vdev_read
return (vdev_read(NULL, (void *)(uintptr_t)ppa->fd,
ret = vdev_probe(vdev_read, vdev_write, (void *)(uintptr_t)fd, &spa);
vdev_create(uint64_t guid, vdev_read_t *vdev_read)
vdev->v_read = vdev_read;
if (vdev_read != NULL) {