Re: [Qemu-devel] [PATCH v8 1/9] block: add bdrv_measure() API

[Max Reitz <mreitz@redhat.com>]

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On 2017-06-14 17:35, Stefan Hajnoczi wrote:
> bdrv_measure() provides a conservative maximum for the size of a new
> image.  This information is handy if storage needs to be allocated (e.g.
> a SAN or an LVM volume) ahead of time.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Alberto Garcia <berto@igalia.com>
> ---
> v6:
>  * Change bdrv_measure() return type to BlockMeasureInfo * [Eric]
>  * Clarify that holes in sparse POSIX files are still counted [Eric]
> ---
>  qapi/block-core.json      | 25 +++++++++++++++++++++++++
>  include/block/block.h     |  2 ++
>  include/block/block_int.h |  2 ++
>  block.c                   | 35 +++++++++++++++++++++++++++++++++++
>  4 files changed, 64 insertions(+)

[...]

> diff --git a/block.c b/block.c
> index fa1d06d..056400a 100644
> --- a/block.c
> +++ b/block.c
> @@ -3446,6 +3446,41 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
>      return -ENOTSUP;
>  }
>  
> +/*
> + * bdrv_measure:
> + * @drv: Format driver
> + * @opts: Creation options for new image
> + * @in_bs: Existing image containing data for new image (may be NULL)
> + * @errp: Error object
> + * Returns: A #BlockMeasureInfo (free using qapi_free_BlockMeasureInfo())
> + *          or NULL on error
> + *
> + * Calculate file size required to create a new image.
> + *
> + * If @in_bs is given then space for allocated clusters and zero clusters
> + * from that image are included in the calculation.  If @opts contains a
> + * backing file that is shared by @in_bs then backing clusters are omitted
> + * from the calculation.

Well, it's a bit weird that qcow2 just doesn't do this, then. :-)

I'm not really complaining (I think it's the best for qcow2 to ignore
this, at least for now), but maybe an s/are/may be/ would be more
appropriate...

(I wouldn't have complained if I hadn't replied to patch 7 anyway.)

Max

> + *
> + * If @in_bs is NULL then the calculation includes no allocated clusters
> + * unless a preallocation option is given in @opts.
> + *
> + * Note that @in_bs may use a different BlockDriver from @drv.
> + *
> + * If an error occurs the @errp pointer is set.
> + */
> +BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
> +                               BlockDriverState *in_bs, Error **errp)
> +{
> +    if (!drv->bdrv_measure) {
> +        error_setg(errp, "Block driver '%s' does not support size measurement",
> +                   drv->format_name);
> +        return NULL;
> +    }
> +
> +    return drv->bdrv_measure(opts, in_bs, errp);
> +}
> +
>  /**
>   * Return number of sectors on success, -errno on error.
>   */
> 



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