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Re: [Qemu-block] [PATCH v5 0/2] qemu-img: Check for backing image if spe

From: John Snow
Subject: Re: [Qemu-block] [PATCH v5 0/2] qemu-img: Check for backing image if specified during create
Date: Mon, 17 Jul 2017 20:38:12 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

Kevin: I took a stab at this 'feature', but if there are any fixups or
changes that need to occur and it's important that it happens before I'm
awake, please feel free to steal these patches and do whatever you need
to them, including setting them on fire.


post-script: I think the only thing that I don't really do here is
attempt to force-open an image to see if it exists if a size is already
provided in order to quiet errors related to locking.

That change would just eliminate a little bit of "this image is locked!"
whining in the case that -u was omitted but a size was provided, which
is mostly QOL.

On 07/17/2017 08:34 PM, John Snow wrote:
> We do not currently guarantee that QEMU will or will not open a
> backing file when creating a new overlay file. Presently, QEMU will
> not open that file if you provide a filesize, because it has no reason
> to want to open it in that case.
> This series makes the contract more explicit: if '-u' is provided to
> create, we will not open the backing image regardless, erroring out if
> a size was not provided.
> In the other case, if '-u' is not provided, we now endeavor to open the
> backing image if possible to check that it exists. For now, if a size
> is provided and the image does not exist, QEMU will only warn to maintain
> compatibility with legacy behavior.
> In the future, QEMU may treat the operation as a failure if '-u' was not
> provided.
> Tests are amended primarily to pass the '-u' flag where it makes sense;
> which is when creating overlays for objects already open by QEMU. These
> will now generally fail to succeed because of image locking. In this
> case, they only warn instead of fail, but this keeps the output cleaner.
> Test 111 is updated to accommodate a new error message.
> 082, 085, 139, 156 and 158 add '-u' just to suppress warnings.
> John Snow (2):
>   blockdev: move BDRV_O_NO_BACKING option forward
>   qemu-img: Check for backing image if specified during create
>  block.c                    | 96 
> +++++++++++++++++++++++++---------------------
>  blockdev.c                 | 11 +++---
>  qemu-img-cmds.hx           |  4 +-
>  qemu-img.c                 | 16 +++++---
>  tests/qemu-iotests/082     |  4 +-
>  tests/qemu-iotests/082.out |  4 +-
>  tests/qemu-iotests/085     |  2 +-
>  tests/qemu-iotests/111.out |  1 +
>  tests/qemu-iotests/139     |  2 +-
>  tests/qemu-iotests/156     |  2 +-
>  tests/qemu-iotests/158     |  2 +-
>  11 files changed, 81 insertions(+), 63 deletions(-)

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