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Normal SSD will expire after some number of reads and writes. Why is this? And how did they fix it for DRAM?

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Two different technologies of chips.. explained on wikipedia. Basically RAM is volatile- it stores its context using an eleictrical charge. SSD is non-volatile it stores its bit without charge-changing the materials property physically- which after time will burn out. Similar to Rewrittable CDroms. –  ppumkin Jan 6 '12 at 12:04
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Flash does use electric charges (ppumpkin's comment is wrong). However, they're stored in a Floating Gate. A Floating Gate is electrically isolated. The upside is that it discharges very, very slowly (many years). The downside is that to get electrons in the Floating Gate, you need to apply a very high voltage. (Hot Electron Injection). This causes damage to the isolation.

In DRAM, the electric charges aren't isolated like in Flash cells. They're in fact stored so loosely that they leak away in seconds. A circuit internal to the DRAM chip periodically reads all cells, and rewrites them. That resets 0.9 to 1 and 0.1 to 0. This periodic refresh consumes a bit of energy, and explains why DRAM memory is gone when you turn off the power. But the energy levels per write are so low that this doesn't damage the DRAM at all.

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@MarkBooth: Being less volatile still equals volatile. I don't see your point. That has nothing to do with the discussion, which is write endurance.The question was about write endurance not about whether it absolutely volatile inside of 5 minutes. –  surfasb Jan 7 '12 at 2:52
    
@MarkBooth: Again, this isn't a discussion about security either. But, did you know that a pig can scream at 115 decibels? See the analogy? –  surfasb Jan 7 '12 at 22:45
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