Re: [Qemu-devel] [RFC PATCH 4/4] qcow2: Add full image preallocation option

[Kevin Wolf]

Am 12.11.2013 um 08:47 hat Hu Tao geschrieben:
> This adds a preallocation=full mode to qcow2 image creation, which
> creates a non-sparse image file.
> 
> Signed-off-by: Hu Tao <address@hidden>
> ---
>  block/qcow2.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 359030f..d3ca6cf 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1385,7 +1385,13 @@ static int qcow2_change_backing_file(BlockDriverState 
> *bs,
>      return qcow2_update_header(bs);
>  }
>  
> -static int preallocate(BlockDriverState *bs)
> +enum prealloc_mode {
> +    PREALLOC_OFF = 0,
> +    PREALLOC_METADATA,
> +    PREALLOC_FULL,
> +};
> +
> +static int preallocate(BlockDriverState *bs, enum prealloc_mode mode)
>  {
>      uint64_t nb_sectors;
>      uint64_t offset;
> @@ -1394,9 +1400,12 @@ static int preallocate(BlockDriverState *bs)
>      int ret;
>      QCowL2Meta *meta;
>  
> +    assert(mode != PREALLOC_OFF);
> +
>      nb_sectors = bdrv_getlength(bs) >> 9;
>      offset = 0;
>  
> +    /* First allocate metadata in _really_ big chunks */
>      while (nb_sectors) {
>          num = MIN(nb_sectors, INT_MAX >> 9);
>          ret = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num,
> @@ -1424,6 +1433,11 @@ static int preallocate(BlockDriverState *bs)
>          offset += num << 9;
>      }
>  
> +    /* Then write zeros to the cluster data, if requested */
> +    if (mode == PREALLOC_FULL) {
> +        bdrv_zero_init(bs->file, offset, bdrv_getlength(bs));
> +    }

bdrv_zero_init() is completely optional. It is only implemented for
raw-posix and errors are not checked. You can't assume that after this
point anything is preallocated.

Kevin