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Comment by jerf | Hacker News Reader

Anybody have some detailed technical insight into why the failure rate has been so high? (i.e., I can guess as well as the next guy, I'm asking for knowledge.) One of their initial promises was reliability due to not having moving parts, and, well, that sounds pretty reasonable as far as marketing copy goes. Why isn't it true?

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jerf, the oddest part is that if you read through the de-duplication and redundancy designs of modern SSD controllers (like the SandForce SF-2k series) not only do the drives do automatic de-duplication and compression on the fly, the scatter the bits evenly across the memory arrays (in parallel) to control even wear-and-tear of R/W cycles on the memory.

Then they typically write the bits to multiple locations or an entire section of the drive is maintained for parity information, sort of like an on-disk RAID5 setup where the individual blocks of NAND can be though of as "disks". For example, a 120GB drive typically has something like 150-160GB of storage space on the disk. This is not unlike what spindle disks do with a large section of disk not allocated to the format operation and kept behind for failed sectors that are re-allocated by the controller.

So all that work and the failure rates are still too high for comfort. It's really surprising to me and suggests that all of this needs more baking time.

I have no idea what the failure rates are in lower-end drives like the Kingstons and other drives that are likely rebranded garbage. I imagine it's not any better :)

the interesting this is that Macbook Air are all SSDs - so unless OSX is doing something terribly creative, why arent we seeing the same kind of return/failure rates on Macbook Airs ?

There's a tradeoff between speed and reliability, and the Samsung SSDs that I believe are used in the Macbook Airs lean towards reliabilty at the expense of having non-random IO no better than a normal hard drive. People who buy SSDs by themselves, on the other hand, are often just going down a list of offerings comparing IO speed to price and not having any good way to look at the reliability.

I haven't seen that they aren't.

That, and you never own a MacBook Air more than a couple years, because Steve Says To Buy A New One Then.

chuckle I was expecting the indignant Apple downvotes =)

They fail (or rather you hear about them failing) because they are new. I have graveyards filled with spinny disks that failed, but it's hardly newsworthy because everybody knows drives fail. Has anybody put half HDs and half SSDs in a data center of any size? Anecdotes and data and all that.

It is a physical quality of flash memory that each flash cell has only a limited number of writes it can handle before it fails. If you want to know the dirty details, what happens is that electrons eventually get trapped in the dielectric of a flash cell and ruin everything. This cannot be fixed unless someone invents a new electron capture proof dielectric.

Flash memories use all kinds of electronics to ameliorate this problem, but in the end most they can do is try to equalize the number of writes each cell experiences, which results in the drive working at close to its full capacity for as long as possible but then failing very quickly afterwards.

Mind you, there have been people working on clearing up trapped charges. I doubt there's been much usable progress (IIRC, the methods involve some kind of irradiation of the dielectric, which is difficult to perform in situ.)

If this was the failure mode experienced by most drives that fail, though, it wouldn't happen for years and even after you were no longer able to write to the drive you would still be able to read from it. This might be how the more reliable Intel or Samsung drives end up failing, but all evidence is that the failure modes of the highest performance but less reliable drives seem to be more interesting than that.

Why do you think it would not happen for years? As I mentioned in a previous post there is a trade off between price memory size and failure rate so some of the higher density memories will have much lower life than what you may be used to.

And no you would not be able to read from it. When a flash cell wears off the ability to read it degrades or disappears. Depending on the memory you still may be able to recover the data using ECC or the like, but this ability tends to disappear as large numbers of cells start dying.

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