Can we write the data as a single track, that way they could spin very slowly, like a vinly record.
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The encoding of data has a minimum physical size it can be and still be reliably decoded. CDs and HDDs are what you get when you balance reliability and read speed.
The language of a hard disk is binary. It’s only 0’s and 1’s. Every single process your computer goes through ultimately comes down to a 0 or a 1. You need a bajillion 0’s and 1’s to make whatever you’re doing, so it needs to run super fast to get through all of that data in a timely manner. You could make one that spins really slow, but you wouldn’t be able to put much data on it.
First of all, the speed of spinning equal to the speed of reading data. Slow spinning – less data per millisecond.
As to the single track – what happens when you delete a 1MB file from that track, but need to write a 2MB file instead? There is not enough space in that part of the track where the 1MB file used to be.
People have no patience.
When they click “next song” or “open file”, they want immediate response. If the disk spun at 60RPM it would take as much as a whole second before the device/computer responded. That’s unacceptable, so the disks need to spin faster, about 500 to 1000 times faster for computers and 10 times faster for a CD.
Let’s take the most recent 20TB HDDs as an exemple. They are made of 9 platters, each of which can be written on both sides, so 18 surfaces to write to.
That means 21,990,232,555,520 Bytes, or 175,921,860,444,160 Bits.
Each side of a platter is then about 9,773,436,691,342 Bits.
Each of these Bits have to be written, that’s a whole lot of them, it has to spin fast to write all of that quickly.
We do write the data as a single track but you are restricted in how small you can make the little bits. Making the little bits smaller increases the rate you can read data at a certain speed, but also just ramping up the speed is a nice way to get more data out without needing fancier platters
There’s also the problem of latency. Your standard hard drive spins at 7200 RPM, about 120 times per second. If you request a file on it that happens to have just passed the read head then you need to wait for it to come all the way around, this will take about 8 milliseconds which for a processor that completes instructions in fractions of nanoseconds is basically an eternity
So we spin them faster to reduce latency. Old high speed HDDs could spin at 15,000 RPM so it only took 4 ms for the disk to come back around, they could also read out data about twice as fast as a 7200 HDD built with the same size bits on its platter.
You could imagine a CD as a vinyl record that’s been hit with a combination of a growth ray and a shrink ray.
The growth ray is because a vinyl track has about 1500 ft of track; that is the distance the needle has to travel to go from the beginning to the end of the vinyl. Meanwhile, a CD is in the realm of 3.7 *miles*, and other discs are only longer. To fully read a CD, the stylus needs to cover so much more distance.
The shrink ray is because a CD is physically smaller. It has a smaller stylus (a beam of light instead of a needle) and it takes up less space (less than 5 inches instead of 10 to 12 inches). The bigger a disc is, the faster one spin moves a part of the disc; for one spin of a vinyl record, the CD needs to spin more than 2 times to cover the same distance. It does this by spinning faster.
The combination of needing to cover more distance and needing to spin faster to make the disc move the same speed is what creates the requirement for such a high spin speed on digital disk readers. It also helps that the light beam is more accurate than the needle, so spinning faster doesn’t mess up the data, and a light beam isn’t mechanically touching the disc so it won’t scratch the disc while reading it faster.
The reason we need so much distance on the CD? This is the digital data definitions I see other answers talking about. Relying on digital data requires more instances of reading than a vinyl that just holds the shape of the speaker’s movement like an original vinyl would, which might sound worse, but the more points of information means the data read will be more consistent or higher quality.
The time between when a program decides it wants a block of data until it gets that block needs to be as short as possible. A big chunk of that time is waiting for the disk to rotate around until the block passes under the reading head of the drive. Sometimes you’ll be lucky and the block is right there as the drive is ready for it and other times you have to wait almost a whole rotation. On average, you have to wait half a turn^(*). This time is called the “rotational latency”.
A fast 7200 RPM drive has to wait, on average, half of one 7200th of a minute, or 4.2 ms (thousandths of a second). Slower 5400 RPM drives average 5.6 ms. These numbers are roughly a third of the total time taken to access a block of data from the disk so keeping them down is an important part of computer performance.
You might think that accessing a block every 13 ms is very fast but it’s fewer than 80 blocks per second, glacially slow in computer terms. Computers try to keep files arranged with their blocks all together so they can be read in order, without having to wait for the disk to rotate around. But with many programs running at once accessing many different files, accessing blocks essentially in random order from all over a disk is very common.
* Actually drives with smart queuing can do better. On a busy computer there will often be a long stream of requests coming in for blocks from all over the disk. Drives that support “tagged command queuing” are sent the whole list and can rearrange the order in which the blocks are retrieved, potentially allowing them to plan a tour of the disk that avoids so much waiting.
I’m going to go back to old hard drives to make this simple, and counting a drive with only one platter (disk), but the general idea still applies.
A hard disk is concentric rings, each divided up into sectors that hold your data. For discussion, our disk has 1,000 tracks (rings) of 64 sectors each for a total of 64,000 sectors. Each sector hold 512 bytes of data, so it can hold 32,768,000 bytes (32 MB) on one side, or double that if we read from both sides. So we have a 64 MB hard disk (that was big at one time).
Now we sandwich the disk between two read/write heads and spin it like a record, which is about one rotation every two seconds (rounding). So every two seconds we read one track. We have a thousand tracks. It’s going to take us over half an hour to read the 64 MB on that hard disk. That’s very slow.
And that’s just for reading the whole thing at one time. Think of when we just try to start reading a file. Wherever the read head happens to be now, we have to wait for the first sector of the file to come under it for it to read, which can be anywhere from zero to two seconds. Ouch.
Files are also not always nicely lined up in adjacent sectors. A file’s sectors could be all over the place. So think of a clock, and the disk is spinning in the direction the hands on a clock go. Your file uses four sectors (2 kilobytes). The first sector is at 3 o’clock, second at 1`2, third at 6, fourth at 9. So we read the first sector (at 3), then we wait a second and a half for the disk to come around to the second sector (at 12), then another second for the third (at 6), and then a half second for it to come around to the fourth (at 9). We just waited three seconds to read 2 kilobytes of data.
Now we spin it as fast as a modern hard disk at 7,200 rpm, or 120 rotations per second. We can read the whole disk in just over 8 seconds. It will take us at most 1/120th of a second for any one sector to come around under our read head. The spread out file above will take 1/80th of a second to read.
Compact discs ARE a single track, a spiral that goes from the inside out. An audio disc does spin relatively slowly, 200RPM-500RPM, but they spin at that rate much higher than LP because the sample rate and encoding of CD requires about 3 million bit transitions between “pit” and “land” per second to play digital audio. Were they to use newer Blu-Ray technology with much smaller bits, they could spin slower.
A hard drive instead requires access to data on-demand. Understand that the data is already packed as densely as the present technology allows. Slowing the rotation speed would impact performance in two ways:
-The amount of data passing under the read head would slow, increasing the transfer time.
-The amount of time that you have to wait for the correct data position to rotate around to underneath the head increases (latency)
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