Re: [BUG] two raid consistency bugs

[Zhang Boyang <zhangboyang.id@gmail.com>]

Hi,

On 2026/7/15 05:40, Qu Wenruo wrote:
>> At first power failure during transaction N, metadata trees of
>> generation N are written to disk A, but super is not committed. Nothing
>> is written to disk B.
>>
>> At second power failure during a different transaction N,
> 
> If it's a different transaction, why it will still have the same transid N?
> 

Because the power failure occurred just before writing superblock 
(transid N). The superblock on disk still has transid N-1.

After reboot, looking at superblock which transid is N-1, btrfs has no 
idea of transid N existed previously, so it uses transid N for new 
transcation.

>> nothing is
>> written to disk A, but metadata trees and super is committed to disk B.
>>
>> This creates a ambiguous generation N in two disks. Currently btrfs
>> can't detect this, and can lead to severe damages.

By the way, I came up another solution:

If generation mismatch between devices (or log-tree mismatch) is 
detected at mount time, set a dirty flag in superblock on device which 
is behind.

If dirty flag is set for a device, disable read load balancing for that 
device. So (meta)data only read from latest device(s).

If dirty flag is detected, ask user to run a scrub. The dirty flag is 
cleared after a successful scrub.


Zhang Boyang