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A pseudo-SLC flash primer: Benefits and drawbacks

Regardless of its use, pseudo-SLC represents a credible option for organizations considering multibit flash over SLC flash. The technology makes MLC and TLC function like SLC.

The use of pseudo-SLC is a growing trend because it uses a multibit architecture to deliver single-bit storage, providing a device that incorporates benefits from both worlds.

The enterprise storage market is brimming with flash drives that come in a variety of shapes and sizes, yet organizations might still lack the storage they need for specific uses. This can be especially true when trying to fill the niche between single-level cell flash, which holds one data bit per memory cell, and chips that squeeze in multiple bits per cell, such as triple-level cell (TLC) flash.

SSDs with SLC flash offer better performance, endurance and reliability than their multibit counterparts. In contrast, multibit storage provides greater densities at a lower cost per gigabyte than SLC storage. Choosing between the two often means sacrificing one set of benefits for another. That's where pseudo-SLC comes in.

What is pseudo-SLC?

In some situations, such as industrial settings, organizations might require storage that delivers SLC capabilities, but they can't easily accommodate the high price tag or capacity limitations. Their only options are to make SLC work no matter what the cost, settle on a multibit drive despite performance and life span concerns, or turn to pseudo-SLC.

Pseudo-SLC attempts to provide a middle ground between SLC and multibit drives. It starts with a multibit drive, such as a standard TLC drive, but stores only one bit per cell, rather than the usual two, three or four bits. In this way, the drive can handle greater capacities than SLC flash and costs less per gigabyte. It also delivers better performance, endurance and reliability than the typical multibit drive.

Pseudo-SLC is not a new concept. It was first introduced with planar multilevel cell (MLC) drives, long before flash took off. The advent of 3D technologies, however, combined with TLC and quad-level cell (QLC) structures, has given a big boost to the pseudo-SLC concept. Pseudo-SLC can be implemented on any multibit flash without needing to change the cell architecture or the manufacturing process. Instead, the changes are within the controller's firmware, which is programmed to accommodate single-bit cells.

Chart of NAND flash evolution

Psuedo-SLC advantages and disadvantages

Admins who consider pseudo-SLC should carefully evaluate the pros and cons, taking into account how they will implement the drive. Pseudo-SLC offers three primary benefits:

  1. It delivers greater performance than a multibit drive because it doesn't incur the read/write overhead that comes with multiple bits per cell. The more bits in a cell, the longer it takes to read/write data.
  2. Pseudo-SLC flash contains only one bit per cell, so the cells are not damaged as quickly as multibit cells and are less prone to bit errors, resulting in greater endurance and reliability.
  3. TLC and QLC flash drives are achieving higher densities than ever, especially with the advent of 3D NAND. Pseudo-SLC can take advantage of this density to deliver greater capacities at a lower cost per gigabyte than what is possible with regular SLC drives.

The technology also has challenges.

Pseudo-SLC attempts to provide a middle ground between SLC and multibit drives.

Pseudo-SLC has more capacity than standard SLC but not as much as a multibit drive. For example, it reduces the available capacity of a TLC drive by 66.6%, which means the pseudo-SLC drive has a higher per-gigabyte cost.

Although pseudo-SLC drives can provide excellent performance, endurance and reliability, those benchmarks are still not as strong as in SLC. A pseudo-SLC drive is not built with the same cell and device architecture as a native SLC drive. As a result, the pseudo-SLC drive might be susceptible to some of the same issues that come with a multibit drive. For example, multibit cells are smaller and squeezed closer together, which can accelerate cell erosion and bit error rates. With pseudo-SLC, only the controller and firmware change, not the cells themselves. They still adhere to a multibit architecture, even if they store only one bit.

Top uses

Pseudo-SLC is rarely used for regular enterprise applications. It can be a good fit in other places, however, especially as an alternative to SLC. In some cases, it could prove a better option than either a pricier SLC drive that holds little data or a multibit drive that doesn't meet performance, endurance and reliability requirements.

In fact, these requirements could mean that an organization has only two viable options: SLC or pseudo-SLC. Costs and capacity are typically what drive a move from SLC to pseudo-SLC, especially if decision-makers believe they can justify tradeoffs in performance, endurance and reliability.

For example, admins could use pseudo-SLC flash in industrial settings, medical equipment or vehicles such as aircraft, ships, automobiles or trucks. They could also use it for smaller SSDs, network or graphics cards, SD memory cards or IoT devices.

Some vendors offer multibit SSDs that include pseudo-SLC partitions to boost write performance or support write-intensive applications. Often, admins can reconfigure these partitions to accommodate specific workloads.

Which vendors offer pseudo-SLC?

A growing number of vendors provide storage components that incorporate pseudo-SLC capabilities, including the following:

  • ATP Electronics sells a line of embedded SSDs with integrated pseudo-SLC. The drives include advanced controller firmware technologies that provide customizable settings to support uses across different industries.
  • Hyperstone offers flash memory controllers for a range of devices. The controllers support a pseudo-SLC mode that provides manufacturers with greater design flexibility.
  • Sabrent has integrated pseudo-SLC caching into all its flash drives to support consumer, prosumer and enterprise workloads. The cache is large and can help absorb sudden workload bursts.
  • Silicon Power sells industrial pseudo-SLC flash SSDs that come with an error-correcting code engine that includes low-density parity check capabilities to maximize endurance.
  • Smart Modular Technologies offers pseudo-SLC drives based on either MLC or TLC flash. Smart Modular implements vendor-specific commands and algorithms within the flash controller firmware to approximate native SLC behavior.
  • Toradex sells a line of embedded MultiMediaCards (eMMCs) that can be set to enhanced mode, which enables pseudo-SLC capabilities. Enhanced mode is available for both planar MLC eMMCs and 3D TLC eMMCs.

Robert Sheldon is a technical consultant and freelance technology writer. He has written numerous books, articles and training materials related to Windows, databases, business intelligence and other areas of technology.

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