Wednesday, Intel announced it's joining Toshiba in the PLC (Penta-Level Cell, meaning 5 bits stored per individual NAND cell) club. Intel has not yet commercialized the technology, so you can't go and buy a PLC SSD yet—but we can expect the technology will lead eventually to higher-capacity and cheaper solid state drives.
To understand how and why this works, we need to go over a little bit of SSD design history. One of the most basic architectural features of a solid state disk is how many bits can be stored in each individual NAND cell. The simplest and most robust design is SLC—Single Layer Cell—in which each floating-gate NAND cell is either charged or not, representing a 1 or a 0. SLC flash can be written at very high speed and typically survives several times more write cycles than more complex designs can. (Endurance levels are specified per drive, but National Instruments uses 100K, 20K, and 3K as sample program/erase cycle endurance levels for SLC, eMLC, and MLC drives here.)
Although SLC flash is high performance, high endurance, and high reliability, it's also extremely expensive to manufacture. SSDs didn't hit the consumer market until MLC—Multi-Layer Cell—flash became widely available. Naturally, the storage industry being what it is, they confused things from here. These are the industry terms for the various NAND storage levels:
- SLC—Single Layer Cell. One bit stored per cell. Typically only found in small cache layers, or extremely high-performance enterprise SSDs.
- MLC—Multi Layer Cell. In the real world, this refers specifically to two bits per cell. Examples include early consumer drives such as Intel X-25M and modern high-performance drives such as Samsung 860 Pro.
- eMLC—enterprise Multi Layer Cell. This is, effectively, just MLC with write speeds throttled in order to reduce error rates. Still only two bits stored per cell.
- TLC—Triple Layer Cell. Three bits stored per cell. Most modern consumer drives, such as Samsung 860 EVO and Western Digital Blue, are TLC drives.
- QLC—Quadruple Layer Cell. Four bits stored per cell. Used by a few high-capacity, low-cost consumer SSDs such as Samsung's 860 QVO and Intel's 660P.
- PLC—Penta Layer Cell, because an acronym for "quintuple" would have collided with 4-bit QLC. Five bits stored per cell. This is new technology that Intel and Toshiba have debuted this quarter.
This slide from last year claims that Intel's floating gate cells have a wider read window—and therefore more accurate results that require fewer retries—than the typical charge trap cells used by most manufacturers. Intel Corporation This slide mostly seems to be making the point that Intel wants to help you cram as many bits of data as possible into each physical server rack. Intel Corporation As both bit density per cell and cell density per cubic centimeter increase, the amount of space required to store each terabyte of data decreases. Intel Corporation
Intel also differentiates itself from competitors by sticking with the floating-gate cell design used in early SLC devices, instead of the less expensive charge-trap design the rest of the industry has shifted to. It's unclear to casual researchers which technology is actually better from a technical perspective, but Intel argues that the floating gates can be manufactured at a higher density, meaning it can pack more cells into the same physical area.
Unfortunately, while PLC SSDs will likely be bigger and cheaper, they'll probably also be slower. Modern SSDs mostly use TLC storage with a small layer of SLC write cache. As long as you don't write too much data too fast, your SSD writes will seem as blazingly fast as your reads—for example, Samsung's consumer drives are rRead More – Source
