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[libvirt-users] Autodetecting backing file properties when using vol-create-as



Hi all,
experimenting with vol-create-as, I think it should autodetect some data - filesize and backing file format, specifically. However, the current implementation require us to specify both filesize and backing file format.

Considering that qemu-img already autodetects these data, there are any reason for the lack of autodetect by libvirt? Should I open a bugzilla issue?

Below you can find a practical example of what I mean. System is CentOS Linux release 7.6.1810 (Core) with libvirt-4.5.0-10.el7_6.4.x86_64

Please let me know if I am missing something.
Thanks.

# create base file
[root singularity images]# qemu-img create base.qcow2 8G -f qcow2
Formatting 'base.qcow2', fmt=qcow2 size=8589934592 cluster_size=65536 lazy_refcounts=off refcount_bits=16
[root singularity images]# qemu-img info base.qcow2
image: base.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 17K
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

# create overlay1.qcow2 via qemu-img; note how backing file format is autodetected ("-f qcow2" regards the overlay file itself) [root singularity images]# qemu-img create -b /var/lib/libvirt/images/base.qcow2 overlay1.qcow2 -f qcow2 Formatting 'overlay1.qcow2', fmt=qcow2 size=8589934592 backing_file=/var/lib/libvirt/images/base.qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
[root singularity images]# qemu-img info overlay1.qcow2
image: overlay1.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 17K
cluster_size: 65536
backing file: /var/lib/libvirt/images/base.qcow2
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

# try the same with virsh vol-create-as; note how you must specify filesize and the backing file format is *wrong* (leading to an unusable overlay disk) [root singularity images]# virsh vol-create-as default overlay2.qcow2 8G --format qcow2 --backing-vol /var/lib/libvirt/images/base.qcow2
Vol overlay2.qcow2 created
[root singularity images]# qemu-img info overlay2.qcow2
image: overlay2.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 17K
cluster_size: 65536
backing file: /var/lib/libvirt/images/base.qcow2
backing file format: raw
Format specific information:
    compat: 0.10
    refcount bits: 16


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