Addition of PCIe 4.0 and BiCS5 NAND

This morning, Kioxia is updating their BG series of M.2 2230 SSDs for OEMs with the addition of the new BG5 family of drives. The BG5 series, which is the latest in the company’s line of stamp-sized SSDs, sees Kioxia rework both NAND and the underlying controller to use newer technologies. As a result, the latest iteration of the drive achieves an overall higher performance thanks to the combination of PCIe 4.0 support as well as the switch to Kioxia’s latest BiCS5 NAND. But in an unexpected twist, the BG series is no longer a single-chip design; instead, NAND and the controller on the BG5 are now separate packages.

Kioxia’s BG series of SSDs, which have long been part of prefabricated systems, have been a favorite among OEMs for the past many years due to their small size – typically the M2. 2230 or less – as well as their low price. In particular, the DRAM-free design of the drive keeps overall component costs down, and it allowed Kioxia to simply stack the NAND matrices on top of the controller, giving the SSDs their small footprint. The simple design and the tight thermal tolerances in such a stacked design also mean that the power consumption has also been kept quite low. The resulting performance of the drives is very entry-level, and therefore rarely remarkable, but for a drive that is not much larger than a stamp, it fills a small role.

After just over two years since BG4 was introduced, the main update to BG5 is the addition of PCIe 4.0 support. Where the BG4 was a PCIe 3.0 x4 drive, the BG5 is PCIe 4.0 x4, which at this point gives the drive more bus bandwidth than it could ever hope to use. Truth be told, I was a bit surprised to see that the BG5 went PCIe 4.0 given the limited performance impact on a boot level drive and the tight power limits, although there are some second-order benefits of PCIe 4.0. In particular, any OEM that ends up allocating only two lanes to the drive (something that happens every now and then) will still get the equivalent of PCIe 3.0 x4 speeds out of the drive, which in turn is still high enough to drive the drive at almost full performance. This underscores one of the major improvements offered by higher PCIe speeds: For components that do not require more bandwidth, integrators can instead cut down on the number of lanes.

Speaking of performance, the BG5 drives are rated for higher throughput than their predecessor. Kioxia’s official press release offers only a single set of numbers, so these are almost certainly for the 1TB configuration, but for that drive, they estimate it at 2900MB / sec readings and 3500MB / sec readings – the latter just cross the boundaries of PCIe 3.0 x4. Random writings and readings are rated at 450K IOPS and 500K IOPS, respectively. As always, these numbers are against writing to the drive’s SLC cache, so sustained write throughput eventually decreases.

As this is a DRAM-free drive, there is no significant caching / buffer layer on the package to speak of. Instead, Kioxia, like its predecessor, relies on Host Memory Buffer (HMB) technology to improve the performance of their drives. HMB is not used to cache user data, but is instead used to cache mapping information about the contents of the drive to speed up access. Along with the latest generation of this technology, Kioxia has also updated their controller to support NVMe 1.4.

The new PCIe 4.0 controller supports Kioxia’s BiCS5 generation of TLC NAND, which is a 112L design. BiCS5 has been shipping for a while now, so it’s largely a known quantity, but the time has finally come for it to seep into the BG series of drives. BiCS5 was a relatively modest increase in density compared to BiCS4, so it is not so surprising here that Kioxia retains the largest BG5 configuration of 1 TB, which would mean stacking 8 of the 1Tbit matrices.

But perhaps the biggest change with the BG5 is not the specifications of the controller or NAND alone, but rather the fact that the two parts are solely to begin with. A fixed component of the BG5 series’ design has been the small package made possible by stacking the memory and controller together in a single package. But from Kioxia’s accompanying product photo, we can clearly see that NAND and the controller are separate packages. Kioxia did not mention this change, so we can only wonder if it is for simplicity of construction (no TSVs for the controller) or perhaps the heat cut off by a PCIe 4.0 controller. But somehow it’s a big change in how the little drive is assembled.

As a result of this change, the BGA M.2 1620 form factor – which delivered the single-chip package in a solder-down package – is gone. Instead, the smallest form factor is now the detachable M.2 2230 version. The stamp size M.2 2230 form factor has long been the main element of the series, as it is what we have seen in Microsoft’s Surface products and other thin and light designs over the years. Since the form factor here does not change, the use of multiple packages should not change things much for many OEMs. And for OEMs who need physically larger drives for compatibility reasons, Kioxia also formally offers a 2280 design. A simple two-chip solution on such a large circuit board is not remarkable, but it would allow the BG5 to be easily inserted into systems designed to take (and typically use) 2280 drives.

As these are OEM drives, no pricing information is available. The drives are currently testing for Kioxia’s customers, so expect to see them land in commercial products by 2022.