diff --git a/block/mirror.c b/block/mirror.c
index c2146c1..043bb7b 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -80,6 +80,12 @@
     uint64_t bytes;
 } MirrorOp;
 
+typedef enum MirrorMethod {
+    MIRROR_METHOD_COPY,
+    MIRROR_METHOD_ZERO,
+    MIRROR_METHOD_DISCARD,
+} MirrorMethod;
+
 static BlockErrorAction mirror_error_action(MirrorBlockJob *s, bool read,
                                             int error)
 {
@@ -319,6 +325,22 @@
     }
 }
 
+static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
+                               unsigned bytes, MirrorMethod mirror_method)
+{
+    switch (mirror_method) {
+    case MIRROR_METHOD_COPY:
+        return mirror_do_read(s, offset, bytes);
+    case MIRROR_METHOD_ZERO:
+    case MIRROR_METHOD_DISCARD:
+        mirror_do_zero_or_discard(s, offset, bytes,
+                                  mirror_method == MIRROR_METHOD_DISCARD);
+        return bytes;
+    default:
+        abort();
+    }
+}
+
 static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
 {
     BlockDriverState *source = s->source;
@@ -385,11 +407,7 @@
         int ret;
         int64_t io_bytes;
         int64_t io_bytes_acct;
-        enum MirrorMethod {
-            MIRROR_METHOD_COPY,
-            MIRROR_METHOD_ZERO,
-            MIRROR_METHOD_DISCARD
-        } mirror_method = MIRROR_METHOD_COPY;
+        MirrorMethod mirror_method = MIRROR_METHOD_COPY;
 
         assert(!(offset % s->granularity));
         ret = bdrv_block_status_above(source, NULL, offset,
@@ -427,22 +445,11 @@
         }
 
         io_bytes = mirror_clip_bytes(s, offset, io_bytes);
-        switch (mirror_method) {
-        case MIRROR_METHOD_COPY:
-            io_bytes = io_bytes_acct = mirror_do_read(s, offset, io_bytes);
-            break;
-        case MIRROR_METHOD_ZERO:
-        case MIRROR_METHOD_DISCARD:
-            mirror_do_zero_or_discard(s, offset, io_bytes,
-                                      mirror_method == MIRROR_METHOD_DISCARD);
-            if (write_zeroes_ok) {
-                io_bytes_acct = 0;
-            } else {
-                io_bytes_acct = io_bytes;
-            }
-            break;
-        default:
-            abort();
+        io_bytes = mirror_perform(s, offset, io_bytes, mirror_method);
+        if (mirror_method != MIRROR_METHOD_COPY && write_zeroes_ok) {
+            io_bytes_acct = 0;
+        } else {
+            io_bytes_acct = io_bytes;
         }
         assert(io_bytes);
         offset += io_bytes;
@@ -635,7 +642,7 @@
                 continue;
             }
 
-            mirror_do_zero_or_discard(s, offset, bytes, false);
+            mirror_perform(s, offset, bytes, MIRROR_METHOD_ZERO);
             offset += bytes;
         }