Intel's high-performance 3D Xpoint (say crosspoint) memory technology promised far-reaching changes for storage, memory, and computer architecture when the company first took the wraps off the technology in mid-2015. Of course, big promises begot high expectations, and despite a hunger for more information, we've heard relatively little about how 3D Xpoint will perform in the intervening time. That all changes today. 3D Xpoint will power Intel's first Optane device, the SSD DC P4800X, and that device demonstrates the wide-ranging potential of Optane to change how we think about the balance between memory and storage in a system.
Optane from the ground up
The SSD DC P4800X launching today packs 375GB of 3D Xpoint storage on a PCIe add-in card form factor. Intel is keeping the physical details of its Xpoint media under wraps, but we do know some of the basics of the stuff now. The first Xpoint dies are being fabricated on a 20-nanometer process and can hold 128 Gbits each. Intel says that because the Xpoint medium itself relies on low-resistance and low-capacitance interconnects in the back end of the device, it can switch a thousand times faster than NAND. Consequently, Xpoint delivers much lower access times and much higher performance.
3D Xpoint also offers much finer access granularity than NAND. Instead of working on pages with many kilobytes of data as NAND does, Intel says the structure of the Xpoint device allows for bit-level access. Practically, that means Xpoint can work with words of data, so it's a natural fit for delivering cache lines like DRAM does. Even though it's not as fast as DRAM, Xpoint can be manufactured with 10 times the density of that memory, giving it NAND-class capacity. Intel also says that chip-for-chip, Xpoint is a thousand times more durable compared to 3D NAND. Of course, Xpoint is non-volatile like NAND, as well.
Xpoint is just one piece of the puzzle for Optane products, however. Intel has developed a custom controller to take advantage of the unique properties of the Xpoint material. For the DC P4800X, the controller talks over PCI Express using the NVMe protocol. Like an SSD controller, Optane products use multiple channels with multiple dies per channel to exploit parallelism for greater performance. However, Optane's controller ASIC handles both data and commands exclusively in hardware to deliver the full performance of Xpoint media (at least the performance possible on the PCI Express 3.0 bus). The Optane controller can also spread a 4KB read operation over all of its dies, which Intel contrasts with the single-die read handling of NAND.
Thanks to Xpoint's fine-grained access characteristics, Optane devices can also write in place on the underlying medium, unlike NAND's read-modify-write cycle at the block level. In turn, Xpoint should match its higher performance with high endurance.
With a drive as fast as the P4800X, Intel believes that up to 30 drive writes per day is going to be adequate endurance for the types of applications it believes will benefit from Xpoint's performance. With a rated maximum of 12.3 petabytes written, that means the 375GB DC P4800X drive could endure 30 DWPD for three years of constant use by our calculations. It's worth noting that the projected endurance figures in the graphs above come from the 750GB P4800X, not the 375GB drive. Larger drives should enjoy a greater total petabytes written figure if Xpoint endurance scaling is similar to that of NAND. The best-case NAND endurance figure, on the other hand, comes from Intel's DC P3700 800GB and its 14.6 PBW figure, good for 10 DWPD.
The 375GB P4800X uses a seven-channel controller with four dies of 3D Xpoint per channel for 28 dies total. Although Xpoint devices don't require overprovisioning like NAND SSDs, the P4800X still reserves some extra space for ECC, firmware, and reallocation of bad regions of its total storage pool.
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