CODEWITHSUNDEEP
rust
aurexav
aurexav

Reputation: 2875

What's the difference between `flush` and `sync_all`?

I'm curious whether I should use Write::flush or File::sync_all when I finish writing a file.

Upvotes: 9

Views: 5070

Answers (2)

hackape
hackape

Reputation: 19987

Write::flush for on-disk file is actually a no-op on Unix and Windows [source]. It's useless for File, just impl for consistency. This interface is meant for stream that utilizes app-level in-memory buffer before writing into destination, as stated in the doc:

Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

File::sync_data is the kinda like the useful version of flush for File. Under the hood, intermediate buffer is used on kernel-level, and sync_data delegates to fdatasync POSIX call, which does what flush does on app-level, .

File::sync_all does what File::sync_data does, and on top of that, it also ensure metadata about a file is written to disk. It delegates to fsync on POSIX system.

Sidenote: depending on system (e.g. macOS, android, etc.), implementation for File::sync_data and File::sync_all could be exactly the same.

Upvotes: 5

Zeta
Zeta

Reputation: 105945

TL;DR: If you want to "ensure" that the data has been written to the device, then use File::sync_all if you use a File. Note that this isn't necessary though.

The Write::flush implementation for File uses the operating system dependent flush operation, for example std::sys::unix::File::flush, or std::sys::windows::File::flush. Those flush operations do... nothing. Both implementations just return Ok(()).

Why? Because the write() already uses the underlying system call for write() in both cases; the handle-based write on Windows, and the file descriptor-based write on Unix-like systems. At that point, it's out of reach of the Rust environment, save for a system call that's specific to files.

So what is Write::flush useful for? It's useful if you have any kind of buffer before the actual file, for example a BufWriter. If you have a File wrapped by a BufWriter, then you need to use flush to ensure that the bytes get written to the file. While it's useful to keep in mind that BufWriter's Drop implementation also tries(!) to write those bytes, it may or may not work, so you're supposed to call Write::flush there (see BufWriter's documentation).

That being said, sync_all isn't necessary and instead will block your program. The operating system will handle the file system synchronisation. While you can certainly wait for that synchronisation to happen via sync_data or sync_all, you're usually better of with not doing either.

Upvotes: 7

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