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Is bit rot really a threat that I should worry about?
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this post was submitted on 01 Sep 2023
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That's not what storage engineers mean when they say "bitrot".
"Bitrot", in the scope of ZFS and BTFS means the situation where a hard-drive's "0" gets randomly flipped to "1" (or vice versa) during storage. It is a well known problem and can happen within "months". Especially as a 20-TB drive these days is a collection of 160 Trillion bits, there's a high chance that at least some of those bits malfunction over a period of ~double-digit months.
Each problem has a solution. In this case, Bitrot is "solved" by the above procedure because:
Bitrot usually doesn't happen within single-digit months. So ~6 month regular scrubs nearly guarantees that any bitrot problems you find will be limited in scope, just a few bits at the most.
Filesystems like ZFS or BTFS, are designed to handle many many bits of bitrot safely.
Scrubbing is a process where you read, and if necessary restore, any files where bitrot has been detected.
Of course, if hard drives are of noticeably worse quality than expected (ex: if you do have a large number of failures in a shorter time frame), or if you're not using the right filesystem, or if you go too long between your checks (ex: taking 25 months to scrub for bitrot instead of just 6 months), then you might lose data. But we can only plan for the "expected" kinds of bitrot. The kinds that happen within 25 months, or 50 months, or so.
If you've gotten screwed by a hard drive (or SSD) that bitrots away in like 5 days or something awful (maybe someone dropped the hard drive and the head scratched a ton of the data away), then there's nothing you can really do about that.