diff --git a/block/mirror.c b/block/mirror.c
index 1a43304..1a4602a 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -176,6 +176,15 @@ static void mirror_read_complete(void *opaque, int ret)
     aio_context_release(blk_get_aio_context(s->common.blk));
 }

+/* Clip bytes relative to offset to not exceed end-of-file */
+static inline int64_t mirror_clip_bytes(MirrorBlockJob *s,
+                                        int64_t offset,
+                                        int64_t bytes)
+{
+    return MIN(bytes, s->bdev_length - offset);
+}
+
+/* Clip nb_sectors relative to sector_num to not exceed end-of-file */
 static inline int mirror_clip_sectors(MirrorBlockJob *s,
                                       int64_t sector_num,
                                       int nb_sectors)
@@ -184,44 +193,39 @@ static inline int mirror_clip_sectors(MirrorBlockJob *s,
                s->bdev_length / BDRV_SECTOR_SIZE - sector_num);
 }

-/* Round sector_num and/or nb_sectors to target cluster if COW is needed, and
- * return the offset of the adjusted tail sector against original. */
-static int mirror_cow_align(MirrorBlockJob *s,
-                            int64_t *sector_num,
-                            int *nb_sectors)
+/* Round offset and/or bytes to target cluster if COW is needed, and
+ * return the offset of the adjusted tail against original. */
+static int mirror_cow_align(MirrorBlockJob *s, int64_t *offset,
+                            unsigned int *bytes)
 {
     bool need_cow;
     int ret = 0;
-    int chunk_sectors = s->granularity >> BDRV_SECTOR_BITS;
-    int64_t align_sector_num = *sector_num;
-    int align_nb_sectors = *nb_sectors;
-    int max_sectors = chunk_sectors * s->max_iov;
+    int64_t align_offset = *offset;
+    unsigned int align_bytes = *bytes;
+    int max_bytes = s->granularity * s->max_iov;

-    need_cow = !test_bit(*sector_num / chunk_sectors, s->cow_bitmap);
-    need_cow |= !test_bit((*sector_num + *nb_sectors - 1) / chunk_sectors,
+    need_cow = !test_bit(*offset / s->granularity, s->cow_bitmap);
+    need_cow |= !test_bit((*offset + *bytes - 1) / s->granularity,
                           s->cow_bitmap);
     if (need_cow) {
-        bdrv_round_sectors_to_clusters(blk_bs(s->target), *sector_num,
-                                       *nb_sectors, &align_sector_num,
-                                       &align_nb_sectors);
+        bdrv_round_to_clusters(blk_bs(s->target), *offset, *bytes,
+                               &align_offset, &align_bytes);
     }

-    if (align_nb_sectors > max_sectors) {
-        align_nb_sectors = max_sectors;
+    if (align_bytes > max_bytes) {
+        align_bytes = max_bytes;
         if (need_cow) {
-            align_nb_sectors = QEMU_ALIGN_DOWN(align_nb_sectors,
-                                               s->target_cluster_size >>
-                                               BDRV_SECTOR_BITS);
+            align_bytes = QEMU_ALIGN_DOWN(align_bytes,
+                                          s->target_cluster_size);
         }
     }
-    /* Clipping may result in align_nb_sectors unaligned to chunk boundary, but
+    /* Clipping may result in align_bytes unaligned to chunk boundary, but
      * that doesn't matter because it's already the end of source image. */
-    align_nb_sectors = mirror_clip_sectors(s, align_sector_num,
-                                           align_nb_sectors);
+    align_bytes = mirror_clip_bytes(s, align_offset, align_bytes);

-    ret = align_sector_num + align_nb_sectors - (*sector_num + *nb_sectors);
-    *sector_num = align_sector_num;
-    *nb_sectors = align_nb_sectors;
+    ret = align_offset + align_bytes - (*offset + *bytes);
+    *offset = align_offset;
+    *bytes = align_bytes;
     assert(ret >= 0);
     return ret;
 }
@@ -257,10 +261,18 @@ static int mirror_do_read(MirrorBlockJob *s, int64_t sector_num,
     nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors);
     nb_sectors = MIN(max_sectors, nb_sectors);
     assert(nb_sectors);
+    assert(nb_sectors < BDRV_REQUEST_MAX_SECTORS);
     ret = nb_sectors;

     if (s->cow_bitmap) {
-        ret += mirror_cow_align(s, &sector_num, &nb_sectors);
+        int64_t offset = sector_num * BDRV_SECTOR_SIZE;
+        unsigned int bytes = nb_sectors * BDRV_SECTOR_SIZE;
+        int gap;
+
+        gap = mirror_cow_align(s, &offset, &bytes);
+        sector_num = offset / BDRV_SECTOR_SIZE;
+        nb_sectors = bytes / BDRV_SECTOR_SIZE;
+        ret += gap / BDRV_SECTOR_SIZE;
     }
     assert(nb_sectors << BDRV_SECTOR_BITS <= s->buf_size);
     /* The sector range must meet granularity because:
