Re: [Qemu-devel] [PATCH v6] migration/block: use blk_pwrite_zeroes for each zero cluster

[Fam Zheng <famz@redhat.com>]

On Thu, 04/13 10:34, jemmy858585@gmail.com wrote:
> From: Lidong Chen <lidongchen@tencent.com>
> 
> BLOCK_SIZE is (1 << 20), qcow2 cluster size is 65536 by default,
> this may cause the qcow2 file size to be bigger after migration.
> This patch checks each cluster, using blk_pwrite_zeroes for each
> zero cluster.
> 
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Lidong Chen <lidongchen@tencent.com>
> ---
> v6 changelog:
> Fix up some grammar in the comment.
> ---
>  migration/block.c | 35 +++++++++++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/block.c b/migration/block.c
> index 7734ff7..41c7a55 100644
> --- a/migration/block.c
> +++ b/migration/block.c
> @@ -885,6 +885,8 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>      int64_t total_sectors = 0;
>      int nr_sectors;
>      int ret;
> +    BlockDriverInfo bdi;
> +    int cluster_size;
>  
>      do {
>          addr = qemu_get_be64(f);
> @@ -919,6 +921,15 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>                      error_report_err(local_err);
>                      return -EINVAL;
>                  }
> +
> +                ret = bdrv_get_info(blk_bs(blk), &bdi);
> +                if (ret == 0 && bdi.cluster_size > 0 &&
> +                    bdi.cluster_size <= BLOCK_SIZE &&
> +                    BLOCK_SIZE % bdi.cluster_size == 0) {
> +                    cluster_size = bdi.cluster_size;
> +                } else {
> +                    cluster_size = BLOCK_SIZE;
> +                }
>              }
>  
>              if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {
> @@ -932,10 +943,30 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>                                          nr_sectors * BDRV_SECTOR_SIZE,
>                                          BDRV_REQ_MAY_UNMAP);
>              } else {
> +                int i;
> +                int64_t cur_addr;
> +                uint8_t *cur_buf;
> +
>                  buf = g_malloc(BLOCK_SIZE);
>                  qemu_get_buffer(f, buf, BLOCK_SIZE);
> -                ret = blk_pwrite(blk, addr * BDRV_SECTOR_SIZE, buf,
> -                                 nr_sectors * BDRV_SECTOR_SIZE, 0);
> +                for (i = 0; i < BLOCK_SIZE / cluster_size; i++) {
> +                    cur_addr = addr * BDRV_SECTOR_SIZE + i * cluster_size;
> +                    cur_buf = buf + i * cluster_size;
> +
> +                    if ((!block_mig_state.zero_blocks ||
> +                        cluster_size < BLOCK_SIZE) &&
> +                        buffer_is_zero(cur_buf, cluster_size)) {
> +                        ret = blk_pwrite_zeroes(blk, cur_addr,
> +                                                cluster_size,
> +                                                BDRV_REQ_MAY_UNMAP);
> +                    } else {
> +                        ret = blk_pwrite(blk, cur_addr, cur_buf,
> +                                         cluster_size, 0);
> +                    }
> +                    if (ret < 0) {
> +                        break;
> +                    }
> +                }
>                  g_free(buf);
>              }

Sorry for asking this question so late, but, before it gets too late: did you
evaluate the performance impact of this change under real world workload?

Effectively, if no cluster is zero, this patch still splits a big write into
small ones, which is the opposition of usual performance optimizations (i.e.
trying to coalesce requests).

Fam