Re: Questions about how block devices use snapshots

[Zhiyong Ye]

Hi Kevin,

Thank you for your reply and this method works.

May I ask if this 'image-end-offset' field can be shown in the qemu-img 
info too? Because it is also a very useful information whether qcow2 is 
placed on a file or a block device.

Regards

Zhiyong

On 2/21/23 11:58 PM, Kevin Wolf wrote:
> Am 21.02.2023 um 14:27 hat Zhiyong Ye geschrieben:
> > Hi Kevin,
> >
> > Sorry to bother you again.
> >
> > I intend to use this approach for snapshots of block devices, which, as you
> > say, requires a lot of disk space to store snapshot data. So, to save disk
> > space, after each successful external snapshot creation, I want to shrink
> > the block device that stores the backing_file image to the size that qcow2
> > data actually occupies, since it has become read-only. But there is no way
> > to get the actual size of qcow2 when it is stored in a block device.
> >
> > Qemu-img info can easily get the actual size of qcow2 when it is stored in a
> > file using the fstat function, but this will fail and return 0 for block
> > devices. Therefore, it is necessary to implement the method of getting data
> > occupancy inside qcow2. I think there may be two possible ways to do this:
> >
> > - Add a cluster count field @nb_clusters in the BDRVQcow2State for each new
> > cluster allocated and the actual size occupied by qcow2 is: nb_clusters *
> > cluster_size.
> > - Iterate through the refcount block to find the value with the largest host
> > offset, and this is the actual size occupied by qcow2.
> >
> > Since I'm not very familiar with qcow2, may I ask if you have any advice on
> > getting the actual size when using qcow2?
>
> I think what you need is the 'image-end-offset' field from 'qemu-img
> check --output=json'.
>
> Kevin