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Version: qemu-kvm-rhev-2.9.0-16.el7_4.14

qemu-img convert has a bug that it will hang if the source base is smaller than overlay:

qemu-img create -f qcow2 test.qcow2 100m
qemu-img create -F qcow2 -f qcow2 -b test.qcow2 overlay.qcow2
qemu-img resize overlay.qcow2 +20m
qemu-img create -F qcow2 -f qcow2 -b overlay.qcow2 overlay2.qcow2
qemu-img convert overlay2.qcow2 -f qcow2 -O qcow2 combined.qcow2

On RHEL 7.5, qemu-kvm-rhev-2.10.0 has no such issue. This bug is to track the issue for 7.4.z.

Two patches fix the problem in upstream:

commit c61e684e44272f2acb2bef34cf2aa234582a73a9 (tag: block-pull-request)
Author: Eric Blake <eblake>

    block: Exploit BDRV_BLOCK_EOF for larger zero blocks

commit fb0d8654ffc3ea1494067327fc4c4da5d0872724 (refs/bisