cards would have.

Well, first we need to know how big our station wagon is. I hereby arbitrarily declare it to be a 1985 Volvo 240, which has 2.2 cubic metres of cargo capacity.

(That's 77.7 cubic feet, 3,871 Imperial pints, or 9.2 hogsheads. I think I'll stick with metric for the rest of this page.)

I'm also going to assume that the wagon isn't really packed totally full of memory cards, such that they cascade into the front whenever you brake and will avalanche out of the tailgate when it's opened. Let's say they are packed almost to the roof of the car, but in cardboard boxes, which reduce the usable cargo capacity to a nice round two cubic metres.

The type of card also obviously makes a big difference. If you go with CompactFlash, you're not going to be able to fit in as much data as if you use microSD. Since microSD cards are commonplace nowadays, with no price premium over full-sized SD cards, let's say you decide to maximise capacity by using those.

A microSD card is nominally 11 by 15mm by 1mm in size. I'll ignore the notches that reduce the width over half of the card's length, but the microSD cards I've got here aren't even a whole millimetre thick at the end bit with the fingernail-grip ridge. Most of the card is only about half a millimetre thick.

So let's say 11 by 15 by 0.5mm, for a volume of 82.5 cubic millimetres, which is 825 ten-billionths of a cubic metre.

(It was at this stage in the calculation that my eyebrows started rising.)

Assuming uniform and perfect stacking of objects of this volume, with zero air space, you can fit 24,242,424 of them into two cubic metres.

In the real world there'd obviously be air spaces, even if you painstakingly stack the tiny cards in perfect layers. My size approximation, that ignores the more-than-0.5mm height of the thick end of the card, could make the perfect-layers calculation quite inaccurate. But if you're just shovelling cards into the boxes and not stacking them, though, there will be even more empty space between cards, and the thicker ends won't matter much.

To use a few words you may have to hit Wikipedia about - I know I did - a random close pack of monodisperse microSD-shaped objects will be considerably tighter than one for, say, spheres. I wouldn't be surprised if it only reduced the theoretical no-air-space density by 20%, provided you shake the boxes while you're filling them.

So let's stick with a 20% density reduction from random packing, giving 0.8 times the theoretical density of perfectly-packed cards. Or nineteen million, three hundred and ninety-three thousand, nine hundred and thirty-nine cards, in the boxes, in the station wagon.

If you're buying that many memory cards, you probably care about value for money.

When I originally wrote this, eBay prices from probably-not-scam-artist dealers made "8Gb" cards (with unformatted capacity around eight billion bytes) the best value, for about three bucks a gigabyte, a price which your monstrous bulk buy probably would not greatly reduce. (Actually, I wouldn't be surprised if your vast purchase empties some warehouses and pushes prices up.)

Today, in mid-May 2010, 16Gb cards seem to be slightly better dollars-per-gigabyte value than 8Gb ones. So let's buy 19.4 million of those!

This'll cost you more than three-quarters of a brillion dollars, but I'm sure you can find that in the couch cushions. Oh, and that many cards will probably also seriously tax the suspension of even a big pickup truck, let alone a mere wagon. Never mind, though; replace the rear shocks with chunks of wood and she'll be right.

That many 14.9-formatted-gibibyte cards add up to a total capacity of some 288,484,849 gibibytes - which is about 275 pebibytes (to continue use of the new-fangled less-confusing-but-sillier-sounding capacity prefixes).

This wouldn't allow you to back up the entire Internet, but you probably could store the whole of YouTube, Wikipedia, Flickr, Facebook, MySpace, archive.org, Google's Web search database and Google Earth, and still have room for a few years of HDTV video.

Unfortunately, even if your cards and card readers could all manage 50 mebibytes per second of read and write speed, getting all of that data onto and off of the cards at each end of the wagon-trip in no more than 24 hours would require around 68,400 parallel copy operations, at each end. (And I thought it was hectic backing up my Amiga hard drive to 880k floppies.)

But the bandwidth of your station-wagon would be... impressive.

Never mind your T1 lines and cable-Internet download speeds. For your station wagon to deliver data slower than a forty-gigabit-per-second Internet core router, it would have to drive for more than 23 months. If the drive took only 5 days, and you had seventy thousand friends to get the data on and off in only one day at each end, then you'd be looking at an end-to-end speed of 477 gibibytes per second.

This - well, not exactly this, but more realistic things like sneakernetting a few large drives around - is why the wagon-of-tapes is so often cited in lessons about networking. It has very high latency - when you order the data sent to you, it may be days before it arrives, never mind milliseconds - but its bandwidth is immense. And people take advantage of this every day, moving large amounts of data (legal or otherwise) around on MP3 players, USB drives, bare hard drives and so on, rather than clog their Internet pipe for ages to avoid moving some atoms around. Good old UUCP also survives in some remote areas un-served by affordable speedy Internet links; people trekking around carrying storage devices, plus the occasional dial-up modem link, aren't exactly ideal for a game of StarCraft, but can move information quite reliably.

When I used to drive single, less-than-full 44Mb SyQuest carts to the paper-publishing output bureau half an hour away, I often only moved something in the order of a megabyte per minute. But that still beat our dialup upload speeds by about a factor of five. With three almost-full carts, I was pushing four megs a minute!

(I usually didn't drive any data back with me, though. So it was a unidirectional link.)

And then there's the fact that to make data-moving via microSD card obvious, you'd pretty much have to be carrying preposterous numbers of the things. The only real difference between microSD cards and the magical computer storage devices in old sci-fi books is that microSD is a lot smaller than anybody expected, even by the year 2200.

This tininess, and easy access, makes a lot of computer security look ridiculous.

In the olden days, anti-spy forces only had to worry about things like tiny film cameras and silk maps hidden inside innocuous objects. Nowadays, even if they confiscate your laptop you can still walk away with a hundred gibibytes of non-magnetic, shockproof digital storage hidden in your shoe-sole. Sixty gibibytes will fit under your upper lip.

Because I own a lot of tweezers and forceps and never really outgrew the toddler stage, I just verified that a microSD card fits quite nicely in my ear.

I don't think anybody really meant tiny memory cards to be spy-tech. But I'm sure people are using them that way, with or without James-Bond hiding places.

And microSD's power can easily be used for good.

I, for instance, doubt anybody's told those North Korean border guards in the silly hats to look out for tourists who've hidden a copy of Wikipedia in their ear.

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