SCM: NEW-GENERATION STORAGE MEDIA

SCM Definition

Storage Class Memory (SCM) is a new storage medium popular in the industry. Also known as Persistent Memory (PM) or Non- (NVM), SCM has the characteristics of both storage and memory, fast and byte-level access like memory and persistent like traditional storage. The access latency of SCM is less than 1 µs, which is faster than that of NAND flash by two to three orders of magnitude. Further, compared with NAND flash, SCM boasts simple operations that do not require sequential write and erase before write. SCM also outperforms NAND flash in longevity and retention capabilities. Perfectly designed to provide higher performance and reliability, it is no wonder why the industry believes that SCM will be a paradigm shift from conventional storage.

SCM Status

At present, there are a wide variety of SCMs being developed, with the following four types predominantly used in the industry: PCM, ReRAM, MRAM, and NRAM.

Phase-change memory (PCM) employs the electrical conductivity difference of special alloy materials in the crystalline phase and the amorphous phase to represent the two values in a binary system. It provides large capacity with a simple structure at low costs; however, it is sensitive to high temperatures and susceptible to damage in extreme conditions. PCM can be used in the acceleration and large memory scenarios. Because PCM is not as good as DRAM in terms of longevity, the system design of PCM needs to be optimized to prevent wear-out. A common optimization is pairing PCM with DRAM to form a tiered large-capacity memory resource pool. The industry believes that 3D Xpoint jointly developed by and Micron is one of the typical PCM applications in SCM. According to the current technology and market trend, 3D Xpoint is the most prospective SCM in large-scale commercial use in the next three years.

Resistive random-access memory (ReRAM)is a form of non-volatile storage that operates by using different voltages on the anode and cathode to change the resistance of a cell (storage unit) based on the formation and melting of internal conductive wires. Different resistance levels indicate different statuses, which maximizes the theoretical capacity density and minimize the cost. However, a criticism of this type of SCM is that the longevity is short and the read/write performance is unsatisfactory, partly due to the fact ReRAM is considered to be at a very early maturity level. Consequently, ReRAM is mainly used in high-speed scenarios. The representative vendors of ReRAM are HPE and Crossbar, however ReRAM is still considered to be at a very early maturity level. Meantime, ReRAM is one of the base materials of Neuron Chip which is very hot in research area.

Magnetic Random Access Memory (MRAM) is a non-volatile random-access memory of storing data bits by changing the electron spin direction using the magnetic field. Theoretically, MRAM features high performance and longevity, suitable for operations in high-speed cache on the CPU side, for example, L2 cache and LL cache. Despite that the theoretical research is mature, the engineering design is still at an early stage, and the actual capacity density is low. Representative vendor is Everspin.

SCM: NEW-GENERATION STORAGE MEDIA / 1 Nantero's CNT Random Access Memory (NRAM) is a type of non-volatile random access memory that uses carbon nanotubes to control circuit connectivity to indicate different data status. Carbon nanotubes are very small but extremely durable. Theoretically, NRAM can be scaled down to below 5 nm. With outstanding capacity density and longevity and low power consumption, NRAM can be used to replace static random-access memory (SRAM). NRAM is a proprietary technology from the company Nantero and is now processed by other storage chip vendors with the authorization of Nantero. There is still a long way to go to commercialize NRAM products.

Type PCM RRAM MRAM NRAM

Working principles

Latency 100 ns 10 µs 35 ns 20 ns

Endurance 1E7 1E5 1E12 1E9

2 weeks@85°C Retention 20 years@85°C 20 years@85°C 10 years@85°C 5 years@40°C

Cost ☆☆ ☆☆☆ ☆ ☆☆

Capacity 128 Gb 1 Tb 1 Gb 16 Gb density Micron/Intel HP, Crossbar, WD, Samsung (Grandis), Nantero, Fujitsu and Major (Numonyx), Samsung, Hynix, Everspin, Hynix, other vendors vendors IBM/Hynix, Fujitsu, Adesto, and Micron/Hitachi, and authorized by Samsung, and WD Rambµs Qualcomm Nantero

Table 1-1 SCM types and technical features

Considering the requirements on capacity density, storage system costs, and the technology maturity, Huawei focuses on 3D Xpoint and carries out related research and product planning. Nevertheless, Huawei keeps a close eye on the development trends of ReRAM and other SCMs.

SCM Product Forms and Application in a Storage System

High-Performance Block Level Access SSD The SCM SSD, which uses NVMe block interface and is compatible with the native architecture, is the primary application form of SCM. Optane P4800X series launched by Intel is an example. This product has little impact on the system architecture but provides better performance than that of flash SSDs. In addition, SCM SSDs do not require garbage collection, which prevents performance deterioration like NAND SSD after long-time running, and curbs latency at an appropriate level. This type of storage product is increasingly springing up in the industry, and Huawei is also working on the R&D of such products. High-performance SSDs provide the following application forms in a storage system:

SCM: NEW-GENERATION STORAGE MEDIA / 2 • cache: As metadata cache of AFA, SCM SSDs work with the DRAM to build a memory + SCM SSD two-tier cache, which avoids bottlenecks in memory capacity and supports larger user capacity with stable performance.

• Data cache: SCM SSDs serve as the acceleration layer of user data, improving performance in typical application scenarios.

• Main storage: SCM SSDs serve as the storage layer of user data and provide a high-performance storage system to meet the performance requirements of certain scenarios.

Byte-level Access Persistent DIMM The secondary application form of SCM is non-volatile memory that translates file-level I/O semantics into memory level (load, store) semantics for the storage medium in the form of dual in-line memory module (DIMM). This form provides similar access latency (100 ns-level) to memory but larger capacity with persistent capability. It lays the foundation for future storage systems that feature ultra-high-performance. To make good use of this form, the software and hardware architecture of the existing computer system needs to undergo great changes. The industry is exploring all possibilities of non-volatile-memory-based data structure, transaction technologies, hardware architecture, and programming tools. For example, the current inter-CPU memory access is bottlenecked by network latency, leaving the SCM not fully exploited. The latency of even the fastest network access is more than 3 µs, far beyond the latency of SCM. Therefore, Huawei and other IT giants introduce the Gen-Z standard, allowing the SCM to connect to the Gen-Z bus using the independent Gen-Z interface. Each CPU can access the shared SCM with nanosecond-level latency to leverage the SCM advantages.

CPU CPU Computer Accelerators DRAM DRAM DRAM DRAM SOC SOC FPGA FPGA FPGA FPGA Memory Memory Memory Memory MCU DDR DRAM MCU DDR DRAM

Gen-Z Direct Attach, Switched, or Fabric Topology PCIE PCIE

Network Storage Storage Network >3µs Storage Network Pooled Memory I/O I/O I/O

Figure 1-2 Current memory access network Figure 1-3 Gen-Z: New-generation memory access network (Source: Gen-Z Consortium)

Various technologies required by DIMM are still under exploration. Huawei has corresponding frontier research programs in progress and is ready to apply the research results to its storage systems once the technology matures.

Summary

With unique design principles, SCM narrows the latency chasm between I/O devices and memory, and makes memory more persistent than ever. It will definitely disrupt the existing computer architecture and application methods which will ignite innovation enthusiasm in the IT industry. Huawei pays close attention to the SCM industrialization progress and will leverage the new technology to provide customers with innovative storage products.

SCM: NEW-GENERATION STORAGE MEDIA / 3 For More Information To learn more about Huawei storage, please contact the local office or visit http://e.huawei.com/en/products/cloud-computing-dc/storage For Huawei Enterprise Website: please visit http://e.huawei.com

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