NAND FLASH is a popular non-volatile memory, mainly because of its small size, low power consumption and durability. Although this technology is suitable for modern storage, there are many important characteristics to consider when including it as part of a larger system. These characteristics apply to all types of storage, including durability, density and performance, price per gigabyte, error probability and data retention.
When designing a system using NAND FLASH , it is very important to select an appropriate balance of features. The flash controller must also be flexible enough to make appropriate trade-offs. Choosing the correct flash memory controller is essential to ensure that flash memory meets product requirements.
Overview of
flash memory The flash memory cell consists of a modified field effect transistor (FET) with an additional "floating" gate in the insulating layer between the control gate and the channel. The charge is injected into (or removed from the floating gate) by applying high voltage. This changes the gate voltage required to turn on the transistor, which represents the value stored in the cell. The array of these cells constitutes a block, and the entire memory is composed of multiple blocks.
Weigh the pros and cons The following features are different between SLC, MLC, TLC and QLCNAND FLASH.
Endurance: The number of P/E cycles that a single unit can write before it becomes unreliable. Since the cell storing multiple bits is manufactured using the same production process as the cell storing a single bit, the read margin is small. This makes reading the true voltage level more difficult, resulting in a higher reading error rate.
Density: As more bits are stored in each cell, the overall bit density increases accordingly. Since MLC, TLC and QLC memories are usually manufactured on processes with smaller functions and more modern functions, this can be further improved.
Performance: As more and more levels are stored in the unit, programming complexity also increases. Also when reading, it may be necessary to sample the output multiple times to get the correct data. SLC memory has the simplest programming process, and the voltage levels are far apart, so it is easier to distinguish them. Therefore, SLC usually has faster read and write performance than MLC.
Price per GB: Since QLC flash has the highest density, the price per GB is the lowest. The benefits of increased density may be slightly offset by the need for higher levels of over-provisioning to compensate for reduced durability.
Error probability: If multiple bits are stored in a cell, the possibility of errors when reading the stored value is greater. This increases the original error rate of the memory. This can be compensated by more complex error correction methods, so it does not have to be directly converted to errors seen by the system.
Data retention: The ability of flash memory to maintain the integrity of stored data over time when it is not powered on. Every time a flash memory cell is written (a P/E cycle), the oxide layer of the cell will be slightly degraded, and the ability of the cell to retain data will be reduced.
The following table summarizes these general implications.
NAND FLASH is used as the main part of many types of storage products including SSDs, USB drives and SD cards. In order to meet customer expectations in terms of price, performance and reliability, each of the above features must be weighed.
The selection may also depend on the application or host with which the storage unit is communicating. For example, application programs have different access patterns: some may perform a lot of random reads and writes, and some may rely more on large sequential writes, such as video recording. There are also external factors such as temperature, which can change the behavior of the memory cell. The flash memory controller needs to be able to consider all these factors when managing the data in the memory. The firmware in the Hyperstone flash controller can be fine-tuned for specific use cases.