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Flash Storage Innovation

Advantages Of Flash In The Data Center: Not Just A Flash In The Pan
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Experimental & New Flash Type Technology: Non-Volatile Memory

At Storage Visions 2014, panelists stated that RAM will transfer to a new Storage Class Memory, before spinning disks and tape. Storage Class Memory is generally non-volatile solid state memory with fast access and high throughput, and will be composed of one or more new types of flash: Phase Change Memory, Memristor (RRAM), Solid Electrolyte (also called PMC -programmable metallization cell), or Racetrack Memory. For all of these, robustness, density, and power consumption are much better than spinning disks, or even NAND flash.    

In Racetrack Memory, memory is stored as domain walls, in the third dimension, in columns of magnetic material, in a vertical "race track," or long nanowire. Domains are moved around the track using nanosecond pulses of current. The patterns (information) move, though there are no moving parts. The density is 10 to 100x conventional flash because each location stores an entire bit pattern, not just a single bit. This technique is sometimes referred to as spintronics, because electron spin orientation determines each bit’s orientation as the bits are moved along nano-scale wires by spin-polarized electric current. The principal advocate of this technology is IBM, but implementation is at least a few years away.

Phase Change Memory (PCM) is the only type currently in production, principally by Samsung and Micron, though others have produced PCM and proof of concepts. Micron, though, has removed their PCM from production in favor of 3D NAND flash. PCM has faster read/write cycles and longer life time than flash. Its latency is close to DRAM, and is byte addressable, compared to flash that is not.

Memristor, or ReRAM (or even RRAM, Resistive Random Access Memory) is primarily HP’s lab tech idea to increase memory density. HP is partnering with Hynix, but Crossbar claims they will have a postage stamp sized chip storing 1 TB by 2015, by which time they expect mass production. Those chips should also offer 10 times the lifetime of existing NAND flash. HP claims memristor can be more than twice as dense as flash, and uses less power than phase-change, which is up to 20 times less than NAND. Memristor is a contraction of memory + resistor, is non-volatile, and is based on the fact that certain special non-conductors will conduct briefly if a high voltage is applied. Normally, this material breakdown is permanent, but in these specialized materials, that breakdown is reversible. The breakdown in the dielectric (non-conductive material) consists of extraordinarily thin filaments, which could store information at very high densities.  The challenge is to generate only predictable filaments, which means the technology is at least 1-2 years away from common use.  A good summary of the technology and its challenges can be found here.Fig. 4. One analyst, Bernstein Research, sees ReRAM  as being most likely to succeed NAND flash.  Courtesy Bernstein Research.

Solid Electrolyte memory is usually referred to as PMC, and is espoused by its inventor, Michael Kozicki, at ASU, and its spinoff company, Axon Technologies. This technology is also licensed to Infineon and Micron Technology, amongst others. PMC cells are composed of two electrodes, one more chemically active than the other, across a created conductive bridge. This bridge is formed by a current applied across the electrodes.  A very small wire is formed in the gap from anode ions migrating to the cathode, which reduces measured resistance. This process is very similar to electroplating, and is what constitutes a write. An animation is available here

Reading is done by measuring resistance: if it is low, the read is a 1. If the resistance is higher, the reading of the memory state is a 0. Erasing is done by reversing the original current. Compared to standard NAND flash, PMC offers many more write cycles since it does not require as high a current to rewrite cells, resulting in much less degradation over  tens of thousands of rewrites. NAND requires an external circuit to charge cells, which takes a relatively long time to prime, but PMC writes at comparatively high speed. At least one company is providing samples, but it will be a few years before this technology has significant market penetration.
Fig. 5. Some types of Non-Volatile RAM on the horizon, and the chemistry of the switching or memory process. Image courtesy of SNIA.

NAND Flash Keeps Improving: We're Not Dead

As much as many associated with these four nexgen memory devices have predicted flash’s demise, that hasn’t happened. NAND has continued to improve by scaling to 10 -19 nanometers currently, though most common, at the low end of the scale, are flash units made with a 22 to 30 nm process. Samsung has announced that it is producing 3D flash, or V-NAND, so named because the flash is built vertically instead of horizontally, and then stacks the flash cells like miniaturized skyscrapers, up to 24 layers. This furthers economies of scale. There are techniques that overcome limitations of flash, principally the write degradation, such as improved ECC controllers, and firmware that will correct for low signals near flash cell life-cycle end.
Fig. 6. NAND has issues – scaling issues. Image courtesy of Bernstein Research.