diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index 4d20e103e6861208862ef0adf8d5b444b30ca56b..a9b3bac26a4e0894de21241e40afa1a491183fd8 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -1377,6 +1377,29 @@ class LURepairDiskSizes(NoHooksLU):
     self.wanted_instances = [self.cfg.GetInstanceInfo(name) for name
                              in self.wanted_names]
 
+  def _EnsureChildSizes(self, disk):
+    """Ensure children of the disk have the needed disk size.
+
+    This is valid mainly for DRBD8 and fixes an issue where the
+    children have smaller disk size.
+
+    @param disk: an L{ganeti.objects.Disk} object
+
+    """
+    if disk.dev_type == constants.LD_DRBD8:
+      assert disk.children, "Empty children for DRBD8?"
+      fchild = disk.children[0]
+      mismatch = fchild.size < disk.size
+      if mismatch:
+        self.LogInfo("Child disk has size %d, parent %d, fixing",
+                     fchild.size, disk.size)
+        fchild.size = disk.size
+
+      # and we recurse on this child only, not on the metadev
+      return self._EnsureChildSizes(fchild) or mismatch
+    else:
+      return False
+
   def Exec(self, feedback_fn):
     """Verify the size of cluster disks.
 
@@ -1422,6 +1445,9 @@ class LURepairDiskSizes(NoHooksLU):
           disk.size = size
           self.cfg.Update(instance)
           changed.append((instance.name, idx, size))
+        if self._EnsureChildSizes(disk):
+          self.cfg.Update(instance)
+          changed.append((instance.name, idx, disk.size))
     return changed