Re: Scrubbing with BTRFS Raid 5

[Duncan <1i5t5.duncan@cox.net>]

Graham Fleming posted on Tue, 21 Jan 2014 10:03:26 -0800 as excerpted:

> I want to keep playing around with BTRFSS RAID 5 and testing with it...
> assuming I have a drive with bad blocks, or let's say some inconsistent
> parity am I right in assuming that a) a btrfs scrub operation will not
> fix the stripes with bad parity

What I know is that it is said btrfs scrub doesn't work with btrfs raid5/6 
yet.  I don't know how it actually fails (tho I'd hope it simply returns 
an error to the effect that it doesn't work with raid5/6 yet) as I've not 
actually tried that mode, here.

> and b) a balance operation will not be
> successful? Or would a balance operation work to re-write parity?

Balance actually rewrites everything (well, everything matching its 
filters if a filtered balance is used, everything, if not), so it should 
rewrite parity correctly.

AFAIK, all the writing works and routine read works.  It's the error 
recovery that's still only partially implemented.  Since reading just 
reads data, not parity unless there's a dropped device or the like to 
recover from, as long as all devices are active and there's a good copy 
of the data (based on btrfs checksumming) to read, the rebalance should 
just use and rewrite that, ignoring the bad parity.



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"Every nonfree program has a lord, a master --
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