Solid-State Drives: How Do They Work?
Is your computer’s performance slowing down, and are you looking for a way to boost it up? If you have the same problem, replacing your hard disk drive (HDD) with solid-state drives (SSD) would be the best fit for you. It will do some magic things so that your computing experiences will be better soon. Maybe, it makes you wonder how SSD can boost your computer’s performance. So, let’s look further into SSD.
What is an SSD?
In the computer term, there is a difference between memory and storage. Memory works like a person’s short term memory. It needs electrical power to do its job, which is holding the program that the computer-executed. If your computer shuts down, the data stored in memory will disappear. Besides that, storage holds all of the data for your digital life such as apps, files, photos, music, etc. It will keep all of your data even if you switch off the power.
But, there is also a flash-memory device that makes a distinction between memory and storage. The flash-memory device still holds data although it is not linked to any electrical power. Flash-memory devices have no mechanical parts, unlike hard drives that contain spinning platters and turntable-like arms-bearing read-write heads. It builds from transistors and other components you’d found in a computer chip. Consequently, it named solid-state which exploits semiconductor material.
There are two types of flash memory: NOR and NAND. In NOR flash, the cells are wired in a series. Its cell contains more wires, bigger and more complex. Aside from that, the cells are wired in parallel in NAND flash. Its cell requires fewer wires and can be arranged on a dense chip. As a consequence, NAND flash can read and write data much more rapidly and also inexpensive. That’s why NAND flash is the primary type of memory in solid-state drives.
Based on this information, the solid-state drive is a device that uses NAND flash to provide non-volatile, rewritable memory which design to retain its charge state when not powered up, not rely on moving parts or spinning disks and store its data on a semiconductor material. In computers, SSD can replace the traditional hard disk drives(HDD). Because manufacturers make SSD appear like HDD in shape so that both SSD and HDD can be used interchangeably. But, you wouldn’t see platters and actuator arms inside the SSD case.
How SSD Store Data?
The NAND flash of a solid-state drive stores data differently. Recall that NAND flash has transistors arranged in a grid with columns and rows. If a chain of transistors conducts current, it has a value of 1. If it doesn’t conduct current, it’s 0. At first, all transistors are set to 1. But when a save operation begins, the current is blocked to some transistors, turning them to 0. This occurs because of how transistors are arranged. At each intersection of the column and row, two transistors form a cell. One of the transistors is known as a control gate, the other as a floating gate. When the current reaches the control gate, electrons flow onto the floating gate, creating a net positive charge that interrupts the current flow. By applying precise voltages to the transistors, a unique pattern of 1s and 0s emerges.
Reading, Writing, and Erasing Data in SSD
Based on the nature of flash memory operation, data cannot be directly overwritten as it can be in a hard disk drive. When you write data for the first time in SSD, it will directly be written into pages at some block. The SSD controller on the SSD, which manages the flash memory and interfaces with the host system, uses a logical-to-physical mapping system known as logical block addressing(LBA) and that is part of the flash translation layer (FTL). When you want to overwrite data that has been written before, the SSD controller will write the replacement pages and the original pages will become stale(invalid). Then, in the idle period, the SSD controller will move the replacement pages into a new block and erase the original block that contains the original pages so that it will become a free block that can store a new data directly anytime. Here is an illustration of the process of reading, writing, and erasing data in SSD.
From this illustration, you can see that data A-D are written directly to a free-block (Block X). Then when the new data E-H are stored, it will be written in the next free-pages in the same block. But, when data A-D are being overwritten, the SSD controller will write replacement pages (A’-D’) and the original pages (A-D) will become stale. In the idle period, the SSD controller will move the good pages besides the stale data into a new free-block (Block Y) and erase the old-block (Block X) so that the old block will become a free-block.
Flash memory can be programmed and erased only a limited number of times. This is often referred to as the maximum number of program/erase cycles (P/E cycles) that can sustain over the life of the flash memory. A single-level cell (SLC) SSD typically operate between 50.000–10.000 cycles. A multi-level cycles (MLC) SSD typically operate between 3.000–5.000 cycles. A triple-level cycles (TLC) SSD typically operate between 1000 cycles. A lower write amplification is more desirable, as it corresponds to a reduced number of P/E cycles on the flash memory and thus to an increased SSD life.
