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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 15 Jan 2020 22:05:00 -0800
Message-ID: <CAPcyv4jDMsPj_vZwDOgPkfHLELZWqeJugKgKNVKbpiZ9th683g@mail.gmail.com>
Subject: Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig <hch@lst.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Wed, Jan 15, 2020 at 9:39 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
[..]
> >         attempts to minimize software cache effects for both I/O and
> >         memory mappings of this file.  It requires a file system which
> >         has been configured to support DAX.
> >
> >         DAX generally assumes all accesses are via cpu load / store
> >         instructions which can minimize overhead for small accesses, but
> >         may adversely affect cpu utilization for large transfers.
> >
> >         File I/O is done directly to/from user-space buffers and memory
> >         mapped I/O may be performed with direct memory mappings that
> >         bypass kernel page cache.
> >
> >         While the DAX property tends to result in data being transferred
> >         synchronously, it does not give the same guarantees of
> >         synchronous I/O where data and the necessary metadata are
> >         transferred together.
>
> (I'm frankly not sure that synchronous I/O actually guarantees that the
> metadata has hit stable storage...)

Oh? That text was motivated by the open(2) man page description of O_SYNC.