Re [PATCH v3 08/11] mirror: Skip writing zeroes when target is already zero

[Sunny Zhu]

on Thu 24 Apr 2025 19:52:08 -0500, Eric wrote:
>      if (s->zero_target) {
> +        int64_t bitmap_length = DIV_ROUND_UP(s->bdev_length, s->granularity);
> +
>          offset = 0;
>          bdrv_graph_co_rdlock();
>          ret = bdrv_co_is_all_zeroes(target_bs);
> @@ -856,6 +898,7 @@ static int coroutine_fn GRAPH_UNLOCKED 
> mirror_dirty_init(MirrorBlockJob *s)
>          if (ret < 0) {
>              return ret;
>          }
> +        s->zero_bitmap = bitmap_new(bitmap_length);
>          /*
>           * If the destination already reads as zero, and we are not
>           * requested to punch holes into existing zeroes, then we can
> @@ -864,6 +907,7 @@ static int coroutine_fn GRAPH_UNLOCKED 
> mirror_dirty_init(MirrorBlockJob *s)
>          if (ret > 0 &&
>              (target_bs->detect_zeroes != 
> BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP ||
>               !bdrv_can_write_zeroes_with_unmap(target_bs))) {
> +            bitmap_set(s->zero_bitmap, 0, bitmap_length);

when arg->mode != NEW_IMAGE_MODE_EXISTING && bdrv_has_zero_init(target_bs) is 
true
in drive_mirror (This means the target image is newly created), in which case
s->zero_target == false, we still need to execute bitmap_set(s->zero_bitmap, 0, 
bitmap_length) 

>              offset = s->bdev_length;
>          }
>          if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
> @@ -875,6 +919,7 @@ static int coroutine_fn GRAPH_UNLOCKED 
> mirror_dirty_init(MirrorBlockJob *s)
>          while (offset < s->bdev_length) {
>              int bytes = MIN(s->bdev_length - offset,
>                              QEMU_ALIGN_DOWN(INT_MAX, s->granularity));