As flash storage has evolved over the past 10 years, capacity and density have increased, but endurance and performance have both declined.
Pseudo-SLC brings back flash’s pioneer SLC, although the comeback is not a messiah for the masses.
It returns SLC’s performance and long lifetimes, but at a cost – and mostly for exotic applications.
SLC, or single-level cell, was the first generation of flash drives. It works with one bit per cell, and so one 1 or 0. Today’s quad-level cell (QLC) drives store four bits per cell, which makes for 16 possible combinations of 1s and 0s.
That means QLC provides far greater storage density, but there are drawbacks, and hence reasons why manufacturers now sell products that mimic SLC.
Chief among the drawbacks of contemporary flash drives with multiple cell levels is that they are less speedy in performance terms, less reliable and have a shorter lifespan.
SLC avoids all those because, with comparatively little activity in cells, and less room for issues related to multiple voltage signals in a very small area, it brings high performance and long life.
What is pseudo-SLC?
Pseudo-SLC drives take multi-level cell (MLC), triple-level (TLC) and QLC flash dies and build drives with on-board firmware to mimic SLC by only using one voltage level per cell.
That removes some of the disadvantages associated with multiple voltages in a single cell found in more modern flash drive types.
What use cases is pseudo-SLC aimed at?
While most contemporary enterprise use cases are well-suited to MLC, TLC and SLC flash storage, there are some edge cases where SLC is desirable.
These can literally be edge computing workloads, where the extra durability is required. Think cases where compute and storage is sited in a remote location where physical maintenance visits may come few and far between, such as internet of things (IoT) applications and military, oil and gas.
Elsewhere, but less often, SLC-type performance may be the requirement, such as for high-end transactional workloads in the enterprise where some pseudo-SLC can help.
What are the drawbacks of pseudo-SLC?
Pseudo-SLC is created by limiting the voltage level-holding capacity of MLC, TLC and QLC drives. Therefore, it provides less capacity and storage density.
It reduces the capacity of an MLC drive by 50%, TLC by 66% and QLC by 75%, for example.
What does pseudo-SLC cost?
Pseudo-SLC costs a lot more per GB than regular flash. A search of UK Amazon for pseudo-SLC drives in February 2024 came up with 29 products that ranged from 5GB to 960GB, with an average per GB of £11-plus.
That compares very unfavourably with MLC, TLC and QLC drive prices at the same time on Amazon UK of around 75p per GB. Pseudo-SLC is therefore set for very specialised use cases.
Who makes pseudo-SLC?
These are some of the companies that sell pseudo-SLC products, mostly aimed at embedded, remote, industrial and IoT workloads:
ATP Electronics sells a line of embedded SSDs with integrated pseudo-SLC. It claims it can exceed equivalent TLC product endurance by 10x.
Hyperstone offers flash memory controllers that support a pseudo-SLC mode to provides manufacturers with design flexibility.
Sabrent has integrated pseudo-SLC caching into all of its flash drives to support a wide range of workloads.
Silicon Power sells industrial pseudo-SLC flash SSDs that claim 30,000 program/erase cycle per lifetime and based on 3D NAND dies.
Smart Modular Technologies offers pseudo-SLC drives with PCIe, M.2 and SATA connectivity and based on MLC or TLC flash.
Swissbit has its EM-30 series, which can be partitioned by the user into several TLC and pSLC segments and is aimed at POS devices, routers, switches, automotive, medical and industrial applications, as well as IoT and boot for embedded applications.