The Samsung 983 ZET (Z-NAND) SSD Review: How Fast Can Flash Memory Get?

What Is Z-NAND?

When Samsung first announced Z-NAND in 2016, it was a year after 3D XPoint memory was announced and before any Optane products had shipped. Samsung was willing to preview some information about the Z-NAND based drives that were on the way, but for a year and a half they kept almost all information about Z-NAND itself under wraps. Initially, the company would only state that Z-NAND was a high-performance derivative of their V-NAND 3D NAND flash memory. Meanwhile at Flash Memory Summit 2017, they confirmed that Z-NAND was a SLC (one bit per cell) memory, while the company also announced that they were also working on a second generation of Z-NAND will introduce a MLC version of Z-NAND. (for reference, mainstream NAND flash is now almost always 3 bit per cell TLC).

If simply operating existing NAND as SLC was all there is to Z-NAND, then we would also expect Toshiba, WD, SK Hynix to have also delivered their competitors by now. But there are further tweaks required to challenge 3D XPoint. A year ago at IEEE's International Solid State Circuits Conference (ISSCC), Samsung pulled back the veil a bit and shared more information about Z-NAND. The full presentation was not made public, but PC Watch's coverage captured the important details. Samsung's first-generation Z-NAND is a 48-layer part with a capacity of 64Gb. Samsung's mainstream capacity-optimized NAND is currently transitioning from 64 layers to what's officially "9x" layers, most likely 96. There are probably several factors for why Z-NAND is lagging behind by almost two generations of manufacturing tech, but one important element is that adding layers can be detrimental to performance.

Compared to their past few generations of TLC NAND, Samsung's SLC Z-NAND improves read latency by a factor of 15-20x, but program latency is only improved by a factor of 5-7x. Note however that the read and program times shown above denote how long it takes to transfer information between the flash memory array and the on-chip buffers; so that 3µs read time doesn't include transferring the data to the SSD controller, let alone shipping it over the PCIe link to the CPU.

With Samsung using 16kB page sizes for their TLC NAND, the 4kB page size for SLC Z-NAND seems to be a reasonable choice as only a slight shrink in total number of memory cells per page, but the capability to instead operate with a 2kB page size indicates that small page sizes are an important part of the performance enhancements Z-NAND is supposed to offer.

Missing from this data set is information about the erase block size and erase time. Erasing flash memory is a much slower process than the program operation and it requires activating large and power-hungry charge pumps to generate the high voltages necessary. For this reason, all NAND flash memory groups many pages together to form each erase block, which nowadays tends to be at least several megabytes.

Samsung's Z-NAND may be able to offer far better read and program times than mainstream NAND, but they may not have been able to improve erase times as much. And shrinking erase blocks would significantly inflate the die space required for peripheral circuitry, further harming memory density that is already at a steep disadvantage for 48L SLC compared to mainstream 64L+ TLC.

Test System

Intel provided our enterprise SSD test system, one of their 2U servers based on the Xeon Scalable platform (codenamed Purley). The system includes two Xeon Gold 6154 18-core Skylake-SP processors, and 16GB DDR4-2666 DIMMs on all twelve memory channels for a total of 192GB of DRAM. Each of the two processors provides 48 PCI Express lanes plus a four-lane DMI link. The allocation of these lanes is complicated. Most of the PCIe lanes from CPU1 are dedicated to specific purposes: the x4 DMI plus another x16 link go to the C624 chipset, and there's an x8 link to a connector for an optional SAS controller. This leaves CPU2 providing the PCIe lanes for most of the expansion slots, including most of the U.2 ports.

The enterprise SSD test system and most of our consumer SSD test equipment are housed in a StarTech RK2236BKF 22U fully-enclosed rack cabinet. During testing for this review, the front door on this rack was generally left open to allow better airflow, since the rack doesn't include exhaust fans of its own. The rack is currently installed in an unheated attic and it's the middle of winter, so this setup provided a reasonable approximation of a well-cooled datacenter.

The test system is running a Linux kernel from the most recent long-term support branch. This brings in the latest Meltdown/Spectre mitigations, though strategies for dealing with Spectre-style attacks are still evolving. The benchmarks in this review are all synthetic benchmarks, with most of the IO workloads generated using FIO. Server workloads are too widely varied for it to be practical to implement a comprehensive suite of application-level benchmarks, so we instead try to analyze performance on a broad variety of IO patterns.

Enterprise SSDs are specified for steady-state performance and don't include features like SLC caching, so the duration of benchmark runs doesn't have much effect on the score, so long as the drive was thoroughly preconditioned. Except where otherwise specified, for our tests that include random writes, the drives were prepared with at least two full drive writes of 4kB random writes. For all the other tests, the drives were prepared with at least two full sequential write passes.

Our drive power measurements are conducted with a Quarch XLC Programmable Power Module. This device supplies power to drives and logs both current and voltage simultaneously. With a 250kHz sample rate and precision down to a few mV and mA, it provides a very high resolution view into drive power consumption. For most of our automated benchmarks, we are only interested in averages over time spans on the order of at least a minute, so we configure the power module to average together its measurements and only provide about eight samples per second, but internally it is still measuring at 4µs intervals so it doesn't miss out on short-term power spikes.