Re: Writing more than 4096 bytes with O_SYNC flag does not persist all previously written data if system crashes

[Alejandro Colomar]

Hi Ted, Andreas,

On 2026-02-19T08:32:44-0500, Theodore Tso wrote:
> +linux-man
> 
> On Wed, Feb 18, 2026 at 02:55:13PM -0700, Andreas Dilger wrote:
> > If anything, the man page should be updated to be more concise, like:
> > 
> >     "the *just written* output data *on that file descriptor* and associated
> >      file metadata have been transferred to the underlying hardware (i.e.
> >      as though each write(2) was followed by a call to sync_file_range(2)
> >      for the corresponding file offset(s))"
> 
> Yeah, this is an inaccuracy in the man page; the definition of O_SYNC
> from the Single Unix Specification states:
> 
>     O_SYNC    Write I/O operations on the file descriptor shall complete
>                                    ^^^^^^^^^^^^^^^^^^^^^^
>               as defined by synchronized I/O file integrity completion.
> 
> Compare and contrast this to what's in the Linux manpage:
> 
>        O_SYNC Write  operations  on the file will complete according to the re‐
>               quirements of synchronized I/O file integrity completion (by con‐
>               trast with the synchronized I/O data  integrity  completion  pro‐
>               vided by O_DSYNC.)
> 
>               By  the  time  write(2) (or similar) returns, the output data and
>               associated file metadata have been transferred to the  underlying
>               hardware (i.e., as though each write(2) was followed by a call to
>               fsync(2)).  See VERSIONS.
> 
> The parenthetical comment in the second paragraph needs to be removed,
> since fsync specifices that all dirty information in the page cache
> will be flushed out.

Would you mind checking the text in VERSIONS (since there's a reference
to it right next to the text you're proposing to remove)?  I suspect it
will also need to be updated accordingly.  I don't feel qualified to
touch that text by myself.

If you'd write a patch, I'd appreciate that.


Have a lovely day!
Alex

>  From the fsync man page:
> 
>        fsync() transfers ("flushes") all modified in-core data of (i.e.,  modi‐
>        fied buffer cache pages for) the file referred to by the file descriptor
>        fd  to  the  disk device (or other permanent storage device) so that all
>        changed information can be retrieved even if the system  crashes  or  is
>        rebooted.   This  includes  writing  through or flushing a disk cache if
>        present.  The call blocks until the device reports that the transfer has
>        completed.
> 
> I'll also mention that the fsync man page doesn't really talk about
> its interaction with O_DIRECT writes.  This is mentioned in the
> open(2) man page, and in general, people who use O_DIRECT are
> generally expected to know what they are doing.  But in the context of
> O_DIRECT writes, the fsync(2) call is also used to make sure that a
> CACHE FLUSH or equivalent command is sent to the storage device, such
> that the O_DIRECT write is guaranteed to persist after a power
> failure.
> 
> Cheers,
> 
> 					- Ted
> 

-- 
<https://www.alejandro-colomar.es>

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