Re: [BUG] Btrfs scrub sometime recalculate wrong parity in raid5

[Goffredo Baroncelli <kreijack@libero.it>]

Hi

On 2016-07-16 17:51, Jarkko Lavinen wrote:
> On 07/12/2016 05:50 PM, Goffredo Baroncelli wrote:
>> Using "btrfs insp phy" I developed a script to trigger the bug.
> 
> Thank you for the script and all for sharing the raid5 and scrubbing
> issues. I have been using two raid5 arrays and ran scrub occasionally
> without any problems lately and been in false confidence. I converted
> successfully raid5 arrays into raid10 without any glitch.
> 
> I tried to modify the shell script so that instead of corrupting data
> with dd, a simulated bad block is created with device mapper. Modern
> disks are likely to either return the correct data or an error if
> they cannot.


You are right; but doing so we are complicating further the test case:
- my tests show what happen when there is a corruption, but the drive behaves well
- your tests show what happen when there is a corruption AND the drive has a failure

I agree that your simulation is more realistic, but I fear that doing so we are complicating the bug finding.

> 
> The modified script behaves very much like the original dd version.
> With dd version I see wrong data instead of expected data. 

When toy say "I see wrong data", you means with 
1) "cat mnt/out.txt" 
or 2) with "dd if=/dev/loop....." ?

In the first case I see always good data; in the second case I see wrong data but of course no reading error

> With simulated bad block I see no data at all instead of expected data
> since dd quits on read error.
> 
> Jarkko Lavinen
> 


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