Rust io unbuffered: O_DIRECT

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Hi,

I read that Rust IO is unbuffered by default.

When choosing to write directly to disk under Linux, do we still need to specify the O_DIRECT flag when opening the file?

Thank you

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It's not unbuffered on that level. It just means that Rust's std does not add any additional buffering, but your operating system caching is still in effect.

If you use std::fs::File to open a file for reading or writing, and you keep jumping around in the file to read/write at different locations somewhat close to each other, all those read/writes will cause read/write syscalls to the OS. However, these reads/writes will still be subject to regular caching in the operating system.

If you put the File in a BufReader/BufWriter, jumping around in the file to read and write will allow the Rust code to use a memory buffer rather than issue the read/write syscalls (until a flush).

I.e. you don't need to worry about O_DIRECT, O_SYNC et al for regular fs::File operations.

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Thanks,

So, I still need to use O_DIRECT to bypass the OS caching system.

You answered my question!

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