qcow2: Set zero flag for discarded clusters

Instead of making the backing file contents visible again after a discard
request, set the zero flag if possible (i.e. on version >= 3).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 14 insertions, 4 deletions

diff --git a/tests/qemu-iotests/046 b/tests/qemu-iotests/046
index 2d44bbb187..e0be46cf2b 100755
--- a/tests/qemu-iotests/046
+++ b/tests/qemu-iotests/046
@@ -193,6 +193,16 @@ echo "== Verify image content =="
 
 function verify_io()
 {
+    if ($QEMU_IMG info -f "$IMGFMT" "$TEST_IMG" | grep "compat: 0.10" > /dev/null); then
+        # For v2 images, discarded clusters are read from the backing file
+        # Keep the variable empty so that the backing file value can be used as
+        # the default below
+        discarded=
+    else
+        # Discarded clusters are zeroed for v3 or later
+        discarded=0
+    fi
+
     echo read -P 0 0 0x10000
 
     echo read -P 1  0x10000 0x2000
@@ -221,16 +231,16 @@ function verify_io()
     echo read -P 70 0x78000 0x6000
     echo read -P 7  0x7e000 0x2000
 
-    echo read -P 8  0x80000 0x6000
+    echo read -P ${discarded:-8} 0x80000 0x6000
     echo read -P 80 0x86000 0x2000
-    echo read -P 8  0x88000 0x2000
+    echo read -P ${discarded:-8} 0x88000 0x2000
     echo read -P 81 0x8a000 0xe000
     echo read -P 90 0x98000 0x6000
     echo read -P 9  0x9e000 0x2000
 
-    echo read -P 10  0xa0000 0x6000
+    echo read -P ${discarded:-10} 0xa0000 0x6000
     echo read -P 100 0xa6000 0x2000
-    echo read -P 10  0xa8000 0x2000
+    echo read -P ${discarded:-10} 0xa8000 0x2000
     echo read -P 101 0xaa000 0xe000
     echo read -P 110 0xb8000 0x8000