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From: Dave Chinner <david@fromorbit.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
	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>,
	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>
Subject: Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute
Date: Sat, 18 Jan 2020 20:11:05 +1100
Message-ID: <20200118091105.GA9407@dread.disaster.area> (raw)
In-Reply-To: <CAPcyv4jDMsPj_vZwDOgPkfHLELZWqeJugKgKNVKbpiZ9th683g@mail.gmail.com>

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@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.

Ugh. "synchronous I/O" means two different things, depending on
context. In the AIO context, it means "process context waits for operation
completion direct", but in the O_SYNC context, it means "we guarantee
data integrity for each I/O submitted".

Indeed, O_SYNC AIO is a thing. i.e. we can do an "async sync
write" to guarantee data integrity without directly waiting for
it. Now try describing that only using the words "synchronous
write" to describe both behaviours. :)

IOWs, if you are talking about data integrity, you need to
explicitly say "O_SYNC semantics", not "synchronous write", because
"synchronous write" is totally ambiguous without the O_SYNC context
of the open(2) man page...

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com

  parent reply index

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 19:29 [RFC PATCH V2 00/12] Enable per-file/directory DAX operations V2 ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute ira.weiny
2020-01-15 11:37   ` Jan Kara
2020-01-15 17:38     ` Darrick J. Wong
2020-01-15 19:45       ` Ira Weiny
2020-01-15 20:10         ` Dan Williams
2020-01-15 22:38           ` Ira Weiny
2020-01-16  5:39             ` Darrick J. Wong
2020-01-16  6:05               ` Dan Williams
2020-01-16  6:18                 ` Darrick J. Wong
2020-01-16  6:25                   ` Dan Williams
2020-01-18  9:11                 ` Dave Chinner [this message]
2020-01-16 17:55               ` Ira Weiny
2020-01-16 18:04                 ` Darrick J. Wong
2020-01-16 18:52                   ` Ira Weiny
2020-01-16 22:19                     ` Darrick J. Wong
2020-01-17 11:58                     ` Jan Kara
2020-01-10 19:29 ` [RFC PATCH V2 02/12] fs/xfs: Isolate the physical DAX flag from effective ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 03/12] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 04/12] fs/xfs: Clean up DAX support check ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 05/12] fs: remove unneeded IS_DAX() check ira.weiny
2020-01-16  9:38   ` Jan Kara
2020-01-16 18:47     ` Ira Weiny
2020-01-10 19:29 ` [RFC PATCH V2 06/12] fs/xfs: Check if the inode supports DAX under lock ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 07/12] fs: Add locking for a dynamic inode 'mode' ira.weiny
2020-01-13 22:12   ` Darrick J. Wong
2020-01-14  0:20     ` Ira Weiny
2020-01-14  1:03       ` Darrick J. Wong
2020-01-15 19:08         ` Ira Weiny
2020-01-16  5:40           ` Darrick J. Wong
2020-01-16 18:54             ` Ira Weiny
2020-01-10 19:29 ` [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs ira.weiny
2020-01-13 22:19   ` Darrick J. Wong
2020-01-14  0:35     ` Ira Weiny
2020-01-15  0:57       ` Ira Weiny
2020-01-15 23:52     ` Ira Weiny
2020-01-16  9:24   ` Jan Kara
2020-01-16 19:12     ` Ira Weiny
2020-01-10 19:29 ` [RFC PATCH V2 09/12] fs: Prevent mode change if file is mmap'ed ira.weiny
2020-01-13 22:22   ` Darrick J. Wong
2020-01-14  0:46     ` Ira Weiny
2020-01-14  1:30       ` Darrick J. Wong
2020-01-14 17:53         ` Ira Weiny
2020-01-15 11:34           ` Jan Kara
2020-01-15 18:24             ` Ira Weiny
2020-01-15 10:21   ` David Laight
2020-01-15 17:53     ` Ira Weiny
2020-01-10 19:29 ` [RFC PATCH V2 10/12] fs/xfs: Fix truncate up ira.weiny
2020-01-13 22:27   ` Darrick J. Wong
2020-01-14  0:40     ` Ira Weiny
2020-01-14  1:14       ` Darrick J. Wong
2020-01-14 19:00         ` Ira Weiny
2020-01-14 19:39           ` Ira Weiny
2020-01-10 19:29 ` [RFC PATCH V2 11/12] fs/xfs: Clean up locking in dax invalidate ira.weiny
2020-01-10 19:29 ` [RFC PATCH V2 12/12] fs/xfs: Allow toggle of effective DAX flag ira.weiny

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