Re: [Qemu-devel] [RFC PATCH v2 5/6] qcow2: implement bdrv_preallocate

[Fam Zheng <famz@redhat.com>]

On 2013年11月27日 18:03, Peter Lieven wrote:
> Am 27.11.2013 07:40, schrieb Fam Zheng:
>> On 2013年11月27日 14:01, Hu Tao wrote:
>>> On Wed, Nov 27, 2013 at 11:01:23AM +0800, Fam Zheng wrote:
>>>> On 2013年11月27日 10:15, Hu Tao wrote:
>>>>> Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
>>>>> ---
>>>>>    block/qcow2.c | 7 +++++++
>>>>>    1 file changed, 7 insertions(+)
>>>>>
>>>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>>>> index b054a01..a23fade 100644
>>>>> --- a/block/qcow2.c
>>>>> +++ b/block/qcow2.c
>>>>> @@ -2180,6 +2180,12 @@ static int qcow2_amend_options(BlockDriverState *bs,
>>>>>        return 0;
>>>>>    }
>>>>>
>>>>> +static int qcow2_preallocate(BlockDriverState *bs, int64_t offset,
>>>>> +                             int64_t length)
>>>>> +{
>>>>> +    return bdrv_preallocate(bs->file, offset, length);
>>>>> +}
>>>>> +
>>>>
>>>> What's the semantics of .bdrv_preallocate? I think you should map
>>>> [offset, offset + length) to clusters in image file, and then
>>>> forward to bs->file, rather than this direct wrapper.
>>>>
>>>> E.g. bdrv_preallocate(qcow2_bs, 0, cluster_size) should call
>>>> bdrv_preallocate(qcow2_bs->file, offset_off_first_cluster,
>>>> cluster_size).
>>>
>>> You mean data clusters here, right? Is there a single function to get
>>> the offset of the first data cluster?
>>>
>>
>> There is a function, qcow2_get_cluster_offset.
> This should return no valid offset as long as the cluster is not allocated.
>
> I think you actually have to "write" all clusters of a qcow2 one by one.
> Eventually this write could be an fallocate call instead of a zero write.
>

Yes, I was wrong about qcow2_get_cluster_offset. The logic here is more 
like cluster allocation in qcow2_alloc_cluster_offset. Maybe we can 
reuse that.

Fam