Re: Adding QCOW2 reading/writing support

[Manuel Bentele <manuel-bentele@web.de>]

On 18.04.19 03:05, Ming Lei wrote:
> On Thu, Apr 18, 2019 at 5:04 AM Manuel Bentele <manuel-bentele@web.de> wrote:
>> On 17.04.19 14:16, Hannes Reinecke wrote:
>>> On 4/17/19 1:32 PM, Manuel Bentele wrote:
>>>> Hi,
>>>>
>>>> On 17.04.19 03:35, Ming Lei wrote:
>>>>> Hi,
>>>>>
>>>>> On Wed, Apr 17, 2019 at 5:33 AM Manuel Bentele
>>>>> <manuel-bentele@web.de> wrote:
>>>>>> Hi everyone
>>>>>>
>>>>>> I'm going to implement an in-kernel reading of QCOW2 images.
>>>>>> In the project, I only need the reading of QCOW2 images, but it's
>>>>>> essential to make thoughts for the implementation of the writing, too.
>>>>>> One of the difficulties seems to be the support of making an image
>>>>>> sparse (resizing the disk image).
>>>>> Could you describe this requirement in a bit more detail? Especially
>>>>> why
>>>>> do you want to read/write QCOW2 in kernel?
>>>> Yes, of course. The implementation of reading a QCOW2 disk image
>>>> in-kernel is required for an already existing system in the university
>>>> environment.
>>>> At the moment, the Linux kernel, initramfs, etc. for each client in the
>>>> system is loaded via PXE boot and then the block device with the default
>>>> file system is included with the help of a modified nbd version, called
>>>> dnbd (distributed nbd).
>>>> Due to the fact that the data on the default file system is only for non
>>>> persistent one-time provision of a client, read access is sufficient.
>>>> The user related data is stored on a network storage, as mostly done in
>>>> large scale infrastructures.
>>>>
>>>> Now, the goal is to minimize the network usage and avoid nbd.
>>>> Furthermore, fixed configured and packed boot images should be avoided.
>>>> Therefore, the advantage of the sparse and compression functionality of
>>>> QCOW2 should be used.
>>>> A workaround for that problem could be the local usage of nbd to include
>>>> the QCOW2 disk image as block device, but it involves a lot of
>>>> interaction between user and kernel space and thus an decreasing
>>>> performance. That leads to the motivation to implement the reading of
>>>> QCOW2 disk images directly in the kernel and aim for an merge into the
>>>> mainline kernel source to avoid out-of-kernel-tree maintenance.
>>>>
>>>> If you have any questions related to the described use case or if you
>>>> require more information, please let me know.
>>>> Thanks for your help.
>>>>
>>> cramfs?
>>> Or btrfs with compression enabled?
>>>
>>> Cheers,
>>>
>>> Hannes
>> Thanks for your simple idea to choose cramfs or btrfs with compression
>> enabled.
>> I will suggest that as alternative at the next project meeting.
> Or vdo which provides compression in block device level:
>
> https://github.com/dm-vdo/vdo

Thanks for your hint. I will take a look at it.

Regards,
Manuel