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NAND Flash Memory

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NAND is the most popular type of flash storage memory for USB flash drives, memory cards, and SSDs. It is used in some of the best SSDs in the market today.

This flash memory technology is non-volatile chip-based storage, and unlike DRAM does not require a persistent power source. NAND cell arrays store 1, 2, 3, or 4 bits of data. When the NAND SSD or card is detached from a power source, metal-oxide semiconductors called floating-gate transistors (FGT) provide electrical charges to the memory cells, and data remains intact.

It stores data in memory cell arrays that are defined by transistors. Each transistor has two gates instead of one, like an electrical switch where the current flows between two points. A floating gate and a control gate control the energy flow in a flash memory cell. The control gate captures electrons and moves them as needed into the floating gate.

NAND flash development concentrates on reducing the size of the chips while maintaining or increasing their capacity. This reduces bit costs and increases density, so NAND SSDs can successfully compete with HDD’s. Another feature is connecting cells in series of FTGs, which takes less takes less space than parallel connections and further reduces NAND flash costs.

The I/O is typically at 1 Gb per second or lower. 3D NAND I/O speed is hitting 1.4 GB per second, and newer technologies are pushing NAND I/O speeds to 3.0 GB per second.

Types of NAND

The most common types of NAND are between cells containing 1, 2, or 3 bits a cell. We call these SLC, MLC, and TLC. 3D NAND is also gaining ground and high-performance, high density environments.

  • SLC: Single-Level Cell stores 1 bit in each cell. The architecture delivers the highest endurance, performance, and data integrity of the bunch; and is also the most expensive. Although MLC and TLC are perfectly adequate for many enterprise workloads, consider SLC for mission-critical applications like high transactional databases.
  • MLC: Multi-Level Cell stores 2 bits per cell. This lowers the price-per-capacity over SLC. But because erasures and writes occur twice as often per cell then in 1-bit-per-cell SLC, endurance is lower. MLC is most often found in laptops and PCs, but provide acceptable performance for applications that do not have intensive I/O requirements.
  • eMLC: Enterprise Multi-Level Cell increases MLC endurance. eMLC drives are more expensive than MLC but less expensive than SLC. It still stores 2 bits per cell, but comes with an intelligent controller that manages wear leveling and data placement. This extends its useful life.
  • TLC: Triple-Level Cell stores – you guessed it -- 3 bits per cell. It’s positioned for consumer grade technology, because it is the lowest performing of the three level cell types, and is correspondingly less expensive. However, advances in 3D NAND and sophisticated controllers are positioning TLC to perform in read-heavy enterprise applications.
  • QLC: Quad-Level Cells store 4 bits per cell. However, increasing density by storing more bits per cell has serious disadvantages. The more bits per cell, the more often writes and erasures occur in the cell, which decreases endurance. Endurance rates are rising with wear leveling features included in flash controllers. QLC’s initial P/E (program/erase) cycles were around 100. Today that number has risen to 1000 for QLC. (Typical TLC P/E’s are 3000.) However, voltage is also an issue in QLCs, since voltage changes cause instability in surrounding cells. There are ways to overcome the issue, but the extra technology decreases performance and raises prices.
  • 3D NAND: Flash manufacturers are on a mission to decrease cell sizes in order to pack more chips and thus more capacity on a NAND device. However, shrinking cells using the above cell level technologies resulted in cell to cell interference, which reduced data integrity in NAND flash. Samsung turned its attention to developing multilayered flash, which increases capacity and keeps costs low by vertically stacking memory cells in multiple layers. Most of the market refers to the technology as 3D NAND, and Samsung calls its eDiscovery product line Vertical NAND or V-NAND. With the move from 2 dimensions to 3, 3D NAND devices achieve higher density and lower power consumption, faster reads and writes, and increased endurance.

NAND vs. NOR

NOR and NAND are the two types of flash memory. Both NOR and NAND flash memory are embedded in small electronics like cameras and smartphones, but only NAND is cost-effective and dense enough to serve in flash-based enterprise storage environments.

Neither is a new technology. Toshiba developed NOR and NAND flash memory in the early 1980s, and named them after their respective logic gates. (Truth tables display each logic gate’s function.) NOR is named after NOT-OR, and NAND after NOT-AND. Both types of flash memory store data in memory cells that are created from floating gate transistors (FTG).

  • Deployment. NAND is much more widely deployed than NOR. The exception is in mission- and business-critical enterprise applications. NOR reads faster than NAND, and has considerably greater endurance. It is, however, slower at writing and erasure, and is significantly more expensive.
  • Memory cell connections. NOR flash connects each memory cell to the source line and the other end to a bit line, which allows multiple cells to simultaneously connect to a bit line. Since the cells are connected in parallel, the system can write and read to individual memory cells. NAND flash connects multiple memory cells, usually 8 cells at a time, to create series of cells. Serial connections take less room, but do not allow direct writes to individual memory cells.
  • Read performance. NOR allows the system to write a single machine word even to an erased location, and reads faster than NAND because NOR allows random access to any memory address. This allows the system to read bytes independently of pages and blocks. NAND reads are slower since it supports page and block access, not random access. NAND reads data in blocks, with typical block sizes of hundreds and thousands of bits. This makes NAND flash more like data storage devices then ROM-type memory, since it does not provide random access to data that many applications require.
  • Write and erasure performance. Writes and erasures flip speeds: they are faster in NAND with its smaller cell sizes, and slower on NOR’s larger cells.
  • Endurance. NOR has higher endurance than NAND. NOR cells have 100% known good blocks over the life of the cell. NAND cells typically have 98% good bits when shipped, and end-users know to expect additional bit failures over the cell’s lifetime. NAND manufacturers usually add error correcting code (ECC), which NOR does not need.
  • Density. NOR has only 40% of the density of equivalent NAND flash memory. NOR densities range from 64MB to 2Gb, while NAND ranges between 1Gb to 16Gb.

NAND flash

NAND is capable of higher performance than NOR flash.

NAND Shortage and Pricing

The NAND marketplace is experiencing a challenging cycle of supply and demand. This is not new for this market: NAND flash pricing tends to raise and lower in 2-year cycles.

In 2016 and 2017 (see below), the NAND market experienced a flash chip shortage. The two main factors were rising demand for NAND flash products, and the development of 3D NAND, which uses more chips per device.

SSD prices did not significantly rise. This was likely because manufacturers were only too aware that growing NAND sales were directly related to lowering the price gap between SSDs and hard drives. Accordingly, manufacturers and resellers did not raise prices by a great deal, despite the shortage. Most prices stayed flat or only rose about 6% a quarter.

Prices began stabilizing in 2018 as supplies loosened up. However, by the 4th quarter of 2018, NAND prices dropped steeply thanks to higher manufacturing rates, increased demand for 3D NAND over 2D NAND, and lower sales of new iPhones. In Q1 2019, analysts are generally forecasting a 10% drop in price.

3D NAND is the game changer, since it encompasses SLC, TLC, MLC, and QLC. The deployed base of existing NAND flash is not going anywhere soon, but we are seeing demand steadily rise for 64-layer 3D NAND. If you are undergoing a technology refresh, or simply adding more NAND flash into your data center, look to NAND and 3D NAND for fast processing and growing capacity in a small flash footprint.

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