Re: [Qemu-block] [PATCH] qemu-img: add seek option to dd

[Max Reitz]

On 10.08.2016 17:03, Reda Sallahi wrote:
> This patch adds the seek option which allows qemu-img dd to skip a number of
> blocks on the output before copying the input.
> 
> A test case was added to test the seek option.
> 
> Signed-off-by: Reda Sallahi <address@hidden>
> ---
> Depends on:
> [PATCH v5] qemu-img: add skip option to dd
> 
>  qemu-img-cmds.hx           |   4 +-
>  qemu-img.c                 | 116 
> ++++++++++++++++++++++++++++++++++++++-------
>  qemu-img.texi              |   4 +-
>  tests/qemu-iotests/161     |  72 ++++++++++++++++++++++++++++
>  tests/qemu-iotests/161.out |  51 ++++++++++++++++++++
>  tests/qemu-iotests/group   |   1 +
>  6 files changed, 228 insertions(+), 20 deletions(-)
>  create mode 100755 tests/qemu-iotests/161
>  create mode 100644 tests/qemu-iotests/161.out

On IRC, Kevin has raised concern that qemu-img dd should never truncate
the output image, or at least not until told to do so.

While I do not share his opinion, I do share his concern that people may
fall into this trap. Therefore, I propose doing two things:

First, we should merge a patch which implements conv=notrunc. Without
qemu-img dd actually truncating anything yet, that will be rather
trivial. However, I think that until we have come to a conclusion on how
to handle the case where conv=notrunc has not been passed, we should
actually make conv=notrunc mandatory. That is, the patch implementing
conv=notrunc should also make qemu-img dd error out if it has not been
specified.

I admit this is a rather stupid idea but I think it will do the trick
for now.

Second, this patch should be separated into two parts: The first of
which make qemu-img dd no longer create the output image if it exists
already, and the second of which then implements the seek option. Those
are two different things so I think they should be done in two different
patches.

Max

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