Composition of SSD Leave a comment

The key components of SSD are composed of controller and storage unit. In addition, there are cache and host interfaces.

controller of SSD

Each SSD has a controller that connects the storage unit to the computer. The controller is an embedded processor that executes firmware code. The main functions are as follows:
 error checking and correction (ECC)
 wear leveling
 bad block mapping
 read disturbmanagement
 cache control
 waste recycling
 encryption
Obviously, the controller is the brain of SSD, and the quality of firmware represents its IQ.

Storage unit of SSD

Although some manufacturers have launched products based on higher speed DRAM memory, NAND flash memory is still the most common and occupies an absolutely dominant position. Low end products generally use MLC (multi-level cell) or even TLC (triple level cell) flash memory, which is characterized by large capacity, slow speed, low reliability, low access times and low price. High end products generally use SLC (single level cell) flash memory, which is characterized by mature technology, small capacity, fast speed, high reliability, high access times and high price. But in fact, depending on the internal architecture design of different products, the differences in speed and reliability can also be compensated or even reversed through various technologies.

cache of SSD

NAND flash based SSDs usually have a DRAM based cache, which is similar to ordinary mechanical hard disks, but also stores other data such as wear level data. The data is buffered in DRAM and then written in a centralized manner to reduce the number of writes. One of the special cases is the controller produced by sandforce. It does not contain cache, but its performance is still very good. Because of its simple structure, it can produce smaller SSDs, and the data is more secure when power is off.

Host interface of SSD

The host interface is closely related to the controller, but it is generally not different from the traditional mechanical hard disk, mainly including the following:
Fibre Channel
The speed of the host interface limits the peak speed that SSDs can achieve. But generally this is not the bottleneck.

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