+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.
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
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
+2013-05-05 Paul Eggert <eggert@cs.ucla.edu>
+
+ `write-region-inhibit-fsync' defaults to noninteractive (Bug#14273).
+ * NEWS: Document this.
+
2013-04-24 Tassilo Horn <tsdh@gnu.org>
* themes/tsdh-dark-theme.el (tsdh-dark): Add ido faces and remove
** `initial-buffer-choice' can now specify a function to set up the
initial buffer.
+** `write-region-inhibit-fsync' now defaults to t in batch mode.
+
** ACL support has been added.
+++
*** Emacs preserves the ACL entries of files when backing up.
+2013-05-05 Paul Eggert <eggert@cs.ucla.edu>
+
+ `write-region-inhibit-fsync' defaults to noninteractive (Bug#14273).
+ * fileio.c (syms_of_fileio): Implement this.
+ * 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.
+
2013-05-04 Stefan Monnier <monnier@iro.umontreal.ca>
* minibuf.c (Fread_minibuffer, Feval_minibuffer): Move to Elisp.
size.
2013-04-26 Paul Eggert <eggert@cs.ucla.edu>
+
Port better to AIX (Bug#14258).
* lisp.h (ENUM_BF) [__IBMC__]: Make it 'unsigned int' here, too,
to pacify AIX xlc.
immediate_quit = 0;
- /* fsync appears to change the modtime on BSD4.2.
- Disk full in NFS may be reported here. */
- /* mib says that closing the file will try to write as fast as NFS can do
- it, and that means the fsync here is not crucial for autosave files. */
+ /* fsync is not crucial for auto-save files, since they might lose
+ some work anyway. */
if (!auto_saving && !write_region_inhibit_fsync)
{
- /* Transfer data and metadata to disk, retrying if interrupted. Also,
- ignore EINVAL which happens when fsync is not supported on this
- file. */
+ /* Transfer data and metadata to disk, retrying if interrupted.
+ fsync can report a write failure here, e.g., due to disk full
+ under NFS. But ignore EINVAL, which means fsync is not
+ supported on this file. */
while (fsync (desc) != 0)
if (errno != EINTR)
{
file is usually more useful if it contains the deleted text. */);
Vauto_save_include_big_deletions = Qnil;
+ /* 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.
+ Hence, for now by default fsync is used only when interactive.
+
+ 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 */
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 = noninteractive;
DEFVAR_BOOL ("delete-by-moving-to-trash", delete_by_moving_to_trash,
doc: /* Specifies whether to use the system's trash can.
if (emacs_write (fd, lock_info_str, lock_info_len) != lock_info_len
|| (need_fchmod && fchmod (fd, world_readable) != 0))
err = errno;
- else
- while (fsync (fd) != 0)
- if (errno != EINTR)
- {
- if (errno != EINVAL)
- err = errno;
- break;
- }
+ /* There is no need to call fsync here, as the contents of
+ the lock file need not survive system crashes. */
if (emacs_close (fd) != 0)
err = errno;
if (!err && rename_lock_file (nonce, lfname, force) != 0)
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