Re: [Qemu-devel] [Qemu-block] Can I only commit from active image to corresponding range of its backing file by qemu cmd?

[Max Reitz]

On 27.09.18 16:23, Alberto Garcia wrote:
> On Thu 27 Sep 2018 04:14:15 PM CEST, Max Reitz wrote:
>> On 25.09.18 16:13, Alberto Garcia wrote:
>>> On Thu 13 Sep 2018 08:37:05 PM CEST, Max Reitz wrote:
>>>> First, split .003 into the part we want to commit and the part we
>>>> don't want to commit.  This is a bit tricky without qemu-img dd @seek
>>>> (or a corresponding convert parameter), so we'll have to make do with
>>>> backing=null so we don't copy anything into the output from img.003's
>>>> backing chain.
>>>>
>>>> Or, we would have to use backing=null, but for some reason that
>>>> doesn't work.  I'll have to investigate.
>>>>
>>>> So rebase will need to do:
>>>>
>>>> $ qemu-img rebase -u -b '' img.003
>>>>
>>>> $ qemu-img convert -O qcow2 \
>>>>     "json:{'driver':'raw','offset':0,'size':1048576,\
>>>>            'file':{'driver':'qcow2',\
>>>>                    'file':{'driver':'file','filename':'img.003'}}}" \
>>>>     "json:{'driver':'null-co','size':2097152}" \
>>>>     img.003.commit.000
>>>
>>> If you only want to copy parts of a backing file I think it's much
>>> simpler if you use copy-on-read:
>>
>> And you can do it at runtime!
> 
> How would you do that? As far as I'm aware HMP's qemu-io doesn't allow
> you to enable copy-on-read, and block-stream (which uses copy-on-read)
> doesn't allow you to specify a range.

We have a copy-on-read filter driver now.

Max

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