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
