Vim rewrite whole file on save?

Question

I opened a large file, didn't make a modification, and pressed :w. Then Vim was frozen for a while. Was it simply rewriting the whole file? Is it possible to make it understand that there was no change so that it could just quit? To generalize a little bit, is it possible to let it not rewrite the whole file if I only modified a small part of it?

Answer 1

Instead of :write you should use :update. According to vim's help:

                            *:up* *:update*
:[range]up[date][!] [++opt] [>>] [file]
            Like ":write", but only write when the buffer has been
            modified.  {not in Vi}

As stated, this will only write out the file to disk if there have been modifications known to vim. You would use :write instead if, for example, you have changed or corrupted the file on disk and would like to write vim's buffer contents again.

Answer 2

I did a short check with strace on Linux. I edit README.md and add one character to the last line.

Here is the relevant part:

01: stat("README.md", {st_mode=S_IFREG|0644, st_size=5986, ...}) = 0
02: stat("README.md~", {st_mode=S_IFREG|0644, st_size=5988, ...}) = 0
03: unlink("README.md~")                    = 0
04: rename("README.md", "README.md~")       = 0
05: open("README.md", O_WRONLY|O_CREAT, 0644) = 3
06: ftruncate(3, 0)                         = 0
07: write(3, "![Vim Logo](https://github.com/v"..., 5987) = 5987
08: fsync(3)                                = 0
09: stat("README.md", {st_mode=S_IFREG|0644, st_size=5987, ...}) = 0
10: stat("README.md", {st_mode=S_IFREG|0644, st_size=5987, ...}) = 0
11: fchmod(3, 0100644)                      = 0
12: close(3)                                = 0

The lines 02-04 show how the backup file is created. Here the original file is moved to README.md~ (a already existing README.md~ is deleted in in line 03).

Then the new file README.md is written line 05-12. In line 06 the call to ftruncate truncates the file to zero (irrelevant here, as this is a new file) and then the complete content is written (5987 byte).

Note that depending on the option backupcopy, the backup file README.md~ might be written, instead of just renaming README.md to README.md~.

Here with set nobackup:

01: open("README.md", O_WRONLY|O_CREAT, 0644) = 3
02: ftruncate(3, 0)                         = 0
03: write(3, "![Vim Logo](https://github.com/v"..., 5987) = 5987
04: fsync(3)                                = 0
05: stat("README.md", {st_mode=S_IFREG|0644, st_size=5987, ...}) = 0
06: stat("README.md", {st_mode=S_IFREG|0644, st_size=5987, ...}) = 0
07: fchmod(3, 0100644)                      = 0
08: close(3)                                = 0

Here no backup file is created, but also the file README.md is first truncated to size zero and then the complete content is written (5987 byte).

My conclusion: Vim always writes the entire file.

I'm not aware that there is any option to just write changed parts. Also this would be not trivial to implement.