Re: Scrub on btrfs single device only to detect errors, not correct them?

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-12-07 09:47, Jon Panozzo wrote:
> And I'll throw this question out to everyone:
>
> Let's say I have a means of providing parity for a btrfs device, but
> in a way that's external to btrfs (imagine a btrfs single device as
> part of a hardware or software RAID).  If BTRFS detected an error
> during a scrub, and parity wasn't updated as a result (say the result
> of bitrot on the btrfs device), couldn't parity be used to repair the
> broken bit(s)?  If so, the big question is how to use scrub to
> determine the sector/bit (forgive me if I'm using wrong terminology)
> at the block level that needs to be fixed.  I think my theory is sound
> in principle, but not sure if it's possible to correlate a scrub-found
> uncorrectable error to a physical location on the block device.
>
In theory, this is possible, but it's _really_ tricky to do right. 
BTRFS uses it's own internal block addressing that is mostly independent 
from what's done at the block device level, which makes things 
non-trivial to map to actual addresses.  On top of that, it's 
non-trivial to get a address for a block that failed the scrub 
operation.  It's probably easier to just run a check on the lower-level 
device if scrub reports errors.  If that fails, then it's probably 
fixable by the lower level directly, if it passes, then the issue is 
probably a bug in BTRFS.



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