Commit cbee2131 authored by Paul Eggert's avatar Paul Eggert
Browse files

`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
parent fd3a9a6b
2013-05-05 Paul Eggert <>
`write-region-inhibit-fsync' defaults to noninteractive (Bug#14273).
* cmdargs.texi (Initial Options):
* files.texi (Customize Save): Document this.
2013-05-04 Glenn Morris <>
* calendar.texi (Importing Diary): Mention diary-from-outlook-function.
......@@ -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
......@@ -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
2013-05-05 Paul Eggert <>
`write-region-inhibit-fsync' defaults to noninteractive (Bug#14273).
* NEWS: Document this.
2013-04-24 Tassilo Horn <>
* themes/tsdh-dark-theme.el (tsdh-dark): Add ido faces and remove
......@@ -61,6 +61,8 @@ simply disabling Transient Mark mode does the same thing.
** `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 <>
`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 <>
* minibuf.c (Fread_minibuffer, Feval_minibuffer): Move to Elisp.
......@@ -51,6 +59,7 @@
2013-04-26 Paul Eggert <>
Port better to AIX (Bug#14258).
* lisp.h (ENUM_BF) [__IBMC__]: Make it 'unsigned int' here, too,
to pacify AIX xlc.
......@@ -4979,15 +4979,14 @@ This calls `write-region-annotate-functions' at the start, and
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)
......@@ -6069,11 +6068,29 @@ in the buffer; this is the default behavior, because the auto-save
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
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 */
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.
......@@ -437,14 +437,8 @@ create_lock_file (char *lfname, char *lock_info_str, bool force)
if (emacs_write (fd, lock_info_str, lock_info_len) != lock_info_len
|| (need_fchmod && fchmod (fd, world_readable) != 0))
err = errno;
while (fsync (fd) != 0)
if (errno != EINTR)
if (errno != EINVAL)
err = errno;
/* 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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