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

From: Peter Lieven
Subject: Re: [Qemu-block] callout to *file in bdrv_co_get_block_status
Date: Sat, 18 Mar 2017 17:16:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Am 17.03.2017 um 15:51 schrieb Paolo Bonzini:
> On 17/03/2017 12:24, Fam Zheng wrote:
>> On Fri, 03/17 12:16, Paolo Bonzini wrote:
>>> 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.
>> I think from qemu-img convert's perspective, it doesn't care about the *file
>> status if the metadata already speaks, because, like you said, the data 
>> shows up
>> as zeroes.
> That's already the case: *file is only examined if the metadata  says

Maybe Fam meant in qemu-img this info is not that necessary because it will 
skip zeroes inside
a datablock anyway. I don't know why the lseek is soo slow, but the 
optimization could be to identify
the case where this extra info of the *file containing zeroes is really useful 
and then only call out for
it in those cases.


> Paolo

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