The solid-state disk structure of SSD：controller algorithm- firmware- NAND flash

Source: 　　Editor: Lucky Update Time :2018-11-25

SSD solid-state disks have been a major advance in storage technology over recent years, which brings about disruptive changes in main storage of computer. Upgrading SSD is not only a slight improvement in the aspect of performance，it also will change the experience of using computer with revolutionary random access speed, excellent multitasking ability, excellent durability and reliability. There is no doubt that SSD will be the leading role of future storage. Now, the composition of SSD, related technologies related to SSD and usage skills of SSD are briefly introduced to help more users understand what real SSD is.

First, we should understand the most basic SSD components, controller, NAND flash and firmware algorithm.

The speed of SSD far exceeds hard disk drive:

The traditional hard disk drive (HDD) is mainly operated by mechanical driving head, including necessary mechanical parts such as motor, disk, magnetic head and rocker arm, which must be moved to the access position on the rapidly rotating disk, and at least 95% of the time is consumed on the action of mechanical components.

However, SSD is different from mechanical structure，which requires no moving parts. The kind of SSD, which mainly consist of master controls and flash memory chips, can visit any location of drives at a faster rate and greater accuracy. While traditional hard disk drive must rely on the main shaft host, magnetic head, and magnetic head arm to find their positions, SSDS replace physical spinning disks with integrated circuits, and the time and delay of accessing data far exceeds the hard disk drive. SSD has such "amazing speed", entirely thanks to the internal components: the master control, flash memory and firmware algorithm.

The relationship of controller, flash memory and firmware algorithm

The three most important components of SSD are NAND flash, controller, and firmware. NAND is responsible for important storage tasks, and controllers and firmware work together to accomplish complex and equally important tasks, such as managing data storage, maintaining SSD performance, and using life.

Master control:

The controller is a kind of embedded microchip (such as CPU in the computer), whose function is like the command center, and can send out all the operation requests of SSD -- from reading and writing data to executing garbage collection and consumption balance algorithm, so as to guarantee the speed and cleanliness of SSD. It can be said that the master control is the brain center of SSD.

The current mainstream controllers are Marvell, SandForce, Samsung, Indilinx:

Marvell is powerful in all respects, especially in performance and speed. Its typical model is Marvell 88SS9187 master control, mainly used on the SSD of Plextor, SanDisk and crucial.

The performance of Sand Force is good, and its characteristics is able to support compressing data. For example, a compressible data of 10M can be compressed into 5M when written to the hard disk but still occupies a space of 10M, which can accelerate the writing speed. The biggest characteristic is can prolong the lifetime of the SSD, but the CPU utilization will be high and rate will gradually decrease as the use of hard disk. The representative model is sf-2281, which is used on SSDS including Intel, Kingston, ADATA and other brands.

The master control of Samsung is only used on their own SSD and are very powerful in performance, which is not worse than Marvell. Currently, the master control of Samsung has been developed to the fifth generation of MEX, mainly used in the Samsung 840EVO.

After Indilinx was purchased by OCZ, it was mainly used in the SSD of OCZ, which had a good comprehensive reading and writing performance. And its typical model is Barefoot 3.

Firmware algorithm:

The firmware of the SSD is the most important component to ensure SSD performance, which is used to drive the controller. The master controller will use the control program in the firmware algorithm of SSD to perform automatic signal processing, loss balancing, error correction code (ECC), bad block management, garbage collection algorithm, communication with host equipment (such as computer), and data encryption. Since the firmware redundancy is stored in NAND flash, when the SSD manufacturer issues an update, it is necessary to manually update the firmware to improve and extend the function of the SSD.

Developing high-quality firmware requires not only sophisticated engineering, but also perfect integration of NAND flash, controller, and the rest of SSD components. In addition, the most advanced technologies in the areas of NADN characteristics, semiconductor technology and controller characteristics must be mastered. The better the quality of the firmware, the more accurate and efficient the whole SSD. Currently, not many SSD manufacturers have independent research and development of firmware, only Intel/ crucial/Plextor/OCZ/ Samsung and some manufacturers.

NAND flash memory：

The data of SSD users is stored in NAND flash memory, which is the storage medium of SSD. The main cost of SSD is concentrated on NAND flash memory. NAND flash memory determines not only the lifetime of the SSD, but also its performance. The particle structure of NAND flash memory and its working principles are very complex, and we'll continue to publish a series of articles, which focuses on flash memory. Here, we mainly have a basic understanding of SLC, MLC and TLC flash memory, which customers usually purchase.

SLC/MLC/TLC flash memory：

The kinds of flash memory state

In the past few years, the technology of NAND flash memory has been developing rapidly. In a short time, we have seen significant progress in NAND technology, from the enterprise-level SLC flash to the consumption-level MLC flash, to the current emerging TLC flash. How