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you can already send fragmented data in one system call without copying it to a single buffer.

Indeed you can, but we've found it useful to use MSG_MORE when using state machines, where different states are responsible for different parts of the reply. (Plenty of examples in states*.c here: https://gitlab.com/nbdkit/libnbd/-/tree/master/generator?ref...)

Doing more system calls isn't really a good idea for performance.

Also if you're doing asynchronous writes you typically can only have one write in-flight at any time, you should aggregate all other buffers while that happens.

Though arguably asynchronous writes are often undesired due to the complexity of doing flow-control with them.

Actually, with newer Linux kernels and io_uring, it appears it is now possible to do multiple writes asynchronously concurrently, by annotating each one with a sequencing constraint.

Whether that's really useful or not depends on whether you do the associated buffer management work.

Consider using a user-space buffer instead, syscalls are slow so bonus points for limiting them are on the table. If you want to avoid resizing an array have a look at the vectored io syscalls (readv etc.).

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