Solid State Drives (SSDs) are storage devices constructed from an array of solid-state electronic storage chips, consisting of a control unit and a storage unit (FLASH chip, DRAM chip). SSDs share the same interface specifications, definitions, functions, and usage as traditional hard disks, and they also match ordinary hard disks in terms of product shape and size. Due to their robustness and efficiency, SSDs are widely used in various fields, including military, automotive, industrial control, video surveillance, network monitoring, network terminals, electric power, medical, aviation, and navigation equipment.

SSD Architecture

The three most important components of the SSD are the NAND flash, the controller, and the firmware. The NAND flash is responsible for important storage tasks, while the controller and firmware need to work together to manage complex and equally important tasks such as data storage, maintaining SSD performance, and ensuring longevity.

Controller

Controller: The controller is responsible for managing data read and write operations, performing error correction, optimizing performance, and extending the lifespan of the flash memory.

NAND Flash Chips

NAND flash memory chips serve as the primary storage medium for SSDs, comprising millions or even tens of billions of memory cells. The number of these cells determines the data capacity of the flash chip. Each memory cell’s main structure is a floating gate transistor, which "confines"  electrons and controls their movement in and out of the cell. The change in its high and low levels determines the stored data, which can be either a 0 or a 1. The NAND flash chip connects to the controller chip via a Serial Peripheral Interface (SPI) to facilitate data reading and writing.

There are also different types of flash memory particles. Currently, the following types of particles are common:

•  SLC (Single Level Cell): Each cell stores 1 bit of information and has only two voltage variations, 0 and 1. This simple structure allows fast voltage control, resulting in a long life and strong performance. Its P/E (Program/Erase) life is between 10,000 and 100,000 times. However, the SLE’s disadvantages include low capacity and high cost because each cell can only store 1 bit of information.

•  MLC (Multi-Level Cell): Each cell stores 2 bits of information, requiring more sophisticated voltage control with four variations: 00, 01, 10, and 11. This complexity results in lower write performance and reliability. The P/E cycle life of MLC ranges from 3,000 to 5,000 cycles, depending on the manufacturing process.

•  TLC (Triple Level Cell): Triple-Level Cell (TLC) NAND, each storage cell can hold 3 bits of information. Increasing the number of bits per cell reduces the cost and increases storage capacity. However, its Program/Erase (P/E) cycle count is only around 3,000, so its performance and durability are not very high, but its cost is lower than SLC and MLC. 

•  QLC (Quad-Level Cell): Each storage cell can hold 4 bits of information. This further reduces costs and increases storage capacity compared to TLC NAND. However, QLC NAND has an even lower Program/Erase (P/E) cycle count, typically around 1,000 cycles, resulting in lower performance and durability. Despite these drawbacks, QLC technology is popular in consumer products due to its affordability and high storage density.

Cache

SSDs are usually adopted with a cache of some capacity to increase the speed of reading and writing data. The cache can be DRAM (Dynamic Random-Access Memory) or part of the flash memory itself. The presence of cache can effectively reduce the latency of data access.

Interface

Currently, there are several common SSD interfaces:

•  SATA: The SATA interface is the most common SSD interface in the consumer-grade market, with transmission speeds around 500-550 MB/s, offering significantly better performance than the HDD.

•  mSATA: mSATA is a compact version of the SATA interface; it adheres to the SATA standard but in a smaller form factor, making it ideal for ultrathin notebooks, tablets, and other compact devices.

•  NGFF: It uses the M.2 interface but operates through the SATA channel, connecting to the southbridge and indirectly communicating with the CPU, resulting in slower performance.

•  NVMe: NVMe is a high-speed storage interface designed for modern SSDs. It uses the PCIe bus for data transfer, offering low latency, high concurrency, low CPU usage, and excellent scalability, significantly boosting storage performance and efficiency.

•  U.2: A proprietary interface communicating directly with the CPU through the PCIe channel. This type of hard disk requires a special PCIe to the U.2 interface adapter card.

How SSDs Work

The operation of Solid State Drives (SSDs) primarily revolves around the storage mechanism of flash memory. Flash memory is a non-volatile storage medium capable of retaining data even without power. Its basic storage unit is a floating gate transistor, which represents binary information (0 or 1) by storing a charge in the floating gate under controlled voltage.

In SSDs, data read and write operations are managed by the controller. Reading data is straightforward; the controller reads data from a specified address and transfers it to the host system. Writing data is more intricate due to the slower write speed of flash memory, which requires an erase operation before new data can be written.

Flash memory erase operations typically occur at the “block" level, while writes are performed on a "page" basis. This complicates SSD write performance compared to traditional HDDs, as each write operation necessitates prior block erasure. To solve this problem, controllers use Garbage Collection and Wear Leveling techniques to optimize performance and extend flash memory life.

•  Garbage Collection: When data on some pages of an SSD becomes invalid (e.g., due to file deletion or modification), Garbage Collection reorganizes valid data, writes it to new blocks, and erases old blocks to reclaim space.

•  Wear Leveling: Since flash memory has a limited write life, the controller uses wear leveling to ensure that all blocks are written roughly the same number of times, preventing certain blocks from being damaged prematurely.

Application Scenarios of SSDs

SSDs find extensive applications across diverse fields, leveraging their superior performance and capabilities:

I. Personal Computers and Laptops

•  System Boot-Up and Application Loading: SSDs have faster read and write speeds, which can significantly shorten system startup time and application loading time, enhancing the user experience.

•  Data Storage: Ideal for users requiring rapid access to and processing of large data volumes, such as designers and gamers, SSDs provide an efficient data storage solution.

II. Data Centers and Servers:

•  High-Performance Storage Needs: SSDs meet the intensive read/write demands of data centers and servers, supporting high-speed data operations.

•  Data Backup and Recovery: Fast read/write speeds facilitate quicker data backup and recovery, improving data center reliability and availability.

III. Enterprise-Level Applications

•  Big Data Processing: SSDs with high read/write speeds and large capacities cater to big data processing needs, including data analysis and mining.

•  Real-time Data Processing: For scenarios that require real-time data processing, such as financial transactions, online games, etc., SSDs are able to provide low-latency data access capabilities to ensure real-time data.

IV. Embedded System and Industrial Control

•  High Reliability and Stability: Embedded systems and industrial control areas require high reliability and stability of equipment, SSDs are suitable for these environments due to their design without mechanical parts and high shock and vibration resistance.

•  Longevity and Durability: SSDs usually have a longer life than mechanical hard disks and can withstand more read and write operations, making them suitable for embedded systems and industrial control devices that require long hours of operation and frequent reads and writes.

V. Other Fields

•  Military and Aerospace: In these areas, there are extremely high requirements for data security and stability, and SSDs are widely used in these scenarios because of their high-speed read/write and durable characteristics.

•  Video Surveillance and Network Monitoring: With the popularization of video surveillance technology, a large amount of video data needs to be stored and processed. The high-speed read/write capability of SSDs is able to meet these needs and provide a stable video playback and storage solution.

•  Connected Car: SSDs are being used to meet the requirements for high-performance graphics, data storage and communications, providing flash solutions that help deliver high-performance storage and reliability in a wide range of in-vehicle applications, including driver assistance technologies, entertainment systems, navigation systems and 3D mapping.