Since NAND flash storage was first introduced into enterprise computing, we’ve seen a rapid explosion in the types and capabilities of flash products that can now be deployed in servers, HCI solutions and storage arrays. QLC is the next evolution of cost reducing, space increasing flash technology. What is it and what can we expect it to deliver?
Flash storage works by storing an electrical charge in a cell and using the presence of this charge to determine whether the cell is storing a “0” or a “1”. Flash devices lay down billions of cells on a silicon substrate that can then be used to store gigabytes and now terabytes of information. The original flash design stored a single bit in one cell, otherwise known as SLC or Single Level Cell technology. Flash developers soon worked out that a cell could store multiple states by having a range of voltages in each cell and so MLC was developed, with each cell storing four states and therefore capable of recording two bits of binary data – 00, 01, 10 and 11. TLC (Triple Level Cell) extends this to eight states and values from 000 to 111. QLC goes another step further, doubling the states and adding the ability to store an additional bit. Now we can represent values from 0000 to 1111.
There are a number of challenges that arise with increasing the bit density per cell. First, writing flash is a destructive process, with the integrity of each cell being slightly damaged when it is written. This means that flash has a finite lifetime for writes – a feature called endurance and measured in the number of P/E (program/erase) cycles that can be performed on the NAND flash itself. Cells are programmed in blocks, rather than individually, requiring some clever algorithms and software to manage updates – one reason why flash is overprovisioned. P/E cycles for SLC are around 100,000, MLC around 10,000 and TLC around 1,000. Each generation results in an order of magnitude worse endurance. For QLC we’ve yet to see whether the P/E cycle will be in the 100 range or whether manufacturers have managed to perform some magic and make the technology more reliable.
Second, as more data is stored in each cell, the contents must be read before writing, because the change to a single bit requires knowing what value was already present. As the technology has evolved, writing to SLC, MLC and TLC has become progressively slower. QLC may be even worse than previous generations, which has an impact on latency more than throughput.
It’s a simple question, why develop another, less reliable media? Surely what we have is good enough? Like any storage technology, there’s a desire to condense more data into a smaller space at a lower cost. Look at the improvements the hard drive manufacturers have been making for years. The same applies to flash. The move from TLC to QLC gains 1/3 more capacity for the same number of cells, allowing for greater densities and larger capacity drives. The industry is driving down cost and increasing capacity every year with a range of technologies, many of which work together. TLC, for example, has been combined with 3D-NAND and we’re likely to see the same for QLC.
The storage industry has also built diversification into storage media. In the HDD market, we have performance and capacity drives, small format (2.5″ and 1.8″) and a range of cheaper solutions that use lower specification interfaces like SATA. The same applies to flash. At the technology has developed, we have seen a divergence of offerings covering high and low endurance, high and low performance and of course ranges in capacity. With each of these, there’s a cost profile that matches the requirements to the price. As an example, look at the latest Nytro 3000 drives from Seagate. There are 43 models with varying capacity, endurance and features such as encryption.
Obviously, the only issue with QLC will be significantly reduced endurance and that may be reflected in how the technology is used.
Who’s Doing QLC?
Both Toshiba and Western Digital have already announced the (see related links below) development of QLC, based on 64-layer 3D-NAND using their BiCS3 technology (BiCS is Bit Cost Scaling, Toshiba’s name for 3D-NAND). This promises 768Gb (96GB) die capacity, which translates to 1.5TB in a chip when combining 16 die into a single package. The manufacturing process for these chips has likely been reduced from the 1Xnm process seen in 2D scalar NAND to something more like 40nm, in order to get reliability. We can then expect to see this shrink to increase capacities and with BiCS4 an increase to 96 layers.
Samsung has pre-announced a 1Tb die with the potential to turn their QLC product into a 128TB drive, stacking 32 die per package. Further details seem scarce, however hopefully we will start to see more details on performance and endurance. SK Hynix doesn’t appear to be jumping on QLC just yet, while Micron is apparently developing QLC using 64 layer technology (link) but I can’t find any more specific details at the moment.
The Architect’s View
The range of NAND flash products is overwhelming as the industry races to produce ever faster, cheaper and more reliable products. With such as range of flash to choose from, we’re going to see the emergence of more “hybrid all-flash” platforms that will once again take us into the tiering scenarios of previous shared storage. What will be interesting to see is the developments that will be made by having high endurance flash as the cache layer and QLC as the capacity layer. SDS solutions like VMware Virtual SAN already do this today, but perhaps at not a granular enough level that we could start seeing in shared storage platforms.
While capacity hard drives will continue for some time yet, QLC flash looks to squeeze out 10K drives, leaving 7.2K RPM bulk media as the main product line for the HDD manufacturers. Existing 15K & 10K drives will probably become niche products, reserved for price-conscious solutions where flash is still too expensive.
What’s next after QLC? PLC perhaps (pentuple – quintuple would be confusing)? I suspect we’ve a way to go with the technology we have, so let’s see how TLC and QLC pan out first, rather than getting too far ahead of ourselves!
- Western Digital Announces Four-Bits-Per-Cell (X4) Technology on 3D NAND (WDC press release, published 24 July 2017)
- Toshiba Develops World’s First QLC BiCS FLASH 3D Memory with 4-Bit-Per-Cell Technology (Toshiba press release, published 27 June 2017)
- Samsung’s 10-Year Plan Starts with 128TB QLC SSD, 960 Successor (Tom’s Hardware, published 8 August 2017)
- What Micron Technology Aims to Achieve by Introducing QLC 3D NAND (Market Realist, published 22 March 2017)
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