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

From: Paolo Bonzini
Subject: Re: [Qemu-block] callout to *file in bdrv_co_get_block_status
Date: Mon, 20 Mar 2017 12:21:32 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

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) && 
>> So from my view it can only have any effect if the metadata returns 
>> BDRV_BLOCK_DATA, but the protocol driver returns
>> 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


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