The Intel Optane Memory (SSD) Preview: 32GB of Kaby Lake Caching

Last week, we took a look at Intel's first product based on their 3D XPoint non-volatile memory technology: the Optane SSD DC P4800X, a record-breaking flagship enterprise SSD. Today Intel launches the first consumer product under the Optane brand: the Intel Optane Memory, a far smaller device with a price that is 20 times cheaper. Despite having "Memory" in its name, this consumer Optane Memory product is not a NVDIMM nor is it in any other way a replacement for DRAM (those products will be coming to the enterprise market next year, even though the obvious name is now taken). Optane Memory also not a suitable replacement for mainstream flash-based SSDs, because Optane Memory is only available in 16GB and 32GB capacities. Instead, Optane Memory is Intel's latest attempt at an old idea that is great in theory but has struggled to catch on in practice: SSD caching.

Optane is Intel's brand name for products based on the 3D XPoint memory technology they co-developed with Micron. 3D XPoint is a new class of non-volatile memory that is not a variant of flash memory, the current mainstream technology for solid state drives. NAND flash memory—be it older planar NAND or newer 3D NAND flash—has fundamental limits to performance and write endurance, and many of the problems get worse as flash is shrunk to higher densities. 3D XPoint memory takes a radically different approach to non-volatile storage, and it makes different tradeoffs between density, performance, endurance and cost. Intel's initial announcement of 3D XPoint memory technology in 2015 came with general order of magnitude comparisons against existing memory technologies (DRAM and flash). Compared to NAND flash, 3D XPoint is supposed to be on the order of 1000x faster with 1000x higher write endurance. Compared to DRAM, 3D XPoint memory is supposed to be about 10x denser, which generally implies it'll be cheaper per GB by about the same amount. Those comparisons were about the raw memory itself and not about the performance of an entire SSD, and they were also projections based on memory that was still more than a year from hitting the market.

3D XPoint memory is not intended or expected to be a complete replacement for flash memory or DRAM in the foreseeable future. It offers substantially lower latency than flash memory but at a much higher price per GB. It still has finite endurance that makes it unsuitable as a drop-in replacement for DRAM without some form of wear-leveling. The natural role for 3D XPoint technology seems to be as a new tier in the memory hierarchy, slotting in between the smaller but faster DRAM and the larger but slower NAND flash. The Optane products released this month are using the first-generation 3D XPoint memory, along with first-generation controllers. Future generations should be able to offer substantial improvements to performance, endurance and capacity, but it's too soon to tell how those characteristics will scale.

The Intel Optane Memory is a M.2 NVMe SSD using 3D XPoint memory instead of NAND flash memory. 3D XPoint allows the Optane Memory to deliver far higher throughput than any flash SSD of equivalent capacity, and lower read latency than a NAND flash SSD of any capacity. The Optane Memory is intended both for OEMs to integrate into new systems and as an aftermarket upgrade for "Optane Memory ready" systems: those that meet the system requirements for Intel's new Optane caching software and have motherboard firmware support for booting from a cached volume. However, the Optane Memory can also be treated as a small and fast NVMe SSD, because all of the work to enable its caching role is performed in software or by the PCH on the motherboard. 32GB is even (barely) enough to be used as a Windows boot drive, though doing so would not be useful for most consumers.

Intel Optane Memory uses a PCIe 3.0 x2 link, while most M.2 PCIe SSDs use the full 4 lanes the connector is capable of. The two-lane link allows the Optane Memory to use the same B and M connector key positions that are used by M.2 SATA SSDs, so there's no immediate visual giveaway that Optane Memory requires PCIe connectivity from the M.2 socket. The Optane Memory is a standard 22x80mm single-sided card but the components don't come close to using the full length. The controller chip is far smaller than a typical NVMe SSD controller, and the Optane Memory includes just one or two single-die packages of 3D XPoint memory. The Optane Memory module has labels on the front and back that contain a copper foil heatspreader layer, positioned to cool the memory rather than the controller. There is no DRAM visible on the drive.

The performance specifications of Intel Optane Memory have been revised slightly since the announcement last month, with Intel now providing separate performance specs for the two capacities. Given the PCIe x2 link it's no surprise to see that sequential read speeds are substantially lower than we see from other NVMe SSDs, with 900 MB/s for the 16GB model and 1350 MB/s for the 32GB model. Sequential writes of 145 MB/s and 290 MB/s are far slower than consumer SSDs are usually willing to advertise, but are typical of the actual sustained sequential write speed of a good TLC NAND SSD. Random read throughput of 190k and 240k IOPS is in the ballpark for other NVMe SSDs. Random write throughput of 35k and 65k IOPS are also below the peak speeds advertised my most consumer SSDs, but on par with mainstream TLC and MLC SSDs respectively for actual performance at low queue depths.

Really it's the latency specifications where Optane Memory shines: the read latency of 7µs and 9µs for the 16GB and 32GB respectively are slightly better than even the enterprise Optane SSD DC P4800x, while write latency of 18µs and 30µs are just 2-3 times slower. The read latencies are completely untouchable for flash-based SSDs, but the write latencies can be matched by other NVMe controllers, but only because they cache write operations instead of performing them immediately.

The power consumption and endurance specifications don't look as impressive. 3.5W active power is lower than many M.2 PCIe SSDs and low enough that thermal throttling is unlikely to be a problem. The 1W idle power is unappealing, if not a bit problematic. Many M.2 NVMe SSDs will idle at 1W or more if the system is not using PCIe Active State Power Management and NVMe Power States. The Optane Memory doesn't even support the latter and will apparently draw the full 1W even in a well-tuned laptop. Since these power consumption numbers are typically going to be in addition to the power consumption of a mechanical hard drive, an Optane caching configuration is not going to offer decent power efficiency.

Meanwhile write endurance is rated at the same 100GB/day or 182.5 TB total for both capacities. Even though a stress test could burn through all of that in a week or two, 100GB/day is usually plenty for ordinary consumer use. However, a cache drive will likely experience a higher than normal write load as data and applications will tend to get evicted from the cache only to be pulled back in the next time they are loaded. More importantly, Intel promised that 3D XPoint would have on the order of 1000x the endurance of NAND flash, which should put these drives beyond the write endurance of any other consumer SSDs even after accounting for their small capacity.