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

[Nir Soffer]

On Sat, Mar 4, 2017 at 12:15 AM, Nir Soffer <address@hidden> wrote:
> On Sat, Mar 4, 2017 at 12:02 AM, John Snow <address@hidden> wrote:
>>
>>
>> On 03/03/2017 04:38 PM, Nir Soffer wrote:
>>> On Fri, Mar 3, 2017 at 3:51 PM, Stefan Hajnoczi <address@hidden> 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?
>>>
>>
>> It's an upper bound for the property being measured, which is current
>> allocation size, not maximum potential size after growth.
>
> From my point of view, this is the minimal size you must allocate if you
> want to convert the image to logical volume.

Thinking more about this, max-size is correct for this use case, the
maximum size of the image, used as the minimal allocation.

>
>>
>>>>
>>>> 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.
>>>
>>
>> Measure what? This is strictly less descriptive even if "max-size" isn't
>> a verb.
>
> measure-size?
>
>>> 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!
>>>
>>
>> It sounds like you want something different from what was intuited by
>> Maor Lipchuck. There are two things to estimate:
>>
>> (A) An estimate of the possible size of an image after conversion to a
>> different format, and
>> (B) An estimate of the possible size after full allocation.
>>
>> I got the sense that Maor was asking for (A), but perhaps I am wrong
>> about that. However, both are "maximums" in different senses.
>
> Both are minimum when you have to allocate the space.
>
> Maor ask about A because he is working on fixing allocation when
> converting existing files, but we also have other use cases like B.
>
> Nir
>
>>
>> --js
>>
>>>>
>>>> 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
>>>>
>>