When to flush a file in Go?

See here. File.Sync() is syscall to fsync. You should be able to find more under that name.

Keep in mind that fsync is not the same as fflush and is not executed before close.

You generally don't need to call it. The file will be written to disk anyway, after some time and if there is no power failure during this period.

You'll notice that an os.File doesn't have a .Flush() because it doesn't need one because it isn't buffered. Writes to it are direct syscalls to write to the file.

When your program exits(even if it crashes) all files it has open will be closed automatically by the operating system and the file system will write your changes to disk when it gets around to it (sometimes up to few minutes after your program exits).

Calling os.File.Sync() will call the fsync() syscall which will force the file system to flush it's buffers to disk. This will guarantee that your data is on disk and persistent even if the system is powered down or the operating system crashes.

You don't need to call .Sync()