What Micron's Upcoming 3D NAND Means for SSD Capacity, Performance, and Cost
Subject: Storage | February 14, 2016 - 02:51 PM | Allyn Malventano
Tagged: vnand, ssd, Samsung, nand, micron, Intel, imft, 768Gb, 512GB, 3d nand, 384Gb, 32 Layer, 256GB
You may have seen a wave of Micron 3D NAND news posts these past few days, and while many are repeating the 11-month old news with talks of 10TB/3.5TB on a 2.5"/M.2 form factor SSDs, I'm here to dive into the bigger implications of what the upcoming (and future) generation of Intel / Micron flash will mean for SSD performance and pricing.
Remember that with the way these capacity increases are going, the only way to get a high performance and high capacity SSD on-the-cheap in the future will be to actually get those higher capacity models. With such a large per-die capacity, smaller SSDs (like 128GB / 256GB) will suffer significantly slower write speeds. Taking this upcoming Micron flash as an example, a 128GB SSD will contain only four flash memory dies, and as I wrote about back in 2014, such an SSD would likely see HDD-level sequential write speeds of 160MB/sec. Other SSD manufacturers already recognize this issue and are taking steps to correct it. At Storage Visions 2016, Samsung briefed me on the upcoming SSD 750 Series that will use planar 16nm NAND to produce 120GB and 250GB capacities. The smaller die capacities of these models will enable respectable write performance and will also enable them to discontinue their 120GB 850 EVO as they transition that line to higher capacity 48-layer VNAND. Getting back to this Micron announcement, we have some new info that bears analysis, and that pertains to the now announced page and block size:
256Gb MLC: 16KB Page / 16MB Block / 1024 Pages per Block
384Gb TLC: 16KB Page / 24MB Block / 1536 Pages per Block
To understand what these numbers mean, using the MLC line above, imagine a 16MB CD-RW (Block) that can write 1024 individual 16KB 'sessions' (Page). Each 16KB can be added individually over time, and just like how files on a CD-RW could be modified by writing a new copy in the remaining space, flash can do so by writing a new Page and ignoring the out of date copy. Where the rub comes in is when that CD-RW (Block) is completely full. The process at this point is very similar actually, in that the Block must be completely emptied before the erase command (which wipes the entire Block) is issued. The data has to go somewhere, which typically means writing to empty blocks elsewhere on the SSD (and in worst case scenarios, those too may need clearing before that is possible), and this moving and erasing takes time for the die to accomplish. Just like how wiping a CD-RW took a much longer than writing a single file to it, erasing a Block takes typically 3-4x as much time as it does to program a page.
With that explained, of significance here are the growing page and block sizes in this higher capacity flash. Modern OS file systems have a minimum bulk access size of 4KB, and Windows versions since Vista align their partitions by rounding up to the next 2MB increment from the start of the disk. These changes are what enabled HDDs to transition to Advanced Format, which made data storage more efficient by bringing the increment up from the 512 Byte sector up to 4KB. While most storage devices still use 512B addressing, it is assumed that 4KB should be the minimum random access seen most of the time. Wrapping this all together, the Page size (minimum read or write) is 16KB for this new flash, and that is 4x the accepted 4KB minimum OS transfer size. This means that power users heavy on their page file, or running VMs, or any other random-write-heavy operations being performed over time will have a more amplified effect of wear of this flash. That additional shuffling of data that must take place for each 4KB write translates to lower host random write speeds when compared to lower capacity flash that has smaller Page sizes closer to that 4KB figure.
A rendition of 3D IMFT Floating Gate flash, with inset pulling back some of the tunnel oxide layer to show the location of the floating gate. Pic courtesy Schiltron.
Fortunately for Micron, their choice to carry Floating Gate technology into their 3D flash has netted them some impressive endurance benefits over competing Charge Trap Flash. One such benefit is a claimed 30,000 P/E (Program / Erase) cycle endurance rating. Planar NAND had dropped to the 3,000 range at its lowest shrinks, mainly because there was such a small channel which could only store so few electrons, amplifying the (negative) effects of electron leakage. Even back in the 50nm days, MLC ran at ~10,000 cycle endurance, so 30,000 is no small feat here. The key is that by using that same Floating Gate tech so good at controlling leakage for planar NAND on a new 3D channel that can store way more electrons enables excellent endurance that may actually exceed Samsung's Charge Trap Flash equipped 3D VNAND. This should effectively negate the endurance hit on the larger Page sizes discussed above, but the potential small random write performance hit still stands, with a possible remedy being to crank up the Over-Provisioning of SSDs (AKA throwing flash at the problem). Higher OP means less active pages per block and a reduction in the data shuffling forced by smaller writes.
A 25nm flash memory die. Note the support logic (CMOS) along the upper left edge.
One final thing helping out Micron here is that their Floating Gate design also enables a shift of 75% of the CMOS circuitry to a layer *underneath* the flash storage array. This logic is typically part of what you see 'off to the side' of a flash memory die. Layering CMOS logic in such a way is likely thanks to Intel's partnership and CPU development knowledge. Moving this support circuitry to the bottom layer of the die makes for less area per die dedicated to non-storage, more dies per wafer, and ultimately lower cost per chip/GB.
Samsung's Charge Trap Flash, shown in both planar and 3D VNAND forms.
One final thing before we go. If we know anything about how the Intel / Micron duo function, it is that once they get that freight train rolling, it leads to relatively rapid advances. In this case, the changeover to 3D has taken them a while to perfect, but once production gains steam, we can expect to see some *big* advances. Since Samsung launched their 3D VNAND their gains have been mostly iterative in nature (24, 32, and most recently 48). I'm not yet at liberty to say how the second generation of IMFT 3D NAND will achieve it, but I can say that it appears the next iteration after this 32-layer 256Gb (MLC) /384Gb (TLC) per die will *double* to 512Gb/768Gb (you are free to do the math on what that means for layer count). Remember back in the day where Intel launched new SSDs at a fraction of the cost/GB of the previous generation? That might just be happening again within the next year or two.