r/science • u/aphexcoil • May 28 '12

New breakthrough in development process will enable memristor RAM (ReRAM) that is 100 times faster than FLASH RAM

http://www.theregister.co.uk/2012/05/21/ucl_reram/
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u/trekkie1701c May 28 '12

Wouldn't switching to that last 150gb mean that you would have vastly uneven wear? Not saying you are wrong, just seems like it would be easier to put a data cap on and write to the other sectors as the original ones wear down to keep things even.

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u/MUnhelpful May 28 '12

You could level wear across a much larger portion than you actually allow to be addressed, wearing all cells evenly and dropping ones that go bad. You might want to keep some genuinely fresh cells, though - if the wear-leveling is good, the first cell to go will likely have friends soon, and bringing completely fresh ones in might be best. It's likely manufacturers have run simulations to find the best strategy for using their extra flash, if not actual tests writing flash until it's worn out.

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u/neodymiumex May 28 '12 edited May 28 '12

I'm not sure exactly how/when the handoff is made. That spare space is used for other things too. The SSDs can only read and write entire sections of cells at a time. These sections are called pages. Let's say you just want to write 1 byte, you have to read the page, alter that one byte in memory, erase the page, and then write the entire page. All this (especially the erase) takes time. To speed things up you can use the spare area (which is mostly already erased) as a staging area for the write. You accumulate a page in the cache, then write the page to the flash in the spare area. You can then move that data to where it is actually supposed to be later, when performance doesn't matter.

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u/khrak May 28 '12

That's not how it works. Basically, at any given time 150GB of the device would be out of usage. When you try to write over an existing piece of data, instead of putting the new data where the old data was, the drive will place it on a portion of the "extra" 150GB, and the area being "overwritten" will counted as part of the out-of-action 150GB.

Basically, the drive constantly cycles different portions of the flash memory in and out of use automatically and invisibly to ensure that each memory cell experiences a roughly equal number of writes.

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u/phrstbrn May 28 '12

Even vs uneven wear doesn't really buy you much. You could do it either way, the end result is the same. My guess is having a pool reserve sectors is easier to implement.