bug#14273: `write-region-inhibit-fsync' should default to t

[Stefan Monnier]

> How about the attached patch instead?  It causes Emacs to use fsync by
> default only when operating interactively.

That looks OK for now.

> +     fsync can be a significant performance hit.  Often it doesn't
> +     suffice to make the file-save operation survive a crash.  For
> +     batch scripts, which are typically part of larger shell commands
> +     that don't fsync other files, its effect on performance can be
> +     significant so its utility is particularly questionable.
> +     For more on why fsync often fails to work on today's hardware, see:
> +     Zheng M, Tucek J, Qin F, Lillibridge M. Understanding the
> +     robustness of SSDs under power fault. 11th USENIX Conference on
> +     File and Storage Technologies, 2013 (FAST '13), 271-84
> +     http://www.usenix.org/system/files/conference/fast13/fast13-final80.pdf
> +     For more on why fsync does not suffice even if it works properly, see:
> +     Roche X. Necessary step(s) to synchronize filename operations on disk.
> +     Austin Group Defect 672, 2013-03-19
> +     http://austingroupbugs.net/view.php?id=672
> +     * cmdargs.texi (Initial Options):
> +     * files.texi (Customize Save): Document this.

But this longish text does not belong in the commitlog/changelog but in
the code.


        Stefan