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

[Andrei Borzenkov <arvidjaar@gmail.com>]

15.07.2016 00:20, Chris Mason пишет:
> 
> 
> On 07/12/2016 05:50 PM, Goffredo Baroncelli wrote:
>> Hi All,
>>
>> I developed a new btrfs command "btrfs insp phy"[1] to further
>> investigate this bug [2]. Using "btrfs insp phy" I developed a script
>> to trigger the bug. The bug is not always triggered, but most of time
>> yes.
>>
>> Basically the script create a raid5 filesystem (using three
>> loop-device on three file called disk[123].img); on this filesystem 

Are those devices themselves on btrfs? Just to avoid any sort of
possible side effects?

>> it is create a file. Then using "btrfs insp phy", the physical
>> placement of the data on the device are computed.
>>
>> First the script checks that the data are the right one (for data1,
>> data2 and parity), then it corrupt the data:
>>
>> test1: the parity is corrupted, then scrub is ran. Then the (data1,
>> data2, parity) data on the disk are checked. This test goes fine all
>> the times
>>
>> test2: data2 is corrupted, then scrub is ran. Then the (data1, data2,
>> parity) data on the disk are checked. This test fail most of the time:
>> the data on the disk is not correct; the parity is wrong. Scrub
>> sometime reports "WARNING: errors detected during scrubbing,
>> corrected" and sometime reports "ERROR: there are uncorrectable
>> errors". But this seems unrelated to the fact that the data is
>> corrupetd or not
>> test3: like test2, but data1 is corrupted. The result are the same as
>> above.
>>
>>
>> test4: data2 is corrupted, the the file is read. The system doesn't
>> return error (the data seems to be fine); but the data2 on the disk is
>> still corrupted.
>>
>>
>> Note: data1, data2, parity are the disk-element of the raid5 stripe-
>>
>> Conclusion:
>>
>> most of the time, it seems that btrfs-raid5 is not capable to rebuild
>> parity and data. Worse the message returned by scrub is incoherent by
>> the status on the disk. The tests didn't fail every time; this
>> complicate the diagnosis. However my script fails most of the time.
> 
> Interesting, thanks for taking the time to write this up.  Is the
> failure specific to scrub?  Or is parity rebuild in general also failing
> in this case?
> 

How do you rebuild parity without scrub as long as all devices appear to
be present?