The writev syscall can return early and do a partial write.

Note that it is not an error for a successful call to transfer fewer bytes than requested (see read(2) and write(2)).

The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).)

We don't currently handle this.