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

From: Peter Lieven
Subject: Re: [Qemu-block] [Qemu-devel] callout to *file in bdrv_co_get_block_status
Date: Mon, 20 Mar 2017 17:43:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Am 20.03.2017 um 15:05 schrieb Paolo Bonzini:
> On 20/03/2017 14:35, Peter Lieven wrote:
>> Am 20.03.2017 um 14:23 schrieb Paolo Bonzini:
>>> On 20/03/2017 14:13, Peter Lieven wrote:
>>>> Am 20.03.2017 um 13:47 schrieb Peter Lieven:
>>>>> commit 5daa74a6ebce7543aaad178c4061dc087bb4c705
>>>>> Author: Paolo Bonzini <address@hidden>
>>>>> Date:   Wed Sep 4 19:00:38 2013 +0200
>>>>>     block: look for zero blocks in bs->file
>>>>>     Reviewed-by: Eric Blake <address@hidden>
>>>>>     Signed-off-by: Paolo Bonzini <address@hidden>
>>>>>     Signed-off-by: Stefan Hajnoczi <address@hidden>
>>>>> It was introduced while introducing bdv_get_block_status. I don't know 
>>>>> what the real
>>>>> issue was that was addressed with this patch?
>>>> Is it possible that this optimization was added especially for RAW? I was 
>>>> believing that
>>>> raw would forward the get_block_status call to bs->file, but it looks it 
>>>> doesn't.
>>>> If this one here was for RAW would it be an option to move this callout to 
>>>> the raw-format driver
>>>> and remove it from the generic code?
>>> It was meant for both raw and qcow2.
>> Okay, but as Fam mentioned qcow2 Metadata should know that a cluster is 
>> zero. Do you remember
>> what the issue was?
> I said that already---preallocated metadata.  Also, at the time
> pre-qcow2v3 was more important.

Yes, but Fam said that with preallocated metadata the clusters should be zero, 
or was that
not true before qcow2v3?

> Are you using libiscsi, block devices or files?

Its a mixture. raw with libiscsi or lvm and qcow2 and vmdk either with libnfs 
or on local storage.

I stumbled across the issue with lseek on a tmpfs because in the build process 
for our templates
I temporarily have vmdks on a tmpfs and it takes ages before qemu-img convert 
starts to run (it iterates
over every 64kb cluster with that callout to find_allocation and for some 
reason lseek is very slow on tmpfs).


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