block/qcow2: Metadata preallocation for truncate

We can support PREALLOC_MODE_METADATA by invoking preallocate() in
qcow2_truncate().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170613202107.10125-12-mreitz@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2.c

@@ -3077,10 +3077,11 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
                           PreallocMode prealloc, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
+    uint64_t old_length;
     int64_t new_l1_size;
     int ret;
 
-    if (prealloc != PREALLOC_MODE_OFF) {
+    if (prealloc != PREALLOC_MODE_OFF && prealloc != PREALLOC_MODE_METADATA) {
         error_setg(errp, "Unsupported preallocation mode '%s'",
                    PreallocMode_lookup[prealloc]);
         return -ENOTSUP;
@@ -3104,8 +3105,10 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
         return -ENOTSUP;
     }
 
+    old_length = bs->total_sectors * 512;
+
     /* shrinking is currently not supported */
-    if (offset < bs->total_sectors * 512) {
+    if (offset < old_length) {
         error_setg(errp, "qcow2 doesn't support shrinking images yet");
         return -ENOTSUP;
     }
@@ -3117,6 +3120,32 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
         return ret;
     }
 
+    switch (prealloc) {
+    case PREALLOC_MODE_OFF:
+        break;
+
+    case PREALLOC_MODE_METADATA:
+        ret = preallocate(bs, old_length, offset);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Preallocation failed");
+            return ret;
+        }
+        break;
+
+    default:
+        g_assert_not_reached();
+    }
+
+    if (prealloc != PREALLOC_MODE_OFF) {
+        /* Flush metadata before actually changing the image size */
+        ret = bdrv_flush(bs);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret,
+                             "Failed to flush the preallocated area to disk");
+            return ret;
+        }
+    }
+
     /* write updated header.size */
     offset = cpu_to_be64(offset);
     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, size),