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Let’s discuss how drives store data for HDD & SSD!

Almost every day, we use smartphones or computers to save pictures, videos, documents, and other data. Many people also know that data can be saved because there is a component called a "hard disk" in the device, but many people do not know how the hard disk stores these data.

The first thing we need to understand is that there are only 0 and 1 in the computer, so the data we deposit into the hard disk is actually a bunch of 0 and 1. In other words, if we have a way to record 0 and 1 we can record the data, for example, we have 2 light bulbs, one does not light up light can indicate 01, that is, the number 1, both light up 11, that is, the number 2, the larger the number can be represented by more light bulbs.

Hard disks are divided into mechanical hard disks and solid-state hard disks, which store data differently due to their different structures. First, mechanical hard disks, whose most important structure is a magnetic disk coated on both sides, spin at 7,200 revolutions per minute while working.

The purpose of a disk is to record data. On the disk are arranged many small particles of material, which are magnetic material, and can be permanently magnetized and changed poles, which represent the 0 and 1 in the computer binary.

When data is written, a magnetic head three nanometers from the disk uses an electromagnet to record data by changing the polarity of the magnetic material on the disk, which corresponds to either a zero or a one. When the data is read, a reader next to it can recognize the polarity of the magnetic material and restore it to zero or one.

Since the disk rotates to read and write data, the original design was to cut blocks out of concentric circles like disks. These blocks were integrated into a circle that could be accessed by the magnetic head on the robot arm. This cell is the smallest unit of physical storage on a disk, called a sector. It is typically 512 bytes in size, and the circle formed by a sector is called a track.

Therefore, to read a file, the head must be driven by a motor to find the corresponding track, and then wait for the disk to switch to the corresponding sector. This usually takes tens of milliseconds, which slows down the speed of the mechanical hard disk when reading data scattered around the disk.

Solid-state drives are much simpler than the complex structures found in mechanical drives. It mainly depends on the flash chip to store data as the medium, by the master chip to assume the data transfer, and allocate data stored in the flash chip above.

The basic memory unit of flash memory is the floating gate transistor, which is encased in silicon dioxide and insulated up and down, and can hold electrons in the event of a power failure, with the number of electrons above a median value of 0 and below a median value of 1.

The transistor every time before writing data to erase, add a voltage on the P-pole, floating gate original some electrons will be because of the quantum tunneling effect through the insulation layer is sucked out, so that the number of electrons in the floating gate is lower than the median value, reduced to 1; To write a zero, a voltage is applied at the control pole to send electrons through the insulation and back to the floating gate so that the number of electrons is higher than the median value, indicating a zero.

But when it reads, the flash doesn’t know directly how many electrons are in the floating grid. Because applying a certain amount of voltage to the control pole will lead to the two n-poles. The greater the voltage at the control pole, the greater the current between the N poles. However, a floating gate that stores 0 has more electrons than a floating gate that stores 1, which cancels out the voltage at the control pole, so the control pole needs a larger voltage to conduct the two N-poles.

Therefore, when we do not know how many electrons there are in the floating grid, we can add an intermediate voltage to the control pole. If the two N poles conduct, we can deduce that there are fewer electrons in the floating grid and identify it as 1. If there is no conduction, it means that there are more electrons in the floating grid and it is identified as 0.

The traditional SLC has only two states of electrons and can hold only one bit of data. However, MLC, TLC, and QLC have 4~16 states of electron quantity, and one unit can store 2~4 bits.

Multi-order storage units greatly reduce the cost per unit capacity of SSDs but also affect disk life and performance. This is because as the transistor wipes data, the silicon dioxide insulation traps some electrons. The accumulation of these electrons gradually cancels out the voltage at the control pole, making the control pole need more and more voltage to run through the two N-poles. When the offset exceeds the median value, the reading becomes impossible to distinguish between 0 and 1.

However, multi-order storage units are more susceptible to this kind of offset due to the very fine division between different states, so the total number of erasures decreases geometrically from SLC to QLC.

In a mechanical hard disk, the magnetic head reads and writes data by rotating the disk. Therefore, the speed of the disk reading and writing data is closely related to the speed of the disk rotation. The faster the disk rotates, the faster the head can access more areas and the faster the speed of the disk. But it is also the speed is relatively fast, so the anti-earthquake and anti-dust ability are relatively poor, just think of thousands of revolutions per minute of disk shock or hit the dust, even if the magnetic head is just a little bit of a small offset, will also bring irreversible damage.

Solid-state drives, with no mechanical structure, don’t have to worry about that. But solid-state drives also have a drawback: they store data by storing electrons inside storage cells. And an electron, you know, is a very, very small thing, so if you put an electron in the same place and erase it, and you do that over and over again, you’re going to have instability, you’re not going to write an electron. In particular, the upcoming QLC memory chip has four electrons in each memory unit, which greatly reduces the service life due to the interaction between the electrons.

However, a consumer MLC or TLC SSD will last you at least five years and is far better than a mechanical hard drive, with reading and writing speeds up to 10 times faster. In addition, SSDS work quieter and more earthquake-resistant because they don’t have complex mechanical structures.

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