Does Socket IO involve Disk IO?

Question

If one process sends data through a socket to another process on the same machine how likely is it that a disk read/write will occur during transmission? There seems to be a socket file type, are these guaranteed to be in memory provided there is free memory?

Answer 1

Not directly. TCP / UDP network sockets, over localhost, or a UNIX Domain Socket will operate in memory. UNIX Domain Sockets are typically the fastest option outside of dropping into kernel space with a module.

sockets over localhost pipes are nearly as simple as a couple of memcpy's between userspace and kernel space and back. In TCP case, you have the stack overhead.

Both files and sockets share the kernel abstraction of descriptor table, but that doesn't imply an actual file.

Of course, the database may trigger some write to a log, as a result of your transaction.

Answer 2

In the POSIX model, as well as many other kernels, files are not only in disks. Instead, every device is represented by a "special file". They live in directories or some sort of namespace, but accessing them is not disk access, even if they are placed in a directory on disk.

If you have memory pressure, then it's possible for some of your data buffers to get swapped out. But this has nothing to do with the "file" nature of devices. It's just using the disk as additional RAM.

So "Yes, socket I/O is file I/O, but not disk read/write."