+ *
+ * The io engine must transfer in the direction noted by io_u->ddir
+ * to the buffer pointed to by io_u->xfer_buf for as many bytes as
+ * io_u->xfer_buflen. Residual data count may be set in io_u->residual
+ * for a short read/write. Should return 0 for io_u complete, < 0 for
+ * an error, and > 0 for the number of bytes transferred.