Re: [Qemu-devel] [RFC 0/4] qemu-img: add max-size subcommand

[Nir Soffer <nsoffer@redhat.com>]

On Fri, Mar 3, 2017 at 3:51 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> RFCv1:
>  * Publishing patch series with just raw support, no qcow2 yet.  Please review
>    the command-line interface and let me know if you are happy with this
>    approach.
>
> Users and management tools sometimes need to know the size required for a new
> disk image so that an LVM volume, SAN LUN, etc can be allocated ahead of time.
> Image formats like qcow2 have non-trivial metadata that makes it hard to
> estimate the exact size without knowledge of file format internals.
>
> This patch series introduces a new qemu-img subcommand that calculates the
> required size for both image creation and conversion scenarios.
>
> The conversion scenario is:
>
>   $ qemu-img max-size -f raw -O qcow2 input.img
>   107374184448

Isn't this the minimal size required to convert input.img?

>
> Here an existing image file is taken and the output includes the space required
> for data from the input image file.
>
> The creation scenario is:
>
>   $ qemu-img max-size -O qcow2 --size 5G
>   196688

Again, this is the minimal size.

So maybe use min-size?

Or:

    qemu-img measure -f raw -O qcow2 input.img

Works nicely with other verbs like create, convert, check.

Now about the return value, do we want to return both the minimum size
and the maximum size?

For ovirt use case, we currently calculate the maximum size by multiplying
by 1.1. We use this when doing automatic extending of ovirt thin provisioned
disk. We start with 1G lv, and extend it each time it becomes full, stopping
when we reach virtual size * 1.1. Using more accurate calculation instead
can be nicer.

So we can retrun:

{
    "min-size": 196688,
    "max-size": 5905580032
}

Anyway thanks for working on this!

>
> Stefan Hajnoczi (4):
>   block: add bdrv_max_size() API
>   raw-format: add bdrv_max_size() support
>   qemu-img: add max-size subcommand
>   iotests: add test 178 for qemu-img max-size
>
>  include/block/block.h      |   2 +
>  include/block/block_int.h  |   2 +
>  block.c                    |  37 +++++++++
>  block/raw-format.c         |  16 ++++
>  qemu-img.c                 | 196 +++++++++++++++++++++++++++++++++++++++++++++
>  qemu-img-cmds.hx           |   6 ++
>  tests/qemu-iotests/178     |  75 +++++++++++++++++
>  tests/qemu-iotests/178.out |  25 ++++++
>  tests/qemu-iotests/group   |   1 +
>  9 files changed, 360 insertions(+)
>  create mode 100755 tests/qemu-iotests/178
>  create mode 100644 tests/qemu-iotests/178.out
>
> --
> 2.9.3
>