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SSD 2.0: Phase-Change Memory Challenges NAND Flash

Phase-change memory started as a topic of discussion in the 1970s, when it was touted as a possible replacement for magnetic storage because of its high read/write speeds, low volatility, and high storage density. Now, PCM is actively being developed as the next technology after NAND for flash memory, because it has the potential to address NAND's shortcomings as well as the limitations of conventional hard drives.

"There are a number of advantages that PCM has over NAND and hard drives," says Spike Narayan, functional manager of IBM's Science and Technology Group in San Jose, Calif. "First, PCM is a very high-performance device when compared to NAND or hard drives. Second, PCM is non-volatile and energy-conserving. When no information is being read or written, there is no power consumption. This is significant, especially as more data centers look for storage that is energy efficient. Third, PCMs write endurance is far superior to that of NAND. Right now, NAND flash memory has a lifespan of 10,000 to 100,000 write cycles -- but PCM is better by a factor of three to five times."

For writes, phase-change memory uses a medium called chalcogenide, a glassy substance containing sulphur, selenium, or tellurium. These semiconductors can be changed from one phase to another through the application of heat, which automatically erases any data present. Write performance with phase-change memory far surpasses that of NAND, because PCM bits do not have to be erased (as with NAND) before data is written.

"What you have is a two-step write process of NAND being reduced to a single step with PCM," says Cliff Smith, phase-change program manager for Numonyx BV , a developer of memory products that was formed in 2008 by ST Microelectronics, Intel Corp. (Nasdaq: INTC), and Francisco Partners . "We’re envisioning that PCM will enter the storage market, but that it won't replace other existing media. Enterprises will be able to take advantage of PCM's unique capabilities by placing it in front of NAND in an SSD [solid-state disk] array."

Smith says that this architectural approach would allow PCM to perform the quick reads and writes and then push other writes into NANDs, which are better suited for heavy and consolidated block writing. Chunks of data of 512 bytes "are not a good fit for larger blocks of NAND, because it forces you to manage system resources and overhead."

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