uses cloning on CoW filesystems like btrfs

This improves the time it takes to write the changes to disk.

core/oci.go

@@ -47,12 +47,7 @@ func OciExportRootFs(buildImageName string, imageRecipe *ImageRecipe, transDir s
 		return err
 	}
 
-	// cleanup dest
-	err = os.RemoveAll(dest)
-	if err != nil {
-		PrintVerboseErr("OciExportRootFs", 2, err)
-		return err
-	}
+	// create dest if it doesn't exist
 	err = os.MkdirAll(dest, 0755)
 	if err != nil {
 		PrintVerboseErr("OciExportRootFs", 3, err)
@@ -93,7 +88,7 @@ func OciExportRootFs(buildImageName string, imageRecipe *ImageRecipe, transDir s
 	}
 
 	// copy mount dir contents to dest
-	err = rsyncCmd(mountDir+"/", dest, []string{}, false)
+	err = rsyncCmd(mountDir+"/", dest, []string{"--delete"}, false)
 	if err != nil {
 		PrintVerboseErr("OciExportRootFs", 9, err)
 		return err

core/system.go

@@ -703,6 +703,19 @@ func (s *ABSystem) RunOperation(operation ABSystemOperation) error {
 			PrintVerboseErr("ABSystemRunOperation", 4.1, err)
 			return err
 		}
+	} else {
+		PrintVerbose("ABSystemRunOperation: Creating a reflink clone of the old system to copy into")
+		err := os.RemoveAll(systemNew)
+		if err != nil {
+			PrintVerbose("ABSystemRunOperation:err(4.0.2): could not cleanup old systemNew folder %s", err)
+			return err
+		}
+		err = exec.Command("cp", "--reflink", "-a", systemOld, systemNew).Run()
+		if err != nil {
+			PrintVerbose("ABSystemRunOperation:warn(4.0.3): reflink copy of system failed, falling back to slow copy, reason: %s", err)
+			// can be safely ignored
+			// file system doesn't support CoW
+		}
 	}
 
 	err = OciExportRootFs(