Re: [Qemu-devel] [PATCH for-2.10 13/16] block/qcow2: qcow2_calc_size_usage() for truncate

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Mon, Mar 13, 2017 at 10:41:14PM +0100, Max Reitz wrote:
> This patch extends qcow2_calc_size_usage() so it can calculate the
> additional space needed for preallocating image growth.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2.c | 137 +++++++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 98 insertions(+), 39 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 21b2b3cd53..80fb815b15 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2101,7 +2101,15 @@ done:
>      return ret;
>  }
>  
> -static uint64_t qcow2_calc_size_usage(uint64_t new_size,
> +/**
> + * Returns the number of bytes that must be allocated in the underlying file
> + * to accomodate an image growth from @current_size to @new_size.
> + *
> + * @current_size must be 0 when creating a new image. In that case, @bs is
> + * ignored; otherwise it must be valid.
> + */
> +static uint64_t qcow2_calc_size_usage(BlockDriverState *bs,
> +                                      uint64_t current_size, uint64_t new_size,
>                                        int cluster_bits, int refcount_order)
>  {
>      size_t cluster_size = 1u << cluster_bits;
> @@ -2122,47 +2130,97 @@ static uint64_t qcow2_calc_size_usage(uint64_t new_size,
>      refblock_bits = cluster_bits - (refcount_order - 3);
>      refblock_size = 1 << refblock_bits;
>  
> -    /* header: 1 cluster */
> -    meta_size += cluster_size;
> -
> -    /* total size of L2 tables */
> -    nl2e = aligned_total_size / cluster_size;
> -    nl2e = align_offset(nl2e, cluster_size / sizeof(uint64_t));
> -    meta_size += nl2e * sizeof(uint64_t);
> +    if (!current_size) {

This massive if statement effectively makes two functions: the old
qcow2_calc_size_usage() and the new qcow2_calc_size_change(bs) function.

It might be nicer to split the two functions.

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