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

[Johannes Thumshirn]

On 18/05/2022 11:17, Pankaj Raghav wrote:
> On 2022-05-17 14:42, David Sterba wrote:
>> On Mon, May 16, 2022 at 06:54:11PM +0200, Pankaj Raghav wrote:
>>> Superblocks for zoned devices are fixed as 2 zones at 0, 512GB and 4TB.
>>> These are fixed at these locations so that recovery tools can reliably
>>> retrieve the superblocks even if one of the mirror gets corrupted.
>>>
>>> power of 2 zone sizes align at these offsets irrespective of their
>>> value but non power of 2 zone sizes will not align.
>>>
>>> To make sure the first zone at mirror 1 and mirror 2 align, write zero
>>> operation is performed to move the write pointer of the first zone to
>>> the expected offset. This operation is performed only after a zone reset
>>> of the first zone, i.e., when the second zone that contains the sb is FULL.
>> Is it a good idea to do the "write zeros", instead of a plain "set write
>> pointer"? I assume setting write pointer is instant, while writing
>> potentially hundreds of megabytes may take significiant time. As the
>> functions may be called from random contexts, the increased time may
>> become a problem.
>>
> 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?

Thanks,
	Johannes