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Comment by ryao | Hacker News Reader

The mechanism that you describe should be impossible to achieve with one bit-flip. I have no idea how you came to think this.

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There's a lot of misconception spread by FreeNAS forums. I suspect that's where he read it from:

http://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-r...

The author there makes some crazy assumptions like this: "But, now things get messy. What happens when you read that data back? Let's pretend that the bad RAM location is moved relative to the file being loaded into RAM. Its off by 3 bits and move it to position 5."

This is indeed where I read it. When googling to find it again I found a similar story at:

https://pthree.org/2013/12/10/zfs-administration-appendix-c-...

Any good ideas on how one might fix this incorrect "common wisdom"?

Honestly no idea, except just pointing it out when someone mentions it.

The whole ECC recommendation is due to ZFS (unlike other filesystems) providing guarantees to data correctness, but as you know while ZFS can discover data corruption on the disk thanks to checksums, it can't guarantee data correctness in RAM because because as any program it is bound to trust it. That's why ECC RAM is highly recommended.

I think there is some confusion about the impact of bit flips on reads and on writes. Those are separate cases. That blog post is absolutely correct, but it talks about writes. People here are talking about reads.

The forum post and blog both state that data also gets corrupted even more during reads. They also use strange assumption that data would be written to RAM ignoring byte boundary (i.e. shifted by 4 bits).

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