linux-ext4 - Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute

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Date:   Wed, 15 Jan 2020 22:18:04 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute

On Wed, Jan 15, 2020 at 10:05:00PM -0800, Dan Williams wrote:
> On Wed, Jan 15, 2020 at 9:39 PM Darrick J. Wong <darrick.wong@...cle.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.

Eh, that's just me being cynical about software.  Yes, the O_SYNC docs
say that data+metadata are supposed to happen; that's good enough for
another section in the man pages. :)

--D