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Re: [Qemu-devel] Can I only commit from active image to corresponding ra

From: Max Reitz
Subject: Re: [Qemu-devel] Can I only commit from active image to corresponding range of its backing file by qemu cmd?
Date: Thu, 13 Sep 2018 15:22:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 13.09.18 05:33, lampahome wrote:
> I split data to 3 chunks and save it in 3 independent backing files like
> below:
> img.000 <-- img.001 <-- img.002
> img.000 is the backing file of img.001 and 001 is the backing file of 002.
> img.000 saves the 1st chunk of data and img.001 saves the 2nd chunk of
> data, and img.002 saves the 3rd chunk of data.
> Now I have img.003 stores cow data of 1st chunk and img.002 is the backing
> file of img.003.
> The backing chain is like this:
>   img.000 <-- img.001 <-- img.002 <-- img.003
> So that means the data of img.003 saves the same range with img.000 but
> different data.
> I know I can use *`qemu-img commit'* but it only commit the data from
> img.003 to img.002.
> If I use *`qemu-img rebase -b img.000 img.003`*, the data of img.001 and
> img.002 will merge into img.003.
> What I want is only commit the data in img.003 into img.000 because the
> data of the two image are the same range(1st chunk)
> Is there anyway to commit(or merge) data of active image into corresponding
> backing file?

So img.000, img.001, and img.002 all contain data at completely
different areas, and img.003 only contains data where img.000 contains
data as well?

Say like so:

$ qemu-img create -f qcow2 img.000 3M
$ qemu-img create -f qcow2 -b img.000 img.001
$ qemu-img create -f qcow2 -b img.001 img.002
$ qemu-img create -f qcow2 -b img.002 img.003
$ qemu-io -c 'write -P 1 0M 1M' img.000
$ qemu-io -c 'write -P 2 1M 1M' img.001
$ qemu-io -c 'write -P 3 2M 1M' img.002
$ qemu-io -c 'write -P 4 0M 1M' img.003

(img.000 contains 1s from 0M to 1M;
 img.001 contains 2s from 1M to 2M;
 img.002 contains 3s from 2M to 3M;
 img.003 contains 4s from 0M to 1M (the range of img.000))

In that case, rebase -u might be what you want, so the following should
work (although it can easily corrupt your data if it isn't the case[1]):

$ qemu-img rebase -u -b img.000 img.003
$ qemu-img commit img.003

(And then maybe
$ qemu-img rebase -u -b img.002 img.003
to return to the previous backing chain.)


[1] It will corrupt your data if img.001 or img.002 contain any data
where img.003 also contains data; because then that data of img.003 will
be hidden when viewed through img.001 and img.002.

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