Onyx was built using prototype phase-change chips made by Micron, a company working to commercialize the technology. The chips store data in a a type of glass, using small bursts of heat to switch sections of the material between two different states, or phases, that represent digital 1s and 0s. In one phase, the atoms of the glass are arranged in an ordered crystal lattice, in the other they have an amorphous, disorganized arrangement.
Onyx’s performance springs from the much simpler process of writing data to a phase-change chip compared to a flash chip, which stores data as islands of electric charge on chunks of semiconductor, says Swanson. Flash chips cannot rewrite single bits of information—1s or 0s—on demand. Instead they have to erase data in “pages” of a fixed size and then go back to program in the desired data. That limits the technology’s speed. “It requires a flash memory device to have software keep a little log as it goes along of which data is correct,” says Swanson. “With phase-change memory you can just arbitrarily rewrite what you need.”
Sudhanva Gurumurthi, who researches computer architecture at Virginia Tech, says the San Diego project is a valuable demonstration of the true capabilities of phase-change memory chips. “Much research has simulated how they would perform, but this gives insights into complexities a simulation can’t capture,” he says. But it will be the price of the technology that will determine when it becomes a competitive technology, says Gurumurthi.
Gurumurthi’s research suggests that using phase-change memory in combination with flash memory could see the new technology reach the market earlier than the day it is cheap enough to be used in dedicated drives. Simulations showed that adding a small buffer of phase-change memory to a flash-based drive could simplify the process of writing small chunks of data, the kind of operation where flash performs least well. “We found it significantly improves performance,” says Gurumurthi. “That might be enough to offset the cost of adding a small amount of phase-change memory.”