During the three-day virtual Flash Memory Summit for 2020, CEO and founder of NEO Semiconductor, Andy Hsu, gave a detailed presentation covering the company's new X-NAND flash architecture that promises to combine the speed of SLC flash with the density and low pricing of QLC flash.
NEO Semiconductor was founded in 2012 in San Jose, CA, and has twenty memory-related patents to its name. The company first revealed its X-NAND in 2018 as a storage solution for the emerging markets of AI and 5G but has now shared the deep-dive details.
X-NAND promises intriguing performance numbers: The company claims it can do random read and write workloads 3x times faster than QLC flash, and beat it by 27x/14x for sequential read and write workloads, respectively (see above). This is achieved with a far smaller die that's roughly 37% the size of a 16-plane design (see below). There is some flexibility here as speed and die size reduction can be balanced as needed. Still, X-NAND offers particularly high levels of parallelism even for smaller form factors, as you would find in a smartphone or an M.2 drive. The company also claims this can be achieved without impacting endurance or cost, all while consuming very little power.
As the NAND market moves to cheaper but slower flash to increase density, for example, from 3-bit TLC to 4-bit QLC, performance and endurance are inherently reduced. Read and write latencies increase, which can reduce sequential write performance. That's especially detrimental for datacenter and NAS applications.
Consumer QLC drives tend to rely heavily on SLC caching, which consists of part of the native flash operating in single-bit mode. Still, enterprise workloads do not allow for sufficient idle time to migrate written data from the SLC buffer to primary QLC storage.
Instead, X-NAND offers a way for the flash to maintain SLC performance throughout by allowing for simultaneous SLC and QLC write modes (see below).
Hsu was quick to point out that higher-density flash is growing at a rapid rate, citing Western Digital’s anticipation that up to 50% of the market would be comprised of QLC by 2024. His goal with X-NAND was to make sure it used a conventional NAND process with no structural changes, no additional manufacturing costs, and fast development with quick sampling as a solution based on current NAND. That strategy is designed to speed up the adoption of QLC, especially for the data center as flash performance would no longer fall drastically behind I/O speeds. Further, X-NAND programming and erase policies are designed to drastically improve endurance over QLC flash (see below).
X-NAND achieves these gains by going from a 16KB page buffer per plane to a 1KB page buffer per plane, but with sixteen times the planes, as one example.
A plane tends to be the smallest unit of interleaving for flash, with one or more planes per flash die. The page buffer holds data in transit, like read or write data, between the bus and the flash. A flash die is divided into planes containing bit lines or strings of cells (see above), so planar division can reduce the bit line's length, which helps boost performance. This technique is further enhanced by shielding between adjacent bit lines to reduce settling time when reading or verifying a program (see below). Write performance is increased because up to sixteen bit lines can be programmed in parallel.
X-NAND has six primary features: multiple bit line writes, multi-plane QLC programming, program suspend, multi-BL reads, single latch QLC reads, and the aforementioned SLC/QLC parallel programming. Depending on the implementation, this can improve program throughput substantially as multiple planes can be used in the programming sequence (see below).
The use of multiple banks allows for simultaneous SLC and QLC programming, ensuring the SLC pages are never full, while data can be programmed to the QLC pages at SLC speed. The program suspect function enables using the internally shared inter-page buffer data lines or I/O bus to minimize additional latency. Reads are improved by having a plane latch the read per bit line, with DRAM-like refreshing of data in a non-destructive way due to high capacitance.
X-NAND can work with any number of existing NAND layouts, which increases its flexibility and ease of conversion (see below). NEO Semiconductor intends for the tech to be cost-effective, fast, and easy to implement with existing designs.
The company says it is especially useful with higher-density flash like QLC because it can leverage the high capacity with a balance of high performance and lower die area while being affordable with good endurance and power consumption. The tech is aimed at embedded devices, AI, and the cloud, including NAS, data center, and edge computing.
