Re: Adding QCOW2 reading/writing support

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

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.

>> Now, I want to ask you for advice: What is the best approach to achieve
>> this?
>>
>>   * Implement the reading/writing in the device mapper?
>>   * Extend the loop device?
>>   * Create a new subsystem for the processing of sparse devices or images?
>>
>> Or do you have any other idea?
> The 1st idea is that if this feature is really good requirement.
>
> Thanks,
> Ming Lei
Regards,
Manuel Bentele