On Thu, Oct 09, 2014 at 08:07:51AM -0400, Austin S Hemmelgarn wrote: > On 2014-10-09 07:53, Duncan wrote: > >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. > > I'm not 100% certain, but I believe it doesn't actually fix things on disk > when it detects an error during a read,
I'm fairly sure it does, as I've had it happen to me. :) > I know it doesn't it the fs is > mounted ro (even if the media is writable), because I did some testing to > see how 'read-only' mounting a btrfs filesystem really is. If the FS is RO, then yes, it won't fix things. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Great films about cricket: Interview with the Umpire ---
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