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SLC vs MLC vs TLC NAND Flash

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The underlying NAND architecture of SSDs can vary from model to model and this article will explore those differences. Each of the various types of NAND flash – SLC, MLC, eMLC and TLC – have different characteristics that influence which is best for your business storage.

Flash – in all its forms – has revolutionized enterprise data storage. Compared to HDDs, SSDs packing enable storage subsystems and arrays to provide blistering application performance and make short work out of business analytics and other workloads. In PCs and mobile devices, flash memory enables brisk boot times, snappy application start times and fast data transfers.

And since flash storage lacks moving parts, SSDs aren't nearly as susceptible to damage from abrupt movements and physical shocks. They also consume less electricity than HDDs, another bonus that works in their favor.

One downside is that SSDs generally cost more than HDDs on a per-gigabyte (GB) basis, although deduplicaton, compression and other data management techniques can help organizations make the most out of their flash storage investments.

The results of the Enterprise Storage Forum survey, Data Storage Trends 2018, provides insight into flash in the enterprise.

Now, let's dive deeper into NAND flash.

Types of NAND Flash

NAND flash is a type of non-volatile storage architecture used in SSDs and memory cards. It gets its name from the type of the logic gate (NOT-AND) used to determine how digital information is stored in a flash device's chips.

  • SLC

    Single-Level Cell SSDs store one bit in each cell, a design that yields enhanced endurance, accuracy and performance. For critical enterprise applications and storage services, SLC is the go-to flash technology. Of course, it carries the highest price tag.

  • MLC

    Considered the consumer-grade flavor of flash, Multi-Level Cell architectures can store two bits per cell. Although it may seem like packing more than one bit into a memory cell is a good use of space, it comes at the cost of a lower useful life and decreased reliability. MLC SSDs make it possible to economically add flash storage to PCs and laptops, relatively speaking.

  • eMLC

    Enterprise Multi-Level Cell is a hardier version of MLC NAND flash that somewhat bridges the performance and endurance gap between SLC and MLC. eMLC drives costs more than MLC drives, but much less than SLC their counterparts. Although it still stores two bits per cell, an eMLC drive's controller manages data placement, wear leveling and other storage operations in a way that extends an eMLC SSD's useful life.

  • TLC

    The least expensive of the bunch, Triple-Level Cell NAND flash stores three bits per cell and is typically used in consumer-grade electronics with comparatively low performance and endurance requirements. Best suited for read-heavy applications, TLC-based storage components were rarely, if ever, used in business environments but recent improvements in flash architectures, including 3D NAND (more on that later), and endurance-enhancing data placement and error correction techniques have earned the technology a place in read-intensive enterprise storage applications.

How SLC, MLC, eMLC and TLC Stack Up

Poring over an NAND flash specs? Here's how to tell if a flash drive is suitable for the intended workload or use case, starting with the enterprise side of the spectrum down to consumer end. In the chart below, Program/Erase (PE) cycles are a measure of how many read and write operations NAND flash can support. Although they have no mechanical parts that can wear out, SSD drives can still fail.

Each type of NAND flash has a different lifespan, meaning it can accommodate a finite number of P/E cycles before it begins SSDs to degrade and eventually fail. Apart from a manufacturing flaw, power surge or other catastrophic damage that may spell the end of an SSD, of course. This is a prime consideration for the types of storage workloads and applications that an SSD is intended to support.

 SLCeMLCMLCTLC
Bits per Cell1223
Max Program/Erase (PE) Cycles100,00030,00010,0005,000
Reliability/EnduranceExcellentVery GoodVery GoodOK
Cost$$$$$$$$$$$$$$
Use CaseEnterpriseEnterpriseEnthusiast/ConsumerConsumer

NAND vs 3D NAND

One of the biggest innovations to hit the flash storage market is 3D NAND or Vertical NAND (V-NAND). As the term implies, it uses a stacked architecture to arrange the memory cells within an SSD, instead of the planar or flat arrangement in past implementations.

In effect, this architecture enables vendor to pack more capacity into less physical space at lower cost, compared to 2D NAND. It also yields faster speeds, improved longevity and lower power requirements.

Most major SSD vendors today offer 3D NAND SSDs.

For example, Samsung used the technology to pack 8TB of flash storage in its new NVMe NF1 SSD. The Next-Generation Small Form Factor (NGSFF) component measures just 11 cm by 3.05 cm, enough to fill a 2U server with 576TB of all-flash storage capacity, if money is no object. Micron, Toshiba and Western Digital also produce 3D NAND.

How to Choose Between SLC, MLC and TLC SSDs

As a good rule of thumb, IT buyers looking for the highest performing SSD can expect to shell out much more for SLC than eMLC, MLC and TLC. But price isn't everything.

There are a lot of factors to consider when trying to the most bang for your storage buck. An SLC SSD makes sense for critical application workloads and 24/7 databases that demand fast, reliable performance. Of course, not every workload demands blistering speeds and price/performance considerations must also be taken into account.

Want speedy performance but don't expect to place a continual strain on a storage system? eMLC or even MLC may be the way to go. In a PC, an SSD almost always provides a major performance boost, relative to an HDD.

So, it seems pretty clear cut. SLC for demanding enterprise applications, followed by eMLC, MLC and TLC as performance and reliability requirements are loosened.

Not so fast.

As with practically all facets of IT, NAND flash continues to evolve. Nowadays, even TLC is punching above its weight class.

In a 2016 examination of the high-capacity SSD market, Enterprise Storage Forum noted that "things have moved on in the high capacity SSD space, and the stigma around anything that isn't made of SLC has gone. It's now pretty standard for high capacity SSDs for enterprises to make use of MLC or even TLC (triple level cell) flash."

For all practical purposes, "MLC is arguably just as good as SLC" in enterprise use cases, he concluded, a sentiment shared by Jim Handy, a semiconductor analyst at Objective Analysis. "Today's MLC is better than yesterday's SLC," Handy declared.

Controllers can now use powerful error correction algorithms like Low Density Parity Check (LDPC), which may have been around since the 1960s but lacked the processing power to support them, until now.

QLC NAND Inches into the Mainstream

You guessed it. SSD makers aren't stopping at TLC. Intel, Micron and Western Digital are working on bringing QLC, or Quad-Level Cell NAND technology with four bits per cell into the data center.

In May 2018, Intel and Micron announced the first commercially available QLC 3D NAND chips, delivering significantly more storage capacity than TLC NAND flash. "With introduction of 64-layer 4bits/cell NAND technology, we are achieving 33 percent higher array density compared to TLC, which enables us to produce the first commercially available 1 terabit die in the history of semiconductors," said Scott DeBoer, Executive Vice President of Technology Development at Micron in a May 21 announcement.

Micron is already shipping the first SSD with QLC NAND flash, the 2.5 5210 ION SATA SSD, aimed at enterprise workloads, namely big data processing, real-time analytics and artificial intelligence. It's available in limited quantities, with broader availability in the fall of 2018, in capacities ranging from 1.92TB to 7.68TB. Toshiba and Western Digital are also readying QLC-based SSDs.

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