1 files changed, 51 insertions, 1 deletions

diff --git a/block/block-copy.c b/block/block-copy.c
index e83870ff81..5d0076ac93 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -21,12 +21,14 @@
 #include "qemu/units.h"
 #include "qemu/coroutine.h"
 #include "block/aio_task.h"
+#include "qemu/error-report.h"
 
 #define BLOCK_COPY_MAX_COPY_RANGE (16 * MiB)
 #define BLOCK_COPY_MAX_BUFFER (1 * MiB)
 #define BLOCK_COPY_MAX_MEM (128 * MiB)
 #define BLOCK_COPY_MAX_WORKERS 64
 #define BLOCK_COPY_SLICE_TIME 100000000ULL /* ns */
+#define BLOCK_COPY_CLUSTER_SIZE_DEFAULT (1 << 16)
 
 typedef enum {
     COPY_READ_WRITE_CLUSTER,
@@ -342,14 +344,57 @@ void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
     }
 }
 
+static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
+                                                 Error **errp)
+{
+    int ret;
+    BlockDriverInfo bdi;
+    bool target_does_cow = bdrv_backing_chain_next(target);
+
+    /*
+     * If there is no backing file on the target, we cannot rely on COW if our
+     * backup cluster size is smaller than the target cluster size. Even for
+     * targets with a backing file, try to avoid COW if possible.
+     */
+    ret = bdrv_get_info(target, &bdi);
+    if (ret == -ENOTSUP && !target_does_cow) {
+        /* Cluster size is not defined */
+        warn_report("The target block device doesn't provide "
+                    "information about the block size and it doesn't have a "
+                    "backing file. The default block size of %u bytes is "
+                    "used. If the actual block size of the target exceeds "
+                    "this default, the backup may be unusable",
+                    BLOCK_COPY_CLUSTER_SIZE_DEFAULT);
+        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
+    } else if (ret < 0 && !target_does_cow) {
+        error_setg_errno(errp, -ret,
+            "Couldn't determine the cluster size of the target image, "
+            "which has no backing file");
+        error_append_hint(errp,
+            "Aborting, since this may create an unusable destination image\n");
+        return ret;
+    } else if (ret < 0 && target_does_cow) {
+        /* Not fatal; just trudge on ahead. */
+        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
+    }
+
+    return MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
+}
+
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
-                                     int64_t cluster_size, bool use_copy_range,
+                                     bool use_copy_range,
                                      bool compress, Error **errp)
 {
     BlockCopyState *s;
+    int64_t cluster_size;
     BdrvDirtyBitmap *copy_bitmap;
     bool is_fleecing;
 
+    cluster_size = block_copy_calculate_cluster_size(target->bs, errp);
+    if (cluster_size < 0) {
+        return NULL;
+    }
+
     copy_bitmap = bdrv_create_dirty_bitmap(source->bs, cluster_size, NULL,
                                            errp);
     if (!copy_bitmap) {
@@ -960,6 +1005,11 @@ BdrvDirtyBitmap *block_copy_dirty_bitmap(BlockCopyState *s)
     return s->copy_bitmap;
 }
 
+int64_t block_copy_cluster_size(BlockCopyState *s)
+{
+    return s->cluster_size;
+}
+
 void block_copy_set_skip_unallocated(BlockCopyState *s, bool skip)
 {
     qatomic_set(&s->skip_unallocated, skip);