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It's well known that SSDs have a fairly short lifespan. From what I understand, they fail after many re-write cycles; when each bit stored changes as information is moved, replaced, and in some cases deleted.

System memory stores information in a similar way, except it's volatile. However, I would think memory endures even more rewrite cycles than an SSH hard drive would.

Why doesn't memory fail more often? Is it because data in SSDs is persistent and somehow that wears out the components?

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    System memory (RAM) does not store data in a similar way to SSDs (Flash). This is like comparing vinyl records to compact discs, just because they're both round and hold music doesn't make them the same thing. (Yes I know the comparison isn't perfect, but at least I didn't pull out automobiles).
    – Chris S
    May 18, 2011 at 18:26

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Seat of my pants answer: because SSDs store data after the power is turned off.

Regular system ram places a tiny electrical charge in a particular place on the chip. That charge gets continually refreshed as long as power is supplied to the chop. Once power is removed, the charge dissipates. I have heard of forensic techniques that can read from memory some minutes after a system is turned off, but in general I think the charge (and hence the bit) disappears virtually instantaneously.

The memory in SSDs persists after power is turned off. That means that instead of just storing a tiny electrical charge, the chip has to actually physically modify the write location. Thus it makes sense that this could only be done a fixed number of times before the spot wears out. SSDs emplyo many tricks such as write-leveling to spread the writes uniformly across the chip so that one particular spot doesn't get used up more quickly than others.

I guess I kinda think of this as the difference between a capacitor and a battery, I'll be interested to see how others explain this.

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  • Thanks! that makes sense. It'd be interesting to know how the actual process of "modifying the write location" takes place. :)
    – 0x6A75616E
    May 18, 2011 at 18:37
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Why doesn't memory fail more often?

Because RAM and flash storage are two fundamentally-different technologies - so different, in fact, that it's not really even worth attempting to compare them to each other.

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  • Thanks for your answer @Erika. I realize flash and RAM are different, but my question was really aiming at explaining the physical properties that make flash more prone to failure.
    – 0x6A75616E
    May 18, 2011 at 18:00
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    I assume a quick wikipedia journey will explain the difference between flash and dram?
    – Phil Hollenback
    May 18, 2011 at 18:03
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    What's with the attitude? Just explain how they're different instead of posting an answer that effectively says, "I'm too lazy to post the real answer." To be honest, it just sounds like you don't really know the answer. Please contribute if you can, but keep your "this is a stupid question" comments to yourself so other people don't have to discard them every time they try to find the real answer.
    – Homer6
    May 18, 2011 at 18:30
  • they're not so different. both rely on a transistor with the control pin tied to a floating capacitor. this capacitor holds the bit. the difference is that in DRAM the capacitor is in the circuit, and therefore discharges quickly; while in flash it's embedded inside an insulator. writing there needs a huge voltage jolt, that eventually burns the insulator.
    – Javier
    May 18, 2011 at 18:53
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    @Homer6 - if you sense any "attitude" in my answer, then that's your perogative. I had a strong suspicion that the question would get closed (which it did), so I didn't feel that it would be worth putting all that much effort into it. It was not clear from the OPs question that he understood that RAM and SSD were as different as they are, so I made that clear. Assuming that I would be unable to answer the question seems pretty foolish on your part.
    – EEAA
    May 18, 2011 at 18:55

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