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Re: [Qemu-devel] [PATCH 4/6] qemu-img: add-cow will not support convert

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH 4/6] qemu-img: add-cow will not support convert
Date: Thu, 14 Jun 2012 16:24:33 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

Am 14.06.2012 16:17, schrieb Dong Xu Wang:
> On Thu, Jun 14, 2012 at 10:11 PM, Kevin Wolf <address@hidden> wrote:
>> Am 14.06.2012 16:06, schrieb Dong Xu Wang:
>>> On Thu, Jun 14, 2012 at 6:51 PM, Kevin Wolf <address@hidden> wrote:
>>>> Am 13.06.2012 16:36, schrieb Dong Xu Wang:
>>>>> add-cow file can't live alone, must together will image_file and 
>>>>> backing_file.
>>>>> If we implement qemu-img convert operation for add-cow file format, we 
>>>>> must
>>>>> create image_file and backing_file manually, that will be confused for 
>>>>> users,
>>>>> so we just ignore add-cow format while doing converting.
>>>>> Signed-off-by: Dong Xu Wang <address@hidden>
>>>> NACK.
>>>> This stupid "let's drop the feature, it might confuse users" attitude is
>>>> known from Gnome, but not from qemu.
>>>> There's no technical reason to forbid it and anyone who manages to
>>>> create a valid add-cow image will also be able to specify the very same
>>>> options to a convert command. Also, having image format specific code in
>>>> qemu-img is evil.
>>> If I implement add-cow convert command, I am wondering which method should
>>> I use:
>>> 1) create add-cow, and its backing_file, and its image_file,  then we
>>> need 3 files.
>>> 2) create add-cow(with all bitmap marked to allocated), and its
>>> image_file, then we
>>>     need 2 files.
>>> 2) will be easier, I should let .add-cow file can live only with
>>> image_file, without backing_file.
>>> I think both 1) and 2) need add code to qemu-img.c. Or I will have to create
>>> image_file automaticly in add_cow_create function.
>>> Can you give some comments on how to implement convert? Thanks.
>> Just leave it alone and it will work.
>> qemu-img convert takes the same options as qemu-img create. So like for
>> any other image you specify the backing file with -b or -o backing_file,
>> and for add-cow images to be successfully created you also need to
>> specify -o image_file.
> "-o image_file", the image_file should be precreated? Or I should created it
> manually?

Yes, it must already exist. Just like with qemu-img create. To convert
an existing qcow2 image that has a backing file 'base.img' to an add-cow
image, you would use something like:

$ qemu-img create -f raw target.img 4G
$ qemu-img convert -O add-cow -o image_file=target.img -B base.img
source.qcow2 target.add-cow


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