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I have a large file, about 1.5 Gb. When opening the file in vim, I typically will ctrl-c escape the buffering so I don't have to wait for the whole file to load. For one particular file, it was modified by a prepackaged program, which subsequently made it made read-only.

I needed to edit this file somewhere near the beginning, so I did so and then saved it using :wq! (as prompted to do so by vim). When I exited, I noticed that the file had decreased in size from about 1.5Gb to ~200Mb. I believe it only saved what it had loaded before I exited with :wq!.

Is this a known behavior of vim? It seems like a bad design flaw that it would tell you to exit with wq! when a file is read-only, but not warn you that it won't write what hasn't buffered yet.

3

Not sure what is the question here, but this is a known behaviour.
This is the reason why loading only part of the file (with ctrl-c as you did) opens the file in read-only mode, to prevent data losses like the one you described.

You can find more information here: https://stackoverflow.com/questions/908575/how-to-edit-multi-gigabyte-text-files-vim-doesnt-work

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2

Control + C in Vim means the same as in the shell: "cancel everything you're doing as soon as possible". This includes cancelling actually reading the file

When you open a file, Vim will load it in memory ("a buffer"), and :w will write the buffer to disk. If Vim only loaded 200MB in the buffer, it will only write 200MB to disk. This is expected behaviour.

See How can I open very large files with good performance? for tips on how to improve performance.

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