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

Darrick J. Wong
2025-01-17T18:29:46Z

On Fri, Jan 17, 2025 at 10:26:34AM +0000, John Garry wrote:
> 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?

Correct, you'd still need forcealign to make sure that the new
allocations for exchange-range are aligned to awumin.

--D

> 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.)
>
>