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

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> 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