Persistent memory to replace DRAM, but it could take a decade

Wham, bam, hello MRAM, FERAM, and ReRAM

Persistent memories can or will soon match DRAM in terms of speed, which could see it eventually replaced in many applications if one of these technologies can scale up and bring the costs down.

In a recent webinar, the Compute, Memory, and Storage Initiative (CMSI) of the Storage Networking Industry Association (SNIA) examined emerging memory technologies such as MRAM, FERAM, and ReRAM, with answers to questions supplied in a blog afterward.

These memories, all persistent, are already replacing some established memory technologies, including SRAM, NOR flash, and DRAM, at least in some niche applications. But what about mainstream memory used in PCs and servers?

To cut to the chase, the experts believe that DRAM will eventually give way to a new memory type, but it is too early to tell which one will triumph, and when this will happen. Their best guess is the early 2030s.

One benefit of replacing DRAM with persistent memory is obvious; it keeps its content even without power, meaning there is less danger of losing data. However, DRAM has been around for a long time and is cheap to produce and available in high densities – hurdles that persistent memories will also need to surmount.

The webinar pointed out that Optane, which Intel dropped in 2022, proved how difficult it is for rival memory technologies to compete with DRAM (and NAND flash) on price.

One route being taken by many of the makers of these emerging memories is the embedded market. This is seen as "a good path for emerging memory IP," according to the experts, because it allows them to get used in production environments and build up some volume.

"And as the volume and comfort with manufacturing those memories increase, it opens up the possibility down the road of lower costs with higher volume standalone memory as well," the experts wrote.

Those experts on the webinar were Tom Coughlin of Coughlin Associates; Jim Handy, semiconductor analyst at Objective Analysis; and Arthur Sainio, co-chair of the SNIA Persistent Memory Special Interest Group and product marketing director for SMART Modular Technologies.

Another question that arose from the webinar was whether persistent memories will be able to match the speed of DRAM, to which the answer is yes.

"It appears that that has already happened with the hafnium ferroelectrics that SK hynix and Micron have shown," the experts responded. "Ferroelectric memory is a very fast technology and with very fast write cycles there should be every reason for it to go that way."

MRAM (magnetoresistive RAM) might get up to DRAM speeds with both its writes and reads, and reads will also probably rival DRAM speeds with ReRAM (resistive RAM).

"I think that probably most applications are read intensive and so the read (performance) is the real place where the focus is, but it does look like we are going to get there," one of the experts stated.

Another issue could be endurance, or the wear issue familiar from NAND flash. Techniques for wear leveling have been developed and incorporated into the software that manages SSDs to handle this, such that the experts contend that "any emerging memory can get by with an equally low endurance as long as it's put behind the right controller."

But a DRAM replacement will need to have higher endurance. Even with the adoption of practices such as mixing types of memory, such that code goes into memory with low endurance and data into higher endurance memory, the lowest acceptable level would be in the tens of thousands of cycles, the experts conclude.

In terms of host interfaces, most emerging technologies have used a NOR flash or SRAM type interface because these were the memory types used in the embedded applications they are targeting.

There is no reason why they could not adopt the same interface as DRAM, although "there are some special twists because you don't have to refresh emerging memory technologies."

But the SNIA experts see CXL, or compute express link, as the future interface of choice going forward as it can present any type of memory to the host processor.

"One of the beauties of CXL is that you put anything you want to with any kind of interface on the other side of CXL and CXL erases what the differences are. It moderates them so although they may have different performances, it's hidden behind the CXL network," one expert explained.

So it looks like the days of DRAM could be numbered, but even the experts don't know what exactly will replace it, or precisely when.

"DRAM has a transition that we don't know when it's going to happen that it goes to another emerging memory technology," the experts wrote. "We've put it into the early 2030s in our chart, but it could be much later than that." ®

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