Types of SSD
NAND flash comes in many types of based on bits that can be stored in each cell. There are single-level cell (SLC) SSD, multi-level cell (MLC) SSD, triple-level cell (TLC) SSD, quad-level cell (QLC) SSD, and also penta-level cell (PLC) SSD that are still in development.
The most basic type of SSD is the single-level cell (SLC) SSD. SLCs accept one bit per memory cell. SLC store less data, but it has advantages. First, SLCs are the fastest type of SSD. They’re also more durable and less error-prone, so they’re considered more reliable than other SSDs. A multi-level cell (MLC) SSD is a bit slower than SLCs because it takes more time to write two bits onto a cell than just one. They also take a hit in durability and reliability because data is written to the NAND flash more often than with an SLC. A triple-level cell (TLC) SSD write three bits to each cell. They have larger storage in a smaller package than SLC and MLC, but they sacrifice relative speed, reliability, and durability. A quad-level cell (QLC) drives can write four bits per cell. QLC NAND can pack a whole lot more data than other types, but it also reduces the performance and the durability itself. This is especially true when the cache runs out during large file transfers (40 GB or higher). This might be a short-term problem, as manufacturers try to optimize QLCs. The last one is penta-level cell (PLC) which still doesn’t release yet for the customer. Why SLCs are the fastest type? Because SLCs only have to know whether the values are 1 or 0. For MLCs, there are 4 possible values per cell — 00,01,10,11. For TLCs, there are 16 possible values per cell, and so on.
Comparison Between SSD vs HDD
Using SSD besides HDD is an effort to get the best computing performance on your computers. The absence of the HDD’s moving parts (spinning platters and turntable-like arms-bearing read-write heads) in SSD takes a big hit in boosting the computer speed and responsiveness. This table will explain to you why SSD is better than HDD.
From this table, we can see that SSD has faster access time than HDD so programs can run more quickly especially for some programs that need a large amount of data such as operating systems. With no moving parts, SSD generates less heat than HDD, not affected with magnetism, and generates no noise. In consequence, SSDs are more reliable than HDD. SSD uses less power than HDD, which means a lower energy bill for computers or an increase in battery life for laptops.
SSD has many advantages as mentioned before. But, there is no perfect technology. SSD also has disadvantages compared to HDD. First, there’s an issue of longevity. The NAND flash used in SSDs can only be used for a finite number of writes. Why? Because SSDs can’t write a single bit of information without first erasing and then rewriting very large blocks of data at one time. Each time a cell goes through an erase cycle, some charge is left in the floating-gate transistor, which changes its resistance. As the resistance builds, the amount of current required to change the gate increases. Eventually, the gate can’t be flipped at all, rendering it useless. This decaying process doesn’t affect the read capabilities of SSD, because reading only requires checking, not changing, the voltages of cells. As a result, NAND flash can “rot” into a read-only state. Then, the price of SSD is higher than HDD, which is why most of the computers with an SSD only have a hundred gigabytes of storage. A desktop computer will also use extra HDD for additional storage.
For this reason, people are combining SSD and HDD for their digital life. They used SSD in their computers and HDD as external storage to store music, games, and other data. So they can get the best computing performance from SSD and also a relatively inexpensive high capacity storage from HDD. There is also another type of storage that combining these technologies, which are solid-state hybrid drives (SSHD). SSHD has both parts, mechanical moving parts from HDD, and also the semiconductor materials from SSD. It is more affordable than SSD and more reliable than HDD.
Conclusion
In conclusion, a solid-state drive is a device that uses NAND flash to provide non-volatile, rewritable memory. It provides better access time and higher computing performance than HDD. In exchange, you have to prepare more budget for using SSD.
Reference
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Joel Hruska on April 15, 2. (2020, April 15). How Do SSDs Work? Retrieved July 26, 2020, from https://www.extremetech.com/extreme/210492-extremetech-explains-how-do-ssds-work
Paul, I. (2019, October 28). Multi-Layer SSDs: What Are SLC, MLC, TLC, QLC, and PLC? Retrieved July 26, 2020, from https://www.howtogeek.com/444787/multi-layer-ssds-what-are-slc-mlc-tlc-qlc-and-mlc/
Singer, D. (2020, May 18). Understanding the Difference Between Solid State and Hard Disk Drives. Retrieved July 26, 2020, from https://www.liquidweb.com/blog/how-do-solid-state-drives-work/
