Re: [PATCH 0/3] zone-append support in aio and io-uring

[Kanchan Joshi]

On Wed, Jun 17, 2020 at 11:56:34PM -0700, Christoph Hellwig wrote:
> On Wed, Jun 17, 2020 at 10:53:36PM +0530, Kanchan Joshi wrote:
> > This patchset enables issuing zone-append using aio and io-uring direct-io interface.
> >
> > For aio, this introduces opcode IOCB_CMD_ZONE_APPEND. Application uses start LBA
> > of the zone to issue append. On completion 'res2' field is used to return
> > zone-relative offset.
> >
> > For io-uring, this introduces three opcodes: IORING_OP_ZONE_APPEND/APPENDV/APPENDV_FIXED.
> > Since io_uring does not have aio-like res2, cqe->flags are repurposed to return zone-relative offset
>
> And what exactly are the semantics supposed to be?  Remember the
> unix file abstractions does not know about zones at all.
>
> I really don't think squeezing low-level not quite block storage
> protocol details into the Linux read/write path is a good idea.

I was thinking of raw block-access to zone device rather than pristine file
abstraction. And in that context, semantics, at this point, are unchanged
(i.e. same as direct writes) while flexibility of async-interface gets
added.
Synchronous-writes on single-zone sound fine, but synchronous-appends on
single-zone do not sound that fine.

> What could be a useful addition is a way for O_APPEND/RWF_APPEND writes
> to report where they actually wrote, as that comes close to Zone Append
> while still making sense at our usual abstraction level for file I/O.

Thanks for suggesting this. O and RWF_APPEND may not go well with block
access as end-of-file will be picked from dev inode. But perhaps a new
flag like RWF_ZONE_APPEND can help to transform writes (aio or uring)
into append without introducing new opcodes.
And, I think, this can fit fine on file-abstraction of ZoneFS as well.