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Re: aio_fsync() a directory ?
- From: Christoph Hellwig <hch at lst dot de>
- To: Xavier Roche <roche+kml2 at exalead dot com>
- Cc: libc-alpha at sourceware dot org, linux-fsdevel at vger dot kernel dot org
- Date: Wed, 24 Apr 2013 04:48:57 +0200
- Subject: Re: aio_fsync() a directory ?
- References: <513EEF07 dot 5090901 at 3ds dot com> <20130408212729 dot ED7592C085 at topped-with-meat dot com> <5163BA06 dot 30600 at exalead dot com> <20130411225058 dot 989CB2C07B at topped-with-meat dot com> <5167A305 dot 1010506 at exalead dot com> <5174FFBA dot 8030301 at exalead dot com>
This change is completely contrary to real world behaviour. No modern
filesystem I know of implements this behaviour as the default, and performance
with the coresponding mount options (usually -o dirsync on Linux) is terrible
as it forces a write out of the log (or corresponding action on non-log based
filesystems) and in the common case of volatile write caches a cache flush.
Retrospectively claiming this as standards behaviour in a "clarification"
is utterly wrong.
On Mon, Apr 22, 2013 at 11:15:38AM +0200, Xavier Roche wrote:
> Just to close this thread, Geoff Clare has summarized the changes regarding
> aio_fsync() and relationship wrt. directory entry data.
>
> <http://austingroupbugs.net/view.php?id=672>
>
> "In the April 18 teleconference it was agreed that the standard
> should mandate that directory operations are always synchronized
> on conforming file systems, and should include warnings about
> non-conforming configurations. The proposed changes are as follows.
>
> Changes to XBD...
>
> At page 94 line 2581-2588 section 3.376 change:
>
> For read, when the operation has been completed or diagnosed if
> unsuccessful. The read is complete only when an image of the data
> has been successfully transferred to the requesting process. If
> there were any pending write requests affecting the data to be
> read at the time that the synchronized read operation was
> requested, these write requests are successfully transferred prior
> to reading the data.
>
> For write, when the operation has been completed or diagnosed if
> unsuccessful. The write is complete only when the data specified
> in the write request is successfully transferred and all file
> system information required to retrieve the data is successfully
> transferred.
>
> to:
>
> For read operations, when the operation has been completed or
> diagnosed if unsuccessful. The operation is complete only when an
> image of the data has been successfully transferred to the
> requesting process. If there were any pending write requests or
> (if the file is a directory) directory modifications affecting
> the data to be read at the time that the synchronized read
> operation was requested, these requests are successfully
> transferred prior to reading the data.
>
> For write operations and directory modification operations, when
> the operation has been completed or diagnosed if unsuccessful. The
> operation is complete only when the written data or (if the file
> is a directory) modified directory entries have been successfully
> transferred to storage and all file system information required
> to retrieve them is successfully transferred.
>
> At page 107 line 2859 add a new XBD 4.2 section (and renumber the
> current 4.2 and all later 4.x sections):
>
> 4.2 Directory Operations
>
> All file system operations that read a directory or that modify
> the contents of a directory (for example creating, unlinking, or
> renaming a file) shall be completed as defined for synchronized
> I/O data integrity completion (see section 3.376).
>
> <small>Note: Although conforming file systems are required to
> perform all directory modifications as synchronized I/O
> operations, some file systems may support non-conforming
> configurations (for example via mount options) where
> directory modifications are not synchronized. Applications
> that rely on directory modifications being synchronized should
> only be used with such file systems in their conforming
> configuration(s).</small>
>
> Changes to XSH...
>
> At page 574 line 19833 section aio_fsync() change the APPLICATION
> USAGE section from:
>
> None.
>
> to:
>
> Refer to fdatasync() and fsync().
>
> At page 815 line 27215 section fdatasync() append to the first
> paragraph:
>
> If the file is a directory, an implicit fdatasync() is already
> performed on every I/O operation (see XBD 4.2) and consequently
> if fdatasync() is called explicitly it shall take no action and
> shall return the value 0.
>
> At page 815 line 27232 section fdatasync() change the APPLICATION
> USAGE section from:
>
> None.
>
> to:
>
> Although conforming file systems are required to complete all
> directory modifications as defined for synchronized I/O data
> integrity completion, some file systems may support non-conforming
> configurations (for example via mount options) where directory
> modifications are not synchronized. When the file system is
> configured in this way, calls to fdatasync() on directories may
> cause I/O operations to be synchronized, rather than being a no-op.
>
> At page 954 line 31987 section fsync() add a new paragraph to the
> APPLICATION USAGE section:
>
> Since conforming file systems are required to complete all
> directory modifications as defined for synchronized I/O data
> integrity completion (see XBD 4.2), calling fsync() on a directory
> only synchronizes the file attributes such as timestamps. However,
> some file systems may support non-conforming configurations (for
> example via mount options) where modifications to directory
> contents are not synchronized. When the file system is configured
> in this way, calls to fsync() on directories may cause directory
> contents to be synchronized in addition to file attributes.
>
> Changes to XRAT...
>
> At page 3444 line 115531 add a new XRAT A.4.2 section (and renumber
> the current A.4.2 and all later A.4.x sections):
>
> A.4.2 Directory Operations
>
> Earlier versions of this standard did not make clear that all directory
> modifications are performed as synchronized I/O operations, although
> that is the historical behavior and was always intended. Applications
> have no need to call fdatasync() or fsync() on a directory unless
> they want to synchronize the file attributes (using fsync()), provided
> the directory is on a conforming file system. However, since
> applications may wish to use fdatasync() or fsync() to synchronize
> directory modifications on non-conforming file systems, implementations
> are required to support fdatasync() on directories as a no-op on
> conforming file systems."
---end quoted text---