Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Eric Blake <eblake@redhat.com>]

On 4/22/20 10:21 AM, Kevin Wolf wrote:
> If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> undo any previous preallocation, but just adds the zero flag to all
> relevant L2 entries. If an external data file is in use, a write_zeroes
> request to the data file is made instead.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/qcow2.c | 30 ++++++++++++++++++++++++++++++
>   1 file changed, 30 insertions(+)
> 

> @@ -4214,6 +4215,35 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>           g_assert_not_reached();
>       }
>   
> +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
> +        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);

This rounds up beyond the new size...

> +
> +        /* Use zero clusters as much as we can */
> +        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 0);

and then requests that the extra be zeroed.  Does that always work, even 
when it results in pdrv_co_pwrite_zeroes beyond the end of s->data_file? 
  If so,

Reviewed-by: Eric Blake <eblake@redhat.com>

otherwise, you may have to treat the tail specially, the same way you 
treated an unaligned head.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org