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

[Paul Eggert]

Tags: patch

These days it's better to default write-region-inhibit-fsync to t.
Here's a proposed patch, which I hope is self-explanatory.
This change speeds up "make compile-always" by 15% on my platform.

=== modified file 'doc/emacs/ChangeLog'
--- doc/emacs/ChangeLog 2013-04-04 03:46:25 +0000
+++ doc/emacs/ChangeLog 2013-04-26 06:35:33 +0000
@@ -1,3 +1,20 @@
+2013-04-26  Paul Eggert  <eggert@cs.ucla.edu>
+
+       `write-region-inhibit-fsync' now defaults to t.
+       * files.texi (Customize Save): Document this.
+       fsync can be a major performance hit.  Often it doesn't suffice to
+       make the file-save operation survive a crash anyway, so its
+       utility is 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
+
 2013-03-17  Paul Eggert  <eggert@cs.ucla.edu>
 
        doc: convert some TeX accents to UTF-8

=== modified file 'doc/emacs/files.texi'
--- doc/emacs/files.texi        2013-02-25 17:36:03 +0000
+++ doc/emacs/files.texi        2013-04-26 06:35:33 +0000
@@ -705,13 +705,20 @@
 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 programs like Emacs write 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 Emacs
+saves a file.  However, it risks data loss if the operating system
+crashes before committing the cache to disk.
+
+  To lessen this risk, set @code{write-region-inhibit-fsync} to
+@code{nil}: this causes Emacs to 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.
 
 @node Interlocking
 @subsection Protection against Simultaneous Editing

=== modified file 'etc/ChangeLog'
--- etc/ChangeLog       2013-04-24 16:50:14 +0000
+++ etc/ChangeLog       2013-04-26 06:35:33 +0000
@@ -1,3 +1,7 @@
+2013-04-26  Paul Eggert  <eggert@cs.ucla.edu>
+
+       * NEWS: `write-region-inhibit-fsync' now defaults to t.
+
 2013-04-24  Tassilo Horn  <tsdh@gnu.org>
 
        * themes/tsdh-dark-theme.el (tsdh-dark): Add ido faces and remove

=== modified file 'etc/NEWS'
--- etc/NEWS    2013-04-25 17:39:44 +0000
+++ etc/NEWS    2013-04-26 06:35:33 +0000
@@ -61,6 +61,8 @@
 ** `initial-buffer-choice' can now specify a function to set up the
 initial buffer.
 
+** `write-region-inhibit-fsync' now defaults to t instead of to nil.
+
 ** ACL support has been added.
 +++
 *** Emacs preserves the ACL entries of files when backing up.

=== modified file 'src/ChangeLog'
--- src/ChangeLog       2013-04-24 15:09:49 +0000
+++ src/ChangeLog       2013-04-26 06:35:33 +0000
@@ -1,3 +1,8 @@
+2013-04-26  Paul Eggert  <eggert@cs.ucla.edu>
+
+       * fileio.c (syms_of_fileio):
+       `write-region-inhibit-fsync' now defaults to t.
+
 2013-04-24  Kenichi Handa  <handa@gnu.org>
 
        * coding.c (decode_coding_iso_2022): When an invalid escape

=== modified file 'src/fileio.c'
--- src/fileio.c        2013-04-07 16:18:41 +0000
+++ src/fileio.c        2013-04-26 06:35:33 +0000
@@ -6072,8 +6072,9 @@
   DEFVAR_BOOL ("write-region-inhibit-fsync", write_region_inhibit_fsync,
               doc: /* Non-nil means don't call fsync in `write-region'.
 This variable affects calls to `write-region' as well as save commands.
-A non-nil value may result in data loss!  */);
-  write_region_inhibit_fsync = 0;
+Setting this to nil may avoid data loss if the system loses power or
+the operating system crashes.  */);
+  write_region_inhibit_fsync = 1;
 
   DEFVAR_BOOL ("delete-by-moving-to-trash", delete_by_moving_to_trash,
                doc: /* Specifies whether to use the system's trash can.