They also provide atomic read/writes to local files at
arbitrary offset and are useful for multithreaded apps.

May be we should just generalize readv/writev to also
specify offsets.  Something like

struct vio {
	void   *vio_base;
	size_t  vio_len;
	off_t   vio_off;
};

ssize_t vread(int d, const struct vio* vec, int cnt);
ssize_t vwrite(int d, const struct vio* vec, int cnt);