Re: [Qemu-block] [PATCH v3 2/2] qcow2: truncate the tail of the image file after shrinking the image

[Max Reitz]

On 2017-09-28 11:27, 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>
> Reviewed-by: John Snow <address@hidden>
> ---
>  block/qcow2-refcount.c | 22 ++++++++++++++++++++++
>  block/qcow2.c          | 23 +++++++++++++++++++++++
>  block/qcow2.h          |  1 +
>  3 files changed, 46 insertions(+)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 88d5a3f1ad..aa3fd6cf17 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -3181,3 +3181,25 @@ 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;
> +
> +    for (i = size_to_clusters(s, size) - 1; i >= 0; i--) {
> +        uint64_t refcount;
> +        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));
> +            return ret;
> +        }
> +        if (refcount > 0) {
> +            return i;
> +        }
> +    }
> +    qcow2_signal_corruption(bs, true, -1, -1,
> +                            "There are no references in the refcount 
> table.");
> +    return -EIO;
> +}
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 8a4311d338..f08c69ccd9 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -3106,6 +3106,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t 
> offset,
>      new_l1_size = size_to_l1(s, offset);
>  
>      if (offset < old_length) {
> +        int64_t last_cluster, old_file_size;
>          if (prealloc != PREALLOC_MODE_OFF) {
>              error_setg(errp,
>                         "Preallocation can't be used for shrinking an image");
> @@ -3134,6 +3135,28 @@ static int qcow2_truncate(BlockDriverState *bs, 
> int64_t offset,
>                               "Failed to discard unused refblocks");
>              return ret;
>          }
> +
> +        old_file_size = bdrv_getlength(bs->file->bs);
> +        if (old_file_size < 0) {
> +            error_setg_errno(errp, -old_file_size,
> +                             "Failed to inquire current file length");
> +            return old_file_size;
> +        }
> +        last_cluster = qcow2_get_last_cluster(bs, old_file_size);
> +        if (last_cluster < 0) {
> +            error_setg_errno(errp, -last_cluster,
> +                             "Failed to find the last cluster");
> +            return last_cluster;
> +        }
> +        if ((last_cluster + 1) * s->cluster_size < old_file_size) {
> +            ret = bdrv_truncate(bs->file, (last_cluster + 1) * 
> s->cluster_size,
> +                                PREALLOC_MODE_OFF, NULL);
> +            if (ret < 0) {
> +                warn_report("Failed to truncate the tail of the image: "
> +                            "ret = %d", ret);

How about using strerror(-ret) instead?

Max

> +                ret = 0;
> +            }
> +        }
>      } else {
>          ret = qcow2_grow_l1_table(bs, new_l1_size, true);
>          if (ret < 0) {
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 5a289a81e2..782a206ecb 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -597,6 +597,7 @@ int qcow2_change_refcount_order(BlockDriverState *bs, int 
> refcount_order,
>                                  BlockDriverAmendStatusCB *status_cb,
>                                  void *cb_opaque, Error **errp);
>  int qcow2_shrink_reftable(BlockDriverState *bs);
> +int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size);
>  
>  /* qcow2-cluster.c functions */
>  int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
>

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