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Why can Linux do async network IO but not async file IO? Seems like they would be similar. What's so special about file IO?

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When non-blocking socket I/O was implemented the disk was considered to be too fast to warrant the same kind of treatment. At the API level the different is that sockets, pipes and ttys are have an additional "ready for use" state while files are always ready, because the kernel can do something to access them even if it means blocking the thread while the kernel waits for the disks to respond.

Yes, the trick is that disk device is considered "fast", so it's not selectable - it's always ready to read/write, which is a lie. I _feel_ (and think, I don't have any data to prove this) the main problem here is the file system layer, which may or may not need to block _after_ the operation is performed, and this complexity doesn't occur with sockets/pipes.

Having a always-ready state on regular files is a problem since Linux's non-blocking is going around readiness. On Windows (and I believe on Solaris/FreeBSD), completeness model is in place, so you schedule an operation, kernel does _everything_ and then you're resumed upon completion of IO.

The most general layer in Linux (VFS) doesn't really support asynchronicity, so changing this would require a bunch of changes to every tree and out-of-tree FS - not viable. However, the usual suspects (ext, xfs) actually use a bunch of other APIs as well, where the FS itself is often not involved in simple stuff like a read(2). This theoretically would allow async IO on files in many cases; I believe there is ongoing work in this direction, though. For now, only raw / O_DIRECT is async.

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