This morning, Kioxia is updating its BG series of M.2 2230 SSDs for OEMs with the addition of the new BG5 family of drives. The BG5 series, the latest in the postage stamp-sized line of SSDs, sees Kioxia reworking both the NAND and the underlying controller to utilize newer technologies. As a result, the latest version of the drive is getting higher overall performance thanks to the combination of PCIe 4.0 support and the switch to Kioxia’s latest BiCS5 NAND. However, in an unexpected twist, the BG series is no longer a single chip design; instead, the BG5’s NAND and controller are now separate packages.
Long a pre-built systems accessory, Kioxia’s SSD BG series has been a favorite with OEMs for the past few years due to its small size, usually M2. 2230 or less, as well as its low cost. In particular, the drive’s DRAM-free design keeps overall component costs low and allowed Kioxia to simply stack the NAND arrays on top of the controller, giving SSDs their small footprint. Furthermore, the simple design and strict thermal tolerances of such a stacked design mean that power consumption has also been kept quite low. The resulting performance of the units is very basic and therefore rarely noteworthy, but for a unit not much larger than a postage stamp it plays a small role.
It’s been just over two years since the BG4 was introduced, the major update for BG5 is the addition of PCIe 4.0 support. While BG4 was a PCIe 3.0 x4 drive, BG5 is PCIe 4.0 x4, which at this point gives the drive more bus bandwidth than you could ever hope to use. Truth be told, I was a bit surprised to see the BG5 go over to PCIe 4.0 given the limited performance impact on an entry-level unit and the tight power limits, although there are some second-order benefits of PCIe 4.0. In particular, any OEM that ends up assigning only two lanes to the drive (something that happens from time to time) will still get the equivalent of the drive’s PCIe 3.0 x4 speeds, which in turn is still high enough to run the drive at almost full performance. This underscores one of the big improvements that higher PCIe speeds offer: For components that don’t need more bandwidth, integrators can reduce the number of lanes.
|Kioxia BG5 SSD Specifications|
|Capacity||256 GB||512 GB||1 TB|
|Form factor||M.2 2230 or M.2 2280|
|Interface||PCIe Gen4 x4, NVMe 1.4|
|NAND flash||112L BiCS5 3D TLC|
|Sequential reading||3500 MB / s|
|Sequential write||2900 MB / s|
|Random read||500,000 IOPS|
|Random write||450,000 IOPS|
Speaking of performance, the BG5 drives are rated for higher performance than their predecessor. Kioxia’s official press release only offers a single set of figures so it’s almost certainly for the 1TB configuration, but for that drive they are rating it at 2900MB / sec writes and 3500MB / sec reads, this last crossing the limits of PCIe 3.0 x4. Random reads and writes are rated at 450K IOPS and 500K IOPS respectively. As always, these figures are against writing to the drive’s SLC cache, so sustained write performance eventually drops.
As this is a DRAM-free drive, there is no significant caching / buffering layer in the package to speak of. Instead, like its predecessor, Kioxia relies on Host Memory Buffer (HMB) technology to improve the performance of its drive. HMB is not used to cache user data, but is 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 its driver to support NVMe 1.4.
Supporting the new PCIe 4.0 controller is Kioxia’s TLC NAND BiCS5 generation, which is a 112L design. BiCS5 has been shipping for a while so it is a very well known quantity, but the time has finally come for it to leak into the BG series of units. BiCS5 was a relatively modest increase in density over BiCS4, so it’s not too surprising here that Kioxia kept the largest BG5 setting at 1TB, which would mean stacking 8 of the 1Tbit dies.
But perhaps the biggest change with the BG5 is not the specifications of the controller or the NAND alone, but the fact that the two parts are alone to begin with. A staple of the BG5 series design has been the small package enabled by stacking the memory and controller together in one package. But from the product photo provided by Kioxia, we can clearly see that the NAND and the controller are separate packages. Kioxia did not mention this change, so we can only speculate whether it is for simplicity in construction (no TSV for the controller) or perhaps because of the heat unleashed by a PCIe 4.0 controller. But one way or another, it’s a huge change in how the small unit is assembled.
As a result of this change, the BGA M.2 1620 form factor, which supplied the single-chip package in a solder package, has disappeared. Instead, the smallest form factor is now the removable M.2 2230 version. The postage stamp-sized M.2 2230 form factor has long been the staple of the line, as it is what we’ve seen Microsoft’s Surface products and other thin and light designs over the years. Since the form factor here is unchanged, using multiple packages shouldn’t alter things much for many OEMs. And for OEMs who need physically larger units for compatibility reasons, Kioxia also formally offers a 2280 design. A simple two-chip solution on such a large PCB is unremarkable, but would allow the BG5 to be easily inserted into systems. which are designed to support (and generally use) 2280 drives.
As these are OEM units, no pricing information is available. The units are currently being tested for Kioxia customers, so expect to see them land on commercial products in 2022.