[PATCH v4 08/13] btrfs:zoned: make sb for npo2 zone devices align with sb log offsets

[Pankaj Raghav]

On 5/19/22 09:57, Johannes Thumshirn wrote:
>> Unfortunately it is not possible to just move the WP in zoned devices.
>> The only alternative that I could use is to do write zeroes which are
>> natively supported by some devices such as ZNS. It would be nice to know
>> if someone had a better solution to this instead of doing write zeroes
>> in zoned devices.
>>
> 
> I have another question. In case we need to pad the sb zone with a write
> zeros and have a power fail between the write-zeros and the regular 
> super-block write, what happens? I know this padding is only done for the
> backup super blocks, never the less it can happen and it can happen when
> the primary super block is also corrupted.
> 
> AFAIU we're then trying to reach out for a backup super block, look at the
> write pointer and it only contains zeros but no super block, as only the 
> write-zeros has reached the device and not the super block write.
> 
> How is this situation handled?
> 
That is a very good point. I did think about this situation while adding
padding to the mirror superblock with write zeroes. If the drive is
**less than 4TB** and with the **primary superblock corrupted**, then it
will be an issue with the situation you have described for npo2 drives.
That situation is not handled here. Ofc this is not an issue when we
have the second mirror at 4TB for bigger drives. Do you have some ideas
in mind for this failure mode?
> Thanks,
> 	Johannes