4334 Improve ZFS N-way mirror read performance

@@ -547,10 +547,16 @@
                     "both DKIOCGMEDIAINFO{,EXT} calls failed, %d\n",
                     vd->vdev_path, error);
                 pbsize = DEV_BSIZE;
         }
 
+        /*
+         * Determine the rotation
+         */
+        vd->vdev_rotation_rate = VDEV_RATE_UNKNOWN;
+        /* TODO: Implement when there's an ioctl which provides this info. */
+
         *ashift = highbit(MAX(pbsize, SPA_MINBLOCKSIZE)) - 1;
 
         if (vd->vdev_wholedisk == 1) {
                 int wce = 1;