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by amtadt 695 days ago
Self healing is dangerous because it can potentially corrupt good data on disk, if RAM or other system component is flaky.

Repro: supposedly only good copy is copied to ram, ram corrupts bit, crc is recalculated using corrupted but, corrupted copy is written back to disk(s).
2 comments
cesarb 695 days ago
> crc is recalculated using corrupted bit

Why would it need to recalculate the CRC? The correct CRC (or other hash) for the data is already stored in the metadata trees; it's how it discovered that the data was corrupted in the first place. If it writes back corrupted data, it will be detected as corrupted again the next time.

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amtadt 695 days ago
Because CRC is in the on-disk data structure, not in the in-ram data structure. It is stripped upon reading to ram, and created upon writing to disk.

That's how bcachefs is designed right now.

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koverstreet 694 days ago
No, we carefully carry around existing checksums when moving data.

Page cache is a different story, but doesn't apply to what we're talking about here.

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newZWhoDis 695 days ago
That’s why you need ECC RAM.

Our RAM should all be ECC and our OSes should all be on self-healing filesystems.

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