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

[Peter Lieven <pl@kamp.de>]

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.

Peter