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From:   Kanchan Joshi <joshiiitr@gmail.com>
Date:   Mon, 7 Sep 2020 12:31:42 +0530
Message-ID: <CA+1E3r+WXC_MK5Zf2OZEv17ddJDjtXbhpRFoeDns4F341xMhow@mail.gmail.com>
Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
To:     "hch@infradead.org" <hch@infradead.org>
Cc:     Damien Le Moal <Damien.LeMoal@wdc.com>,
        Jens Axboe <axboe@kernel.dk>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Kanchan Joshi <joshi.k@samsung.com>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
        "bcrl@kvack.org" <bcrl@kvack.org>,
        Matthew Wilcox <willy@infradead.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-aio@kvack.org" <linux-aio@kvack.org>,
        "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        SelvaKumar S <selvakuma.s1@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>,
        Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        Naohiro Aota <Naohiro.Aota@wdc.com>
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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