Austin S Hemmelgarn posted on Thu, 09 Oct 2014 07:29:23 -0400 as excerpted:
> Also, you should be running btrfs scrub regularly to correct bit-rot > and force remapping of blocks with read errors. While BTRFS > technically handles both transparently on reads, it only corrects thing > on disk when you do a scrub. AFAIK that isn't quite correct. Currently, the number of copies is limited to two, meaning if one of the two is bad, there's a 50% chance of btrfs reading the good one on first try. If btrfs reads the good copy, it simply uses it. If btrfs reads the bad one, it checks the other one and assuming it's good, replaces the bad one with the good one both for the read (which otherwise errors out), and by overwriting the bad one. But here's the rub. The chances of detecting that bad block are relatively low in most cases. First, the system must try reading it for some reason, but even then, chances are 50% it'll pick the good one and won't even notice the bad one. Thus, while btrfs may randomly bump into a bad block and rewrite it with the good copy, scrub is the only way to systematically detect and (if there's a good copy) fix these checksum errors. It's not that btrfs doesn't do it if it finds them, it's that the chances of finding them are relatively low, unless you do a scrub, which systematically checks the entire filesystem (well, other than files marked nocsum, or nocow, which implies nocsum, or files written when mounted with nodatacow or nodatasum). At least that's the way it /should/ work. I guess it's possible that btrfs isn't doing those routine "bump-into-it-and-fix-it" fixes yet, but if so, that's the first /I/ remember reading of it. Other than that detail, what you posted matches my knowledge and experience, such as it may be as a non-dev list regular, as well. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
