Re: aio_fsync() a directory ?

[Christoph Hellwig <hch@lst.de>]

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."
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