One concern is the potential cost of the RAMCloud. Ousterhout estimates that 2,000 servers could provide 48 terabytes of DRAM storage at $65 per gigabyte. That’s 50 to 100 times more expensive than disks. However, if you look at cost in terms of how many bits you can access per second, DRAM is actually 10 to 100 times cheaper than disk, Ousterhout says. And he projects that by 2020, with improvements in DRAM technology, a RAMCloud could store one to 10 quadrillion bytes at just $6 per gigabyte.
Ousterhout compares the situation to the 1970s, when hard disks supplanted tape drives as the main storage system for computers, not because they were less expensive but because they made computers run more efficiently. “Disks never got cheaper than tape,” Ousterhout says. “I think the same thing’s going to happen with DRAM.”
Another issue with DRAM is that it’s volatile, meaning it only holds information as long as electricity flows to it. So RAMCloud would still use disks as backup storage, along with extra copies of data in DRAM, allowing data lost during a crash to be recovered.
Luiz Barroso, a distinguished engineer at Google, says the Stanford group is tackling a very important problem, and he sees some promise. “The economics of current DRAM technology would rule out RAMCloud as the solution for some important big-data problems, but it could be compelling for more modest-sized workloads,” he says.