New semiconductor memories are quite rare. Today, the semiconductor industry relies on three venerable workhorse technologies: SRAM, DRAM, and NAND. The first two are around fifty years old, while non-volatile NAND (and NOR) flash was invented three decades ago. After around a decade in development, Intel and Micron first announced 3DXP memory in 2015. While this is a new type of memory, the research dates back to the early 2000’s. It appears that 3DXP uses an arsenic doped Se-Ge-Si ovonic threshold switch with a bit-cell formed from a chalcogenide glass (Ge-Sb-Te). The 3DXP memory relies on many techniques and materials pioneered by Numonyx, an joint venture by Intel and STMicro that was acquired by Micron, but the implementation is likely different. Intel and Micron claim that the new type of memory offers attributes of both DRAM and NAND. From a system standpoint, 3DXP is byte-addressable (like DRAM) and non-volatile (like NAND). From a performance standpoint, 3DXP offers worse latency than DRAM, but far better density; it can be vertically stacked (similar to NAND, but with different economics), and has better endurance than NAND.
The first product to use 3DXP was Intel’s P4800X family of SSDs, which offers 375GB or 750GB of 3DXP memory attached via a 4-lane PCIe 3.0 link. The 375GB SSD comprises a proprietary controller that drives seven channels of four 128Gbit 3DXP chips, with about 20% of the physical capacity reserved for ECC, meta-data, and overprovisioning. The P4800X delivers 10X lower latency than comparable NAND-based SSDs and 5-8X better throughput at low queue depths. The endurance of the initial P4800X is 30 drive-writes per day (DWPD), roughly 2X better than NAND-based SSDs. Interestingly, Intel announced that the latest version of the P4800x is rated for 60 DWPD – a 2X improvement over the course of roughly a year. 3DXP SSDs are the fastest currently available storage and initial customers are using 3DXP SSDs for hot storage. For example, Alibaba’s PolarDB uses storage nodes combining five SSDs and a single 3DXP SSD. However, SSDs are merely a stepping stone. The value of 3DXP is masked by the system architecture, specifically the PCIe link and the block-based software interface.
For 3DXP to become a viable complement to DRAM and NAND, it requires a different system and software architecture, which Intel recently announced. As widely expected, Intel will offer 3DXP-based DIMMs (previously codenamed Apache Pass) that use a low-latency DDR4 memory interface, rather than PCIe. The first DIMMs will be available in 128GB, 256GB, and 512GB capacities and work with the next-generation Cascade Lake server processor. Intel has prepared a torturously long marketing name, and for the sake of brevity, we will simply refer to these products as 3DXP DIMMs.