Re: [Qemu-devel] [PATCH 2/2] qcow2: truncate the tail of the image file after shrinking the image

[Max Reitz]

On 2017-09-20 15:58, Pavel Butsykin wrote:
> Now after shrinking the image, at the end of the image file, there might be a
> tail that probably will never be used. So we can find the last used cluster 
> and
> cut the tail.
> 
> Signed-off-by: Pavel Butsykin <address@hidden>
> ---
>  block/qcow2-refcount.c | 21 +++++++++++++++++++++
>  block/qcow2.c          | 19 +++++++++++++++++++
>  block/qcow2.h          |  1 +
>  3 files changed, 41 insertions(+)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 88d5a3f1ad..5e221a166c 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -3181,3 +3181,24 @@ out:
>      g_free(reftable_tmp);
>      return ret;
>  }
> +
> +int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    int64_t i, last_cluster, nb_clusters = size_to_clusters(s, size);
> +    uint64_t refcount;
> +
> +    for (i = 0, last_cluster = 0; i < nb_clusters; i++) {
> +        int ret = qcow2_get_refcount(bs, i, &refcount);
> +        if (ret < 0) {
> +            fprintf(stderr, "Can't get refcount for cluster %" PRId64 ": 
> %s\n",
> +                    i, strerror(-ret));
> +            continue;
> +        }
> +
> +        if (refcount > 0) {
> +            last_cluster = i;
> +        }
> +    }
> +    return last_cluster;
> +}

Wouldn't it make more sense to start from the end of the image?

Max

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