Re: [Qemu-devel] [PATCH v2] qcow2: Implement bdrv_truncate() for growing images

[Kevin Wolf <kwolf@redhat.com>]

Am 28.04.2010 11:24, schrieb Stefan Hajnoczi:
> This patch adds the ability to grow qcow2 images in-place using
> bdrv_truncate().  This enables qemu-img resize command support for
> qcow2.
> 
> Snapshots are not supported and bdrv_truncate() will return -ENOTSUP.
> The notion of resizing an image with snapshots could lead to confusion:
> users may expect snapshots to remain unchanged, but this is not possible
> with the current qcow2 on-disk format where the header.size field is
> global instead of per-snapshot.  Others may expect snapshots to change
> size along with the current image data.  I think it is safest to not
> support snapshots and perhaps add behavior later if there is a
> consensus.
> 
> Backing images continue to work.  If the image is now larger than its
> backing image, zeroes are read when accessing beyond the end of the
> backing image.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
> v2:
>  * Remove superfluous qcow2_grow_l1_table()-related changes
> 
>  block/qcow2.c |   45 +++++++++++++++++++++++++++++++++++++++++----
>  block/qcow2.h |    6 ++++++
>  2 files changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 4fa3ff9..a65af41 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -140,7 +140,7 @@ static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
>  static int qcow_open(BlockDriverState *bs, int flags)
>  {
>      BDRVQcowState *s = bs->opaque;
> -    int len, i, shift;
> +    int len, i;
>      QCowHeader header;
>      uint64_t ext_end;
>  
> @@ -188,8 +188,7 @@ static int qcow_open(BlockDriverState *bs, int flags)
>  
>      /* read the level 1 table */
>      s->l1_size = header.l1_size;
> -    shift = s->cluster_bits + s->l2_bits;
> -    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
> +    s->l1_vm_state_index = size_to_l1(s, header.size);
>      /* the L1 table must contain at least enough entries to put
>         header.size bytes */
>      if (s->l1_size < s->l1_vm_state_index)
> @@ -851,6 +850,42 @@ static int qcow_make_empty(BlockDriverState *bs)
>      return 0;
>  }
>  
> +static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int ret, new_l1_size;
> +
> +    if (offset & 511) {
> +        return -EINVAL;
> +    }
> +
> +    /* cannot proceed if image has snapshots */
> +    if (s->nb_snapshots) {
> +        return -ENOTSUP;
> +    }
> +
> +    /* shrinking is currently not supported */
> +    if (offset < bs->total_sectors * 512) {
> +        return -ENOTSUP;
> +    }
> +
> +    new_l1_size = size_to_l1(s, offset);
> +    ret = qcow2_grow_l1_table(bs, new_l1_size);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* write updated header.size */
> +    offset = cpu_to_be64(offset);
> +    if (bdrv_pwrite(bs->file, offsetof(QCowHeader, size), &offset,
> +                    sizeof(uint64_t)) != sizeof(uint64_t)) {
> +        return -EIO;
> +    }

Please return the right error code here. Also, bdrv_pwrite doesn't
return short writes, so the code can look like this:

ret = bdrv_pwrite(...);
if (ret < 0) {
    return ret;
}

Otherwise I like the patch. It has actually become simple and easy to
understand.

Kevin