Re: Adding QCOW2 reading/writing support

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

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.

Regards,
Manuel