diff --git a/header/src/main/java/org/zstack/header/vm/VmInstanceSpec.java b/header/src/main/java/org/zstack/header/vm/VmInstanceSpec.java
index 7007c592ae..99ee2173b9 100755
--- a/header/src/main/java/org/zstack/header/vm/VmInstanceSpec.java
+++ b/header/src/main/java/org/zstack/header/vm/VmInstanceSpec.java
@@ -847,7 +847,9 @@ public void setBootMode(String bootMode) {
 
     public long getRootDiskAllocateSize() {
         if (rootDiskOffering == null) {
-            return this.getImageSpec().getInventory().getSize();
+            long virtualSize = this.getImageSpec().getInventory().getSize();
+            long actualSize = this.getImageSpec().getInventory().getActualSize();
+            return Math.max(virtualSize, actualSize);
         }
         return rootDiskOffering.getDiskSize();
     }