IBM Advances SSD with Phase-Change Memory Breakthrough

Facing an incessant demand to speed data through computers the latest IBM storage memory advance, announced earlier this week, will ratchet up the speed another notch or two. Scientists at IBM Research have demonstrated storing 3 bits of data per cell using phase-change memory (PCM). Until now, PCM had been tried but had never caught on for a variety of reasons. By storing 3 bits per cell, IBM can boost PCM capacity and speed and lower the cost.

TLCPCMSmall (1)

IBM multi-bit PCM chip connected to a standard integrated circuit board.

Pictured above, the chip consists of a 2 × 2 Mcell array with a 4- bank interleaved architecture, IBM explained. The memory array size is 2 × 1000 μm × 800 μm. The PCM cells are based on doped-chalcogenide alloy and were integrated into the prototype chip serving as a characterization vehicle in 90 nm CMOS baseline technology.

Although PCM has been around for some years only with this latest advance is it attracting the industry’s attention as a potential universal memory technology based on its combination of read/write speed, endurance, non-volatility, and density. Specifically, PCM doesn’t lose data when powered off, unlike DRAM, and the technology can endure at least 10 million write cycles, compared to an average flash USB stick, which tops out at 3,000 write cycles.  Primary use cases will be capturing massive volumes of data expected from mobile devices and the Internet of Things.

PCM, in effect, adds another tier to the storage/memory hierarchy, coming in between DRAM and Flash at the upper levels of the storage performance pyramid. The IBM researchers envision both standalone PCM and hybrid applications, which combine PCM and flash storage together. For example, PCM can act as an extremely fast cache by storing a mobile phone’s operating system and enabling it to launch in seconds. For enterprise data centers, IBM envisions entire databases could be stored in PCM for blazing fast query processing of time-critical online applications, such as financial transactions.

As reported by CNET, PCM fits neatly between DRAM and flash. DRAM is 5-10x faster at retrieving data than PCM, while PCM is about 70x faster than flash. IBM reportedly expects PCM to be cheaper than DRAM, eventually becoming as cheap as flash (or course flash keeps getting cheaper too). PCM’s ability to hold three bits of data rather than 2 bits, PCM’s previous best, enables packing more data into a chip, which lowers the cost of PCM storage and boosts its competitive position against technologies like Flash and DRAM.

Phase change memory is the first instantiation of a universal memory with properties of both DRAM and flash, thus answering one of the grand challenges of our industry,” wrote Haris Pozidis, key researcher and manager of non-volatile memory research at IBM Research –in the published announcement. “Reaching 3 bits per cell is a significant milestone because at this density the cost of PCM will be significantly less than DRAM and closer to flash.”

IBM explains how PCM works: PCM materials exhibit two stable states, the amorphous (without a clearly defined structure) and crystalline (with structure) phases, of low and high electrical conductivity, respectively. In digital systems, data is stored as a 0 or a 1. To store a 0 or a 1 on a PCM cell, a high or medium electrical current is applied to the material. A 0 can be programmed to be written in the amorphous phase or a 1 in the crystalline phase, or vice versa. Then to read the bit back, a low voltage is applied.

To achieve multi-bit storage IBM scientists have developed two innovative enabling technologies: 1) a set of drift-immune cell-state metrics and 2) drift-tolerant coding and detection schemes. These new cell-state metrics measure a physical property of the PCM cell that remains stable over time, and are thus insensitive to drift, which affects the stability of the cell’s electrical conductivity with time. The other measures provide additional robustness of the stored data. As a result, the cell state can be read reliably over long time periods after the memory is programmed, thus offering non-volatility.

Combined these advancements address the key challenges of multi-bit PCM—drift, variability, temperature sensitivity and endurance cycling, according to IBM. From there, the experimental multi-bit PCM chip used by IBM scientists is connected to a standard integrated circuit board

Expect to see PCM first in Power Systems. At the 2016 OpenPOWER Summit in San Jose, CA, last month, IBM scientists demonstrated PCM attached to POWER8-based servers (made by IBM and TYAN® Computer Corp.) via the CAPI (Coherent Accelerator Processor Interface) protocol, which speeds the data to storage or memory. This technology leverages the low latency and small access granularity of PCM, the efficiency of the OpenPOWER architecture, and the efficiency of the CAPI protocol, an example of the OpenPower Foundation in action. Pozidis suggested PCM could be ready by 2017; maybe but don’t bet on it. IBM still needs to line up chip makers to produce it in commercial quantities among other things.

DancingDinosaur is Alan Radding, a veteran information technology analyst and writer. Please follow DancingDinosaur on Twitter, @mainframeblog. See more of his IT writing at and here.



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