From: Eric Blake <eblake@redhat.com>
To: Nir Soffer <nsoffer@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>,
QEMU Developers <qemu-devel@nongnu.org>,
qemu-block <qemu-block@nongnu.org>
Subject: Re: [PATCH] docs: Better mention of qemu-img amend limitations
Date: Thu, 12 Nov 2020 15:10:11 -0600 [thread overview]
Message-ID: <b2e0d546-e28e-264b-38ba-1fe094d8a45f@redhat.com> (raw)
In-Reply-To: <02fe0ea2-a2a0-2618-96f5-df1d7b91d169@redhat.com>
Ping^2
On 10/20/20 10:44 AM, Eric Blake wrote:
> 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
next prev parent reply other threads:[~2020-11-12 21:11 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-23 20:37 [PATCH] docs: Better mention of qemu-img amend limitations Eric Blake
2020-09-23 21:56 ` Nir Soffer
2020-10-20 15:44 ` Eric Blake
2020-11-12 21:10 ` Eric Blake [this message]
2020-11-13 9:49 ` Kevin Wolf
2020-11-13 13:40 ` Eric Blake
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