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Comment by dmix | Hacker News Reader

I believe the whole idea, beyond pure speed of RAM vs SSDs, is they are promoting a programming 'model' where you bypass the operating system I/O (transiting via pages/blocks) and instead directly read/write data (via bytes) to the memory from the application. Which could be useful for particular usecases / subsets of data.

They mention write cycles in the article...

> The existing enterprise Optane SSD DC P4800X initially launched with a write endurance rating of 30 drive writes per day (DWPD) for three years, and when it hit widespread availability Intel extended that to 30 DWPD for five years. Intel is now preparing to introduce new Optane SSDs with a 60 DWPD rating

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The idea is to drive CPU utilization as well.

For example, in the x86 world, pairing NVMe drives with putting portions of the application writing to NVDIMM drives core performance, say, in SQL, from 40% util to 100% util.

Even a single 8Gb DIMM can dramatically increase utilization and performance.

And literally half the bandwidth. NVMe means that you have to write the data, flush the caches over those ranges to ensure that it's actually in DRAM, then instruct the NVMe controller to read it back out of DRAM. With these drives you just write (and maybe flush), and it's done. And they probably have their own dedicated DRAM protocol controller.

And it might even be slightly better than halving the bandwidth needs, since swapping DRAM banks isn't free, so you might be saving on mildly thrashing your DRAM controller when you're using DRAM and the NVMe drive is trying to read at the same time.

Modern DMA is cache coherent (maybe except if you opt-out of it? I'm not even sure you can). It is still costly though.

That level of endurance is great if you've got wear leveling routines like a SSD has but not if writes to the same logical cache line hit the same physical block each time.

Does 3D xpoint have a need for wear leveling? It's a fundamentally different technology than NAND flash. The numbers stated are very close to what you get out of spinning rust in the enterprise space.

It does use wear leveling, especially since the quoted endurance is far lower than promised.

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