Why does the speed slow down when the SSD capacity is full?

ByNorine Cabral 2020-02-20 1103

Below is a schematic diagram of the structure of Yingchi HOF PRO M.2.

Most SSD on the market, like HOF PRO M.2, is mainly composed of master, flash and cache. Different from the mechanical hard disk, SSD relies on electrical signal transmission, and the main control can access multiple data on different flash memory particles in a very short time, so the reading and writing speed of the mechanical hard disk is hundreds or even higher than that of the mechanical hard disk.

The most critical part of SSD is the flash memory particles responsible for storing data, which largely determines the performance and service life of SSD. The saying that "to buy SSD, seven cents is to buy flash memory" shows its importance.

Flash memory was originally born in a silicon wafer. Flash chip manufacturers cut and selected qualified wafers according to the test results, and then packaged them, thus becoming what we are familiar with and commonly known as flash memory.

Since its birth, flash memory particles have experienced four types: SLC, MLC, TLC and QLC. According to the chronological order, SLC is the oldest and QLC is the youngest. They coexist for a long time, but their current applications are very different, and the reasons are closely related to their own characteristics.

In SLC flash memory, a memory cell can only store 1bit (bits) data, that is, a 0 or a 1, so a memory cell has only two states.

In MLC flash memory, a storage unit can be crammed into 2bit data, and 0 and 1 can be arranged and combined, with 10, 11, 00, 01 four states.

And so on, to the latest generation of QLC flash memory, a storage unit has up to 16 storage states, and the storage capacity is 8 times that of SLC flash memory.

But God opened one door and closed another for SSD. Flash-type updates, though bringing a huge increase in capacity, were bought at the expense of speed and lifespan.

Unlike mechanical hard drives, SSD's flash memory can be erased and written for a limited number of times, which is the lifetime of the flash memory. When the number of erases and writes of the flash memory reaches the limit, the flash memory will be damaged and the data stored in it will be lost.

SLC flash memory has 100000 erase and write times, while the latest generation of QLC flash memory has only about 1000. In the case of the same capacity and usage, the theoretical life difference between the two is a full 100 times.

So it can be said that with the development of science and technology, the performance of the new generation of flash memory has gone backwards. But this is actually the active choice of the market. It is only in recent years that SSD has begun to popularize the consumer market, and in the previous long evolution, it was price that hindered the popularity of SSD. As flash memory accounts for most of the cost of SSD, reducing its cost is not only the main breakthrough to reduce the price of SSD, but also the premise to promote the large-scale application of SSD in the consumer market.

What is gratifying to see now is that compared with the SSD, with SLC flash memory, the price of SSD using TLC flash memory has dropped to an affordable price for everyone. At the same time, even if the lifespan is reduced, today's SSD can still be used by ordinary people, and it will not be a problem to use it easily for 3 to 5 years.

Therefore, the main problem with SSD today is not its lifetime, but the sudden decrease in running speed when its capacity is about to be filled.

The cause of this problem is related to the erasing and writing process of flash memory. Unlike mechanical hard drives, SSD cannot directly overwrite old data when writing new data. When new data is written, if there is not enough space, SSD needs to recycle the old data, erase it and then write it. This process leads to write magnification (an introduction to writing magnification can be seen in the "MEMO" column in the official account menu bar).

Write magnification means more complex steps, longer time, and more writes. Therefore, if the SSD is too full, both the running speed and the time life will be affected and will decrease in varying degrees.

So how to avoid the performance degradation caused by too much data storage in SSD?

Entry-level SSD, usually reduces its available capacity and prevents the SSD from being completely full by setting aside some space. This is why many SSD on the market have 240g or 480g of capacity instead of 256g and 512g.

Mid-and high-end SSD, will be equipped with additional cache to reduce write magnification. For example, Yingchi HOF PRO M.2 1TB provides a very large cache up to 1GB.

Of course, the above are mainly the methods adopted by SSD manufacturers, for users, daily use should also develop good habits, timely cleaning of files that are not commonly used, to ensure that SSD has enough space to meet the daily reading and writing needs.


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