@vindex write-region-inhibit-fsync
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.
+secondary storage. Although this can greatly improve performance, it
+risks data loss if the system loses power before committing the cache,
+and on some platforms other processes might not immediately notice the
+file's change.
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
+of data loss or slow notification, partly because many systems do not support
@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 (@pxref{Initial Options, Batch Mode}).
+default value is @code{t}.
Emacs never uses @code{fsync} when writing auto-save files, as these
files might lose data anyway.
@defvar write-region-inhibit-fsync
If this variable's value is @code{nil}, @code{write-region} uses the
-@code{fsync} system call after writing a file. Although this slows
-Emacs down, it lessens the risk of data loss after power failure. If
-the value is @code{t}, Emacs does not use @code{fsync}. The default
-value is @code{nil} when Emacs is interactive, and @code{t} when Emacs
-runs in batch mode. @xref{Files and Storage}.
+@code{fsync} system call after writing a file. If the value is
+@code{t}, Emacs does not use @code{fsync}. The default value is
+@code{t}. @xref{Files and Storage}.
@end defvar
@defmac with-temp-file file body@dots{}
the other is later modified; this will lose both files if the only
copy on secondary storage is lost due to media failure. Second, the
operating system might not write data to secondary storage
-immediately, which will lose the data if power is lost.
+immediately, which will lose the data if power is lost
+or if there is a media failure.
@findex write-region
Although both sorts of failures can largely be avoided by a suitably
-configured file system, such systems are typically more expensive or
-less efficient. In more-typical systems, to survive media failure you
+configured system, such systems are typically more expensive or
+less efficient. In lower-end systems, to survive media failure you
can copy the file to a different device, and to survive a power
-failure you can use the @code{write-region} function with the
+failure (or be immediately notified of a media failure) you can use
+the @code{write-region} function with the
@code{write-region-inhibit-fsync} variable set to @code{nil}.
+Although this variable is ordinarily @code{t} because that can
+significantly improve performance, it may make sense to temporarily
+bind it to @code{nil} if using Emacs to implement database-like
+transactions that survive power failure on lower-end systems.
@xref{Writing to Files}.
+On some platforms when Emacs changes a file other processes might not
+be notified of the change immediately. Setting
+@code{write-region-inhibit-fsync} to @code{nil} may improve
+notification speed in this case, though there are no guarantees.
+
@node File Names
@section File Names
@cindex file names
'frame-visible-' returns nil or 'icon' for it. This can happen, for
example, as part of preview for iconified frames.
++++
+** 'write-region-inhibit-fsync' now defaults to t in interactive mode,
+as it has in batch mode since Emacs 24.
+
\f
* Editing Changes in Emacs 30.1
umask (realmask);
valid_timestamp_file_system = 0;
-
- /* 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 et al. Understanding the robustness of SSDs under power fault.
- 11th USENIX Conf. on File and Storage Technologies, 2013 (FAST '13), 271-84
- https://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
- https://austingroupbugs.net/view.php?id=672 */
- write_region_inhibit_fsync = noninteractive;
}
void
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.
-Setting this to nil may avoid data loss if the system loses power or
-the operating system crashes. By default, it is non-nil in batch mode. */);
- write_region_inhibit_fsync = 0; /* See also `init_fileio' above. */
+By default, it is non-nil.
+
+Although setting this to nil may avoid data loss if the system loses power,
+it can be a significant performance hit in the usual case, and it doesn't
+necessarily cause file-save operations to actually survive a crash. */);
+
+ /* For more on why fsync often fails to work on today's hardware, see:
+ Zheng M et al. Understanding the robustness of SSDs under power fault.
+ 11th USENIX Conf. on File and Storage Technologies, 2013 (FAST '13), 271-84
+ https://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
+ https://austingroupbugs.net/view.php?id=672 */
+ write_region_inhibit_fsync = true;
DEFVAR_BOOL ("delete-by-moving-to-trash", delete_by_moving_to_trash,
doc: /* Specifies whether to use the system's trash can.
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