I've never understood the positioning of these new persistent memory technologies as a replacement for RAM. Isn't it good enough that they are a replacement for Flash? By saying that they can replace RAM, you are setting up a good technology for failure.
Here's why: RAM has to have infinite read/write capability. Not millions, not even billions, but infinite. RAM can be written to very quickly. If it has a 100ns cycle, then you can write 100 million times in 10 seconds. Writing to one location may not be something that is done often, but all it takes is one time, (maybe some weird encryption, compression, or even Bitcoin algorithm that reuses one memory block over and over again), and the memory is dead in the water.
Let's set the goal post a little lower so these memories have a chance.
edit add: did some searching, and one MRAM company, Everspin, claims no wear-out mechanism, or infinite read/write capability, so I guess at least MRAM can replace RAM.
"Meanwhile MRAM and STT MRAM will start to replace SRAM and DRAM within the next few years and probably before RRAM replaces flash memory."
Why would I need both? That is if DRAM/SRAM's replacement is persistant why would a CPU system have any need of other persistent storage, i.e Flash's future substitute? Or conversely, if Flash's future substitutes has DRAM-quick write speeds why would a CPU have any need of DRAM's future substitute? It seems like the future merges the two functions by solving the technical challenges that led to them being seperate in ther first place.