4932 vdev incorrectly expanding on last mirror child -> top-level vdev
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>

*** 785,794 ****
--- 785,805 ----
          if (mvd->vdev_top == mvd) {
                  uint64_t guid_delta = mvd->vdev_guid - cvd->vdev_guid;
                  cvd->vdev_orig_guid = cvd->vdev_guid;
                  cvd->vdev_guid += guid_delta;
                  cvd->vdev_guid_sum += guid_delta;
+ 
+                 /*
+                  * If pool not set for autoexpand, we need to also preserve
+                  * mvd's asize to prevent automatic expansion of cvd.
+                  * Otherwise if we are adjusting the mirror by attaching and
+                  * detaching children of non-uniform sizes, the mirror could
+                  * autoexpand, unexpectedly requiring larger devices to
+                  * re-establish the mirror.
+                  */
+                 if (!cvd->vdev_spa->spa_autoexpand)
+                         cvd->vdev_asize = mvd->vdev_asize;
          }
          cvd->vdev_id = mvd->vdev_id;
          vdev_add_child(pvd, cvd);
          vdev_top_update(cvd->vdev_top, cvd->vdev_top);