diff --git a/lib/test-driver/test-driver.pl b/lib/test-driver/test-driver.pl
index e4df8721936..0fdcf97af67 100644
--- a/lib/test-driver/test-driver.pl
+++ b/lib/test-driver/test-driver.pl
@@ -143,11 +143,11 @@ sub runTests {
 }
 
 
-# Create an empty qcow2 virtual disk with the given name and size (in
+# Create an empty raw virtual disk with the given name and size (in
 # MiB).
 sub createDisk {
     my ($name, $size) = @_;
-    system("qemu-img create -f qcow2 $name ${size}M") == 0
+    system("qemu-img create -f raw $name ${size}M") == 0
         or die "cannot create image of size $size";
 }