- X-NAND makes QLC perform more like SLC.
- X-NAND dramatically improves the endurance of QLC.
It always writes first in SLC mode, and further always at SLC speeds. There's three banks with the first writing incoming data to SLC, a second bank where SLC data is moved to QLC, and a third bank where SLC is erased to make room for more writes. The reason this works so well is because SLC mode tends to have a tPROG (write latency) of ~200µs - this applies to NAND in consumer drives as well, by the way - while QLC in his example is at a total of 6400µs (this is not precise, as newer consumer QLC can manage 1/2 to 1/3 this latency, but he's talking for all pages). Therefore you can write 32 SLC pages (e.g. 32x200µs = 6400µs) while keeping pace with data moving to QLC, therefore avoiding the SLC cache drop-off you usually see. This makes QLC uniquely qualified for this technology but that's
also true of other characteristics - namely, if you're splitting 16KB pages into 4KB I/O chunks/subpages (16 / 4) and then the page buffer into 1KB chunks per plane (4 / 4), 4-bit QLC with 1-bit SLC is a match made in heaven.
As for endurance, typically on a consumer drive you'll have static, dynamic, or a hybrid SLC caching scheme. Dynamic SLC shares a wear zone with native flash, you're converting back-and-forth, you can even increase write amplification with it. Static SLC, however, is dedicated and has its own wear zone, usually in the 30K-40K P/E range (vs. 100K with native SLC or 1.6K with dynamic SLC + QLC). If all the writes are done to this sort of static SLC, depending on workload you can increase overall endurance significantly for a variety of reasons. For example, writing/folding out from SLC is done sequentially and not randomly which reduces write amplification. You can also defer writes or erases. I'm not privy to the exact workings of X-NAND in this capacity (I made a lot of assumptions) but I am in contact with the creator and will certainly be investigating this more. However, it basically offers SLC-like performance with respectable endurance.
There was a query about endurance during FMS, and this was his reply:
"No, the endurance will not be reduced. Because the SLC pages are not erased immediately. Although their data is re-programmed to QLC page immediately, the SLC pages’ erasure will be hold until all the SLC pages in the bank are programmed, then all the SLC pages of the bank will be erased together. Therefore, the endurance is the same as conventional SLC cache. "