Re: [PATCH v6 00/10] block atomic writes

John Garry
2024-04-12T08:16:48Z

On 11/04/2024 20:07, Luis Chamberlain wrote:
>> So if you
>> have a 4K PBS and 512B LBS, then WRITE_ATOMIC_16 would be required to write
>> 16KB atomically.
> Ugh. Why does SCSI requires a special command for this?

The actual question from others is why does NVMe not have a dedicated
command for this, like:
https://lore.kernel.org/linux-nvme/20240129062035.GB19796@lst.de/

It's a data integrity feature, and we want to know if it works properly.

>
> Now we know what would be needed to bump the physical block size, it is
> certainly a different feature, however I think it would be good to
> evaluate that world too. For NVMe we don't have such special write
> requirements.
>
> I put together this kludge with the last patches series of LBS + the
> bdev cache aops stuff (which as I said before needs an alternative
> solution) and just the scsi atomics topology + physical block size
> change to easily experiment to see what would break:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git/log/?h=20240408-lbs-scsi-kludge
>
> Using a larger sector size works but it does not use the special scsi
> atomic write.

If you are using scsi_debug driver, then you can just pass the desired
physblk_exp and sector_size args - they both default to 512B. Then you
don't need bother with sd.c atomic stuff, which I think is what you want.

>
>>>> To me, O_ATOMIC would be required for buffered atomic writes IO, as we want
>>>> a fixed-sized IO, so that would mean no mixing of atomic and non-atomic IO.
>>> Would using the same min and max order for the inode work instead?
>> Maybe, I would need to check further.
> I'd be happy to help review too.

Yeah, I'm starting to think that min and max inode would make life
easier, as we don't need to deal with the scenario of an atomic write to
a folio > atomic write size.

Thanks,
John