Re: Questions about how block devices use snapshots

[Kevin Wolf]

Am 23.02.2023 um 08:35 hat Zhiyong Ye geschrieben:
> 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.

The only way to know the largest offset is by looking at all the
metadata in qcow2. 'qemu-img info' is supposed to be very fast, so we
don't do that there. 'qemu-img check' already looks at all metadata, so
we have the number readily available there.

Kevin

> 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
> > 
>