Re: [Qemu-block] [Qemu-devel] callout to *file in bdrv_co_get_block_status

[Peter Lieven]

Am 20.03.2017 um 12:49 schrieb Fam Zheng:
> On Mon, 03/20 12:21, Paolo Bonzini wrote:
>>
>> On 20/03/2017 03:46, Fam Zheng wrote:
>>> On Fri, 03/17 12:20, Peter Lieven wrote:
>>>> Am 17.03.2017 um 12:16 schrieb Paolo Bonzini:
>>>>> On 17/03/2017 12:11, Peter Lieven wrote:
>>>>>>>> like VMDK or QCOW2 shouldn't we trust the information from the l2 
>>>>>>>> tables in the VMDK or QCOW2?
>>>>>>> It provides additional information, for example it works better with
>>>>>>> prealloc=metadata.
>>>>>> Okay, understood. Can you imagine of a away to conditionally avoid this 
>>>>>> second callout? In my case we have an additional
>>>>>> lseek for each cluster. For a 20GB file this are approx. 327k calls to 
>>>>>> lseek. And if the file has no preallocated metadata
>>>>>> it will likely not improve anything. And even if the metadata is 
>>>>>> prealloced what is the allocation status of the clusters?
>>>>> If the metadata is preallocated, cluster will (or should) show up as
>>>>> zero, speeding up the copy.
>>>> Okay, in this case the second call out to *file will not happen. It only 
>>>> happens if the metadata says it contains data.
>>>> So where does it actually help?
>>>>
>>>> The condition is: (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) && 
>>>> (ret & BDRV_BLOCK_OFFSET_VALID))
>>>>
>>>> So from my view it can only have any effect if the metadata returns 
>>>> BDRV_BLOCK_DATA, but the protocol driver returns
>>>> BDRV_BLOCK_ZERO.
>>>>
>>>> This can only happen if I partially write to a cluster, or am I wrong here?
>>> I think you have a point. The metadata should have said BDRV_BLOCK_ZERO if
>>> protocol would say BDRV_BLOCK_ZERO - there is no reason the format driver 
>>> cannot
>>> know.
>> That's true of qcow2, but many formats (including raw :)) don't know
>> about BDRV_BLOCK_ZERO.
> Raw is a little special, it could have forwarded the call to *file in its
> BlockDriver callback. Most formats with metadata stores zero/nonzero 
> information

I thinks thats the check *file != bs for, right?


> in L1/L2 tables. For qcow2 and VMDK I think it's okay to just trust meta data 
> on
> zero/nonzero.

So what are the really Formats that we don't trust? And is it worth that
we add this (maybe expensive) callout to lseek via find_allocation for any
format altough we already know the answer from the metadata?

Peter