diff --git a/block/mirror.c b/block/mirror.c
index d04db85883..bca99ec206 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -848,23 +848,31 @@ static int coroutine_fn GRAPH_UNLOCKED mirror_dirty_init(MirrorBlockJob *s)
         target_bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP &&
         bdrv_can_write_zeroes_with_unmap(target_bs);
 
+    /* Determine if the image is already zero, regardless of sync mode.  */
     bdrv_graph_co_rdlock();
     bs = s->mirror_top_bs->backing->bs;
+    if (s->target_is_zero) {
+        ret = 1;
+    } else {
+        ret = bdrv_co_is_all_zeroes(target_bs);
+    }
     bdrv_graph_co_rdunlock();
 
-    if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
+    /* Determine if a pre-zeroing pass is necessary.  */
+    if (ret < 0) {
+        return ret;
+    } else if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
         /* In TOP mode, there is no benefit to a pre-zeroing pass.  */
-    } else if (!s->target_is_zero || punch_holes) {
+    } else if (ret == 0 || punch_holes) {
         /*
          * Here, we are in FULL mode; our goal is to avoid writing
          * zeroes if the destination already reads as zero, except
          * when we are trying to punch holes.  This is possible if
-         * zeroing happened externally (s->target_is_zero) or if we
-         * have a fast way to pre-zero the image (the dirty bitmap
-         * will be populated later by the non-zero portions, the same
-         * as for TOP mode).  If pre-zeroing is not fast, or we need
-         * to punch holes, then our only recourse is to write the
-         * entire image.
+         * zeroing happened externally (ret > 0) or if we have a fast
+         * way to pre-zero the image (the dirty bitmap will be
+         * populated later by the non-zero portions, the same as for
+         * TOP mode).  If pre-zeroing is not fast, or we need to punch
+         * holes, then our only recourse is to write the entire image.
          */
         if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
             bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, s->bdev_length);
diff --git a/tests/qemu-iotests/194 b/tests/qemu-iotests/194
index d0b9c084f5..e114c0b269 100755
--- a/tests/qemu-iotests/194
+++ b/tests/qemu-iotests/194
@@ -62,7 +62,8 @@ with iotests.FilePath('source.img') as source_img_path, \
 
     iotests.log('Waiting for `drive-mirror` to complete...')
     iotests.log(source_vm.event_wait('BLOCK_JOB_READY'),
-                filters=[iotests.filter_qmp_event])
+                filters=[iotests.filter_qmp_event,
+                         iotests.filter_block_job])
 
     iotests.log('Starting migration...')
     capabilities = [{'capability': 'events', 'state': True},
@@ -88,7 +89,8 @@ with iotests.FilePath('source.img') as source_img_path, \
 
     while True:
         event2 = source_vm.event_wait('BLOCK_JOB_COMPLETED')
-        iotests.log(event2, filters=[iotests.filter_qmp_event])
+        iotests.log(event2, filters=[iotests.filter_qmp_event,
+                                     iotests.filter_block_job])
         if event2['event'] == 'BLOCK_JOB_COMPLETED':
             iotests.log('Stopping the NBD server on destination...')
             iotests.log(dest_vm.qmp('nbd-server-stop'))
diff --git a/tests/qemu-iotests/194.out b/tests/qemu-iotests/194.out
index 6940e809cd..d02655a514 100644
--- a/tests/qemu-iotests/194.out
+++ b/tests/qemu-iotests/194.out
@@ -7,7 +7,7 @@ Launching NBD server on destination...
 Starting `drive-mirror` on source...
 {"return": {}}
 Waiting for `drive-mirror` to complete...
-{"data": {"device": "mirror-job0", "len": 1073741824, "offset": 1073741824, "speed": 0, "type": "mirror"}, "event": "BLOCK_JOB_READY", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
+{"data": {"device": "mirror-job0", "len": "LEN", "offset": "OFFSET", "speed": 0, "type": "mirror"}, "event": "BLOCK_JOB_READY", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
 Starting migration...
 {"return": {}}
 {"execute": "migrate-start-postcopy", "arguments": {}}
@@ -18,7 +18,7 @@ Starting migration...
 {"data": {"status": "completed"}, "event": "MIGRATION", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
 Gracefully ending the `drive-mirror` job on source...
 {"return": {}}
-{"data": {"device": "mirror-job0", "len": 1073741824, "offset": 1073741824, "speed": 0, "type": "mirror"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
+{"data": {"device": "mirror-job0", "len": "LEN", "offset": "OFFSET", "speed": 0, "type": "mirror"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
 Stopping the NBD server on destination...
 {"return": {}}
 Wait for migration completion on target...
diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 7292c8b342..05274772ce 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -601,13 +601,23 @@ def filter_chown(msg):
     return chown_re.sub("chown UID:GID", msg)
 
 def filter_qmp_event(event):
-    '''Filter a QMP event dict'''
+    '''Filter the timestamp of a QMP event dict'''
     event = dict(event)
     if 'timestamp' in event:
         event['timestamp']['seconds'] = 'SECS'
         event['timestamp']['microseconds'] = 'USECS'
     return event
 
+def filter_block_job(event):
+    '''Filter the offset and length of a QMP block job event dict'''
+    event = dict(event)
+    if 'data' in event:
+        if 'offset' in event['data']:
+            event['data']['offset'] = 'OFFSET'
+        if 'len' in event['data']:
+            event['data']['len'] = 'LEN'
+    return event
+
 def filter_qmp(qmsg, filter_fn):
     '''Given a string filter, filter a QMP object's values.
     filter_fn takes a (key, value) pair.'''