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"what exactly the python's file.flush() is doing?" says you should first f.flush() and then os.fsync(f.fileno()) to make sure the data are written to the disk.
Furthermore, "does close() imply flush() in Python?" claims that f.close() implies a flush().

Now, the question is: should I do

os.fsync(f.fileno())
f.close()

or, does f.close() also implies an os.fsync()?

Here are the doc of Python IO, the doc of Python file close, and the source code. A quick search of 'fsync' returned no relevant info.

Daniel Chin
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2 Answers2

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I checked myself by stracing a test script and no it does not imply an fsync. f.close() implies a f.flush() because it has to. f.flush() writes the internal (user space) buffers to the associated file descriptor and you can not write to a file descriptor after closing it. However writing to the kernel buffers does not guarantee durability and this is where fsync comes into play.

Lucas Crämer
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  • Thank you very much. Therefore it looks like I should do `fsync` before displaying "you may now turn the power off", right? – Daniel Chin Oct 24 '21 at 23:20
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    @DanielChin Shutting down your system will most likely also make all kernel I/O buffering durable, but to be on the safe side you probably should enforce fsync under the assumption there are no major performance concerns. :) – Lucas Crämer Oct 25 '21 at 09:11
  • If you are going to shut it down you can instead use `os.sync()` which takes care of all the write buffers. – Petr Gladkikh Aug 17 '22 at 11:30
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I have tried both os.fsync() then f.close() and os.fsync() only. Both will be giving the same result of immediate data written to the file. The file is updated using either methods despite a sudden power off occur at the main electrical outlet and power back on later.

So, f.close() is not necessary when there's os.fsync().

Shawn Khoo
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