Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Kanchan Joshi <joshiiitr@gmail.com>]

On Fri, Aug 14, 2020 at 1:44 PM hch@infradead.org <hch@infradead.org> wrote:
>
> On Wed, Aug 05, 2020 at 07:35:28AM +0000, Damien Le Moal wrote:
> > > the write pointer.  The only interesting addition is that we also want
> > > to report where we wrote.  So I'd rather have RWF_REPORT_OFFSET or so.
> >
> > That works for me. But that rules out having the same interface for raw block
> > devices since O_APPEND has no meaning in that case. So for raw block devices, it
> > will have to be through zonefs. That works for me, and I think it was your idea
> > all along. Can you confirm please ?
>
> Yes.  I don't think think raw syscall level access to the zone append
> primitive makes sense.  Either use zonefs for a file-like API, or
> use the NVMe pass through interface for 100% raw access.

But there are use-cases which benefit from supporting zone-append on
raw block-dev path.
Certain user-space log-structured/cow FS/DB will use the device that
way. Aerospike is one example.
Pass-through is synchronous, and we lose the ability to use io-uring.

For async uring/aio to block-dev, file-pointer will not be moved to
EoF, but that position was not very important anyway- as with this
interface we expect many async appends outstanding, all with
zone-start.
Do you think RWF_APPEND | RWF_REPORT_OFFSET_DIRECT/INDIRECT is too bad
for direct block-dev. Could you please suggest another way to go about
it?



--
Kanchan