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Re: [PATCH] docs: Better mention of qemu-img amend limitations

From: Eric Blake
Subject: Re: [PATCH] docs: Better mention of qemu-img amend limitations
Date: Tue, 20 Oct 2020 10:44:37 -0500
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On 9/23/20 4:56 PM, Nir Soffer wrote:
> On Wed, Sep 23, 2020 at 11:38 PM Eric Blake <eblake@redhat.com> wrote:
>> Missed during merge resolution of commit bc5ee6da71.
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> ---
>>  docs/tools/qemu-img.rst | 4 ++++
>>  1 file changed, 4 insertions(+)
>> diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
>> index c35bd6482203..2b5891b54db7 100644
>> --- a/docs/tools/qemu-img.rst
>> +++ b/docs/tools/qemu-img.rst
>> @@ -265,6 +265,10 @@ Command description:
>>    --force allows some unsafe operations. Currently for -f luks, it allows to
>>    erase the last encryption key, and to overwrite an active encryption key.
>> +  The set of options that can be amended are dependent on the image
>> +  format, but note that amending the backing chain relationship should
>> +  instead be performed with ``qemu-img rebase``.
> Because of the backing format?

Sorry for not answering this earlier (and now this serves as a ping for
the patch):

Consider if you have:

Base.raw <- Overlay.qcow2

and want to rebase it to:

Base.qcow2 <- Overlay.qcow2

where Base.raw and Base.qcow2 contain identical guest-visible content.
The problem at hand is that 'qemu-amend' needs to know what the backing
format is both before the rebase (to know how to properly read the
existing image) and after the rebase (to ensure that future opens of
Overlay.qcow2 still properly read the backing format).  But there is no
obvious command-line spelling to specify the difference between the
backing format pre-amend and the backing format post-amend.  Maybe we
could add that, but I felt it was easier to just document that instead
of trying to amend the image (which involves multiple steps, and may
require upgrading/downgrading between v2 and v3, which in turn may
affect decisions on efficiency of representing regions of zero) at the
same time as the backing file/format, it's easier to just update the
backing file/format with a stand-alone command.

I can add that to the commit message, if the patch itself is reasonable.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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