Tag Archives: SLC

What are Flash SSDs?

Earlier we used traditional hard disk drives in our computers. These were mechanically spinning magnetic disks with read-write heads. Nowadays, we use SSD or Solid-State Drives that have no moving parts. SSDs can retain data once it is saved without power, as they use NAND flash memory. To increase the data density, the NAND chips are multilayered. That means they can hold upwards of one bit of information per cell. SSDs using multilayered chips are named single-, multi-, triple-, quad-, and penta-level SSDs, according to the number of bits each cell can hold.

Multilayered SSDs have their own advantages and shortcomings and can range from speed to price to reliability. For instance, SLC or Single Level Cells have a lifespan measured in program/erase cycles of about 50,000 to 100,000 and can withstand high-intensity write operations.

MLCs or multi-level cells with two bits per cell can expect a lifespan of about 10,000 cycles and are mostly suitable for enterprise data centers.

TLCs or triple-level cells with three bits per cell can expect a lifespan of about 3,000 cycles and are useful for digital consumer products.

QLCs or quad-level cells with four bits per cell can expect a lifespan of about 2,000 cycles and are suitable for read-heavy operations, streaming media, and content delivery applications.

No data is available for the lifespan of PLCs or penta-level cells with five bits per cell. These SSDs are suitable for long-term storage of data such as in data archives.

Flash SSDs have revolutionized the storage of enterprise data in all its forms. They have enabled faster boot times and the application starts on PCs and mobile devices. They have facilitated the blistering performance of storage arrays in workloads like business analytics. In most performance metrics, flash SSDs have far outshone the older hard disk drives.

Speed aside, flash SSDs offer additional benefits. They are far more durable while being less susceptible to damage from abrupt physical shocks and movements, as compared to the traditional HDDs. Additionally, they use much less power to operate. Even though they cost more than the HDDs per gigabyte, the improved performance of SSDs overcomes their higher expense for most applications.

Flash SSDs store data in their memory cells using a technique called FGT or Floating Gate Metal Oxide Field Effect Transistors that can store a binary 0 or 1. With two gates, each FGT behaves like an electrical switch with current flowing between two points. NAND flash is named so as it uses NOT-AND logic gates. Power is not necessary to retain data in the flash cells, as, in the absence of power, the FGT provides the electrical charge for maintaining the data intact in the memory cells.

Flash SSDs are solid state, meaning they have no mechanical part to wear out. However, SSDs can nonetheless fail. One measure of SSDs is the lifespan or the number of program/erase cycles that a drive can complete before its degradation and failure. This is overcome by using wear-leveling technology, whereby the life of the SSD is prolonged by evenly distributing the program/erase cycle across the total NAND cells in the drive.