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.

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.).