Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

John Garry
2025-01-17T10:26:51Z

On 14/01/2025 23:57, Darrick J. Wong wrote:
>> i.e. RWF_ATOMIC as implemented by a COW capable filesystem should
>> always be able to succeed regardless of IO alignment. In these
>> situations, the REQ_ATOMIC block layer offload to the hardware is a
>> fast path that is enabled when the user IO and filesystem extent
>> alignment matches the constraints needed to do a hardware atomic
>> write.
>>
>> In all other cases, we implement RWF_ATOMIC something like
>> always-cow or prealloc-beyond-eof-then-xchg-range-on-io-completion
>> for anything that doesn't correctly align to hardware REQ_ATOMIC.
>>
>> That said, there is nothing that prevents us from first implementing
>> RWF_ATOMIC constraints as "must match hardware requirements exactly"
>> and then relaxing them to be less stringent as filesystems
>> implementations improve. We've relaxed the direct IO hardware
>> alignment constraints multiple times over the years, so there's
>> nothing that really prevents us from doing so with RWF_ATOMIC,
>> either. Especially as we have statx to tell the application exactly
>> what alignment will get fast hardware offloads...
> Ok, let's do that then. Just to be clear -- for any RWF_ATOMIC direct
> write that's correctly aligned and targets a single mapping in the
> correct state, we can build the untorn bio and submit it. For
> everything else, prealloc some post EOF blocks, write them there, and
> exchange-range them.

I have some doubt about this, but I may be misunderstanding the concept:

So is there any guarantee that what we write into is aligned (after the
exchange-range routine)? If not, surely every subsequent write with
RWF_ATOMIC to that logical range will require this exchange-range
routine until we get something aligned (and correct granularity) - correct?

I know that getting unaligned blocks continuously is unlikely, unless a
heavily fragmented disk. However, databases prefer guaranteed
performance (which HW offload gives).

We can use extszhint to hint at granularity, but that does not help with
alignment (AFAIK).

>
> Tricky questions: How do we avoid collisions between overlapping writes?
> I guess we find a free file range at the top of the file that is long
> enough to stage the write, and put it there? And purge it later?
>
> Also, does this imply that the maximum file size is less than the usual
> 8EB?
>
> (There's also the question about how to do this with buffered writes,
> but I guess we could skip that for now.)