Re: [Qemu-devel] [PATCH v2 2/2] qcow2: truncate the tail of the image file after shrinking the image

[Max Reitz]

On 2017-09-22 11:39, 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 | 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..8dfb5393a7 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;
> +        }

My idea was rather that you just wouldn't truncate the image file if
something fails here.  So in any of these new cases where you currently
just report the whole truncate operation as having failed, you could
just emit a warning and not do the truncation of bs->file.

I can live with the current version, though, so:

Reviewed-by: Max Reitz <address@hidden>

But I'll wait for a response from you before merging this series.

Max

> +        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) {
> +                error_setg_errno(errp, -ret,
> +                                 "Failed to truncate the tail of the image");
> +                return ret;
> +            }
> +        }
>      } 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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