`write-region-inhibit-fsync' defaults to noninteractive.

* cmdargs.texi (Initial Options):
* files.texi (Customize Save): Document this.
* etc/NEWS: Document this.
* src/fileio.c (syms_of_fileio): Implement this.
* src/filelock.c (create_lock_file): If symbolic links don't work, so
we use a regular file as a lock file, do not fsync the lock file;
it's not needed.

Fixes: debbugs:14273

3 files changed, 29 insertions, 8 deletions

diff --git a/doc/emacs/ChangeLog b/doc/emacs/ChangeLog
index 2c47bdd4fad..348100c8408 100644
--- a/doc/emacs/ChangeLog
+++ b/doc/emacs/ChangeLog
@@ -1,3 +1,9 @@
+2013-05-05  Paul Eggert  <eggert@cs.ucla.edu>
+
+        `write-region-inhibit-fsync' defaults to noninteractive (Bug#14273).
+        * cmdargs.texi (Initial Options):
+        * files.texi (Customize Save): Document this.
+
 2013-05-04  Glenn Morris  <rgm@gnu.org>
 
         * calendar.texi (Importing Diary): Mention diary-from-outlook-function.
diff --git a/doc/emacs/cmdargs.texi b/doc/emacs/cmdargs.texi
index 19b439afc7f..e285104b5de 100644
--- a/doc/emacs/cmdargs.texi
+++ b/doc/emacs/cmdargs.texi
@@ -251,7 +251,8 @@ terminal's standard input stream (@code{stdin}) instead.
 but @file{site-start.el} is loaded nonetheless.  It also causes Emacs
 to exit after processing all the command options.  In addition, it
 disables auto-saving except in buffers for which auto-saving is
-explicitly requested.
+explicitly requested, and when saving files it omits the @code{fsync}
+system call unless otherwise requested.
 
 @item --script @var{file}
 @opindex --script
diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index 1f78747eaa6..13fa516af66 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -705,13 +705,27 @@ setting the latter variable, you can control how these modes handle
 final newlines.
 
 @vindex write-region-inhibit-fsync
-  When Emacs saves a file, it invokes the @code{fsync} system call to
-force the data immediately out to disk.  This is important for safety
-if the system crashes or in case of power outage.  However, it can be
-disruptive on laptops using power saving, as it may force a disk
-spin-up each time you save a file.  If you accept an increased risk of
-data loss, you can set @code{write-region-inhibit-fsync} to a
-non-@code{nil} value to disable the synchronization.
+  Normally, when a program writes a file, the operating system briefly
+caches the file's data in main memory before committing the data to
+disk.  This can greatly improve performance; for example, when running
+on laptops, it can avoid a disk spin-up each time a file is written.
+However, it risks data loss if the operating system crashes before
+committing the cache to disk.
+
+  To lessen this risk, Emacs can invoke the @code{fsync} system call
+after saving a file.  Using @code{fsync} does not eliminate the risk
+of data loss, partly because many systems do not implement
+@code{fsync} properly, and partly because Emacs's file-saving
+procedure typically relies also on directory updates that might not
+survive a crash even if @code{fsync} works properly.
+
+  The @code{write-region-inhibit-fsync} variable controls whether
+Emacs invokes @code{fsync} after saving a file.  The variable's
+default value is @code{nil} when Emacs is interactive, and @code{t}
+when Emacs runs in batch mode.
+
+  Emacs never uses @code{fsync} when writing auto-save files, as these
+files might lose data anyway.
 
 @node Interlocking
 @subsection Protection against Simultaneous Editing