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Resistive Ram cache to make Flash fly, say boffins

Wham, bam, thank you, RRam

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SSDs appeal to ordinary computer users because of their speed and silence. Data centre folk appreciate those qualities too, but also like the SSD's very low power consumption. Energy is no small cost for a data centre, where there can be tens of thousands of drives all slurping electricity at once.

But it's not a free ride. SSDs aren't cheap, and they have longevity issues. Write information on them too many times and you'll only be able to read them after that. The fact that large areas of memory rather than single bits need to be written at a time - and erased first, a process that clears an even bigger space - means they are not well-suited to applications that unceasingly write and re-write data.

Solving these problems is something of a Holy Grail in the storage business. Many options exists - or have, at least, been proposed - but they're all many years from replacing wholesale the NAND Flash chips used in today's SSDs.

One Japanese group, from Chuo University, Tokyo, has suggested a half-way house: using one of the new kinds of non-volatile memory, Resistive Ram (RRam) as a buffer between the outside world and the host drive's NAND chips. This "hybrid" SSD uses Flash for bulk storage, and RRam for speed.

RRam uses a feature of certain dielectric materials, which normally exhibit a high resistance, to spontaneously create physical, permanent paths, called "filaments", down which current can flow. All it takes is the application of sufficiently high but not impractical voltage.

The pathways reduce the material's resistance, but it's possible to break them, returning the dielectric to its high resistance state. Re-apply the voltage, new paths form, and the material re-enters low-resistance mode.

It's this ability to maintain either of two states - high and low resistance - that allows the dielectric to store binary information. Hook it up to a controlling transistor and you have a usable non-volatile memory cell.

And it's one that can switch much more quickly than its Flash equivalents can: 10ns compared to 100,000ns. The upshot: its write speed is much, much higher - closer, in fact, to volatile memory like Ram.

Coupling it with Flash puts the RRam in the role of a cache. Controller algorithms store frequently read data in the RRam, from where the information can be gathered more quickly.

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Next page: Cache in hand

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