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

[Alberto Garcia]

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.

Berto