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

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Message-ID: <20200814120422.GA1872@infradead.org>
Date:   Fri, 14 Aug 2020 13:04:22 +0100
From:   "hch@...radead.org" <hch@...radead.org>
To:     Damien Le Moal <Damien.LeMoal@....com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        Kanchan Joshi <joshiiitr@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Kanchan Joshi <joshi.k@...sung.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "bcrl@...ck.org" <bcrl@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        SelvaKumar S <selvakuma.s1@...sung.com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        Javier Gonzalez <javier.gonz@...sung.com>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        Naohiro Aota <Naohiro.Aota@....com>
Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append

On Fri, Aug 14, 2020 at 08:27:13AM +0000, Damien Le Moal wrote:
> > 
> > O_APPEND pretty much implies out of order, as there is no way for an
> > application to know which thread wins the race to write the next chunk.
> 
> Yes and no. If the application threads do not synchronize their calls to
> io_submit(), then yes indeed, things can get out of order. But if the
> application threads are synchronized, then the offset set for each append AIO
> will be in sequence of submission, so the user will not see its writes
> completing at different write offsets than this implied offsets.

Nothing gurantees any kind of ordering for two separate io_submit calls.
The kernel may delay one of them for any reason.

Now if you are doing two fully synchronous write calls on an
O_APPEND fd, yes they are serialized.  But using Zone Append won't
change that.