Re: [Qemu-devel] [PATCH v2] vmdk: Optimize cluster allocation

[Fam Zheng <famz@redhat.com>]

On Tue, 05/06 10:32, Fam Zheng wrote:
> On mounted NFS filesystem, ftruncate is much much slower than doing a
> zero write. Changing this significantly speeds up cluster allocation.
> 
> Comparing by converting a cirros image (296M) to VMDK on an NFS mount
> point, over 1Gbe LAN:
> 
>     $ time qemu-img convert cirros-0.3.1.img /mnt/a.raw -O vmdk
> 
> Before:
>     real    0m26.464s
>     user    0m0.133s
>     sys     0m0.527s
> 
> After:
>     real    0m2.120s
>     user    0m0.080s
>     sys     0m0.197s
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> V2: Fix cluster_offset check. (Kevin)
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/vmdk.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 06a1f9f..98d2d56 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -1037,6 +1037,7 @@ static int get_cluster_offset(BlockDriverState *bs,
>      int min_index, i, j;
>      uint32_t min_count, *l2_table;
>      bool zeroed = false;
> +    int64_t ret;
>  
>      if (m_data) {
>          m_data->valid = 0;
> @@ -1110,12 +1111,20 @@ static int get_cluster_offset(BlockDriverState *bs,
>          }
>  
>          /* Avoid the L2 tables update for the images that have snapshots. */
> -        *cluster_offset = bdrv_getlength(extent->file);
> +        ret = bdrv_getlength(extent->file);
> +        if (ret < 0 ||
> +            ret & ((extent->cluster_sectors << BDRV_SECTOR_BITS) - 1)) {
> +            return VMDK_ERROR;
> +        }
> +        *cluster_offset = ret;
>          if (!extent->compressed) {
> -            bdrv_truncate(
> -                extent->file,
> -                *cluster_offset + (extent->cluster_sectors << 9)
> -            );
> +            ret = bdrv_write_zeroes(extent->file,
> +                                    *cluster_offset >> BDRV_SECTOR_BITS,
> +                                    extent->cluster_sectors,
> +                                    0);

Hi Stefan,

By considering a bdrv_write_zeroes as a pre-write, it in general doubles the
write for the whole image, so it's not a good solution.

A better way would be removing the bdrv_truncate and require the caller to do
full cluster write (with a bounce buffer if necessary).

So let's drop this patch.

Thanks,
Fam