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Depends on the type of I/O (network, file, etc), and the async abstractions that the target operating system supplies.

Only for file access, which wasn't supported by async on Linux until io-uring.

No, any spawn_blocking will run on its own dedicated thread

I missread the comment above and read `async` instead of `sync`, my bad.

(Your response isn't entirely accurate though: it's using a thread pool whose size is configurable, not one thread per spaw_blocking)

Even then I'm pretty sure it's not _a_ dedicated thread, but a thread pool that defaults to a maximum of 512 blocking threads. There's no reason it would bottleneck unrelated writes on unrelated Files through the same thread. I tried to dig into the code here https://news.ycombinator.com/item?id=38180860

Tokio has a thread pool for blocking operations. It's not automatic, though, you have to explicitly use it.

I don't understand why some people think async is difficult to write. It's difficult if language doesn't have good support for it, but rust does. I remember async ruby was pretty weird to write initially.