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IBM has achieved a breakthrough in phase change memory (PCM) that it says will bring it close to the cost of flash and allow it to be used with flash and Dram, for cache, memory and to speed database access, for example.
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The breakthrough – demonstrated to the IEEE in Paris earlier in May 2016 – saw IBM show off triple level cell (TLC) PCM. This is PCM with eight resistance levels per cell, giving three 1/0 bit states in each cell.
IBM fellow Evangelos Eleftheriou said PCM offers read times of 200 to 300 nanoseconds and 2µs to 4µs for writes. Flash storage reads are currently measured in hundreds of µs, while writes are closer to milliseconds.
Eleftheriou also said PCM can provide endurance that it 10,000 time that of flash.
Flash is also bound by having to write entire blocks of cells at a time (known as pages) and has to erase a cell before it can be written to. This is known as the program-erase cycle and is a key issue with flash. PCM, by contrast, can simply overwrite cells.
Dram, used for motherboard memory, is faster than flash, but is volatile – it doesn’t retain data when switched off, for ecample – and is too costly for bulk storage use.
PCM is made of an alloy of germanium, antimony and tellurium, which changes state when a voltage is applied to it. The key state change in PCM has been either crystalline or amorphous.
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Efforts to go beyond two states – and therefore to more bits per cell – have been based on creating states between crystalline and amorphous, but have foundered on the difficulty of ensuring a crystal state written in the memory would be consistent over time and thus readable.
But IBM has got around that issue by reading the relative resistance of different material states rather than the state itself. This has allowed up to eight states per cell.
So, when will PCM make its debut in enterprise systems and where?
“We have the IP and we could license it to a manufacturer. From the technical point of view we have solved the problems. There are no more roadblocks and it is now ready for market,” said Eleftheriou.
He said that for the time being, because of cost, that PCM would likely occupy a layer in systems where better performance than flash is required, such as a faster layer of storage for metadata or in-memory database use, or as a lower cost extension to Dram-based memory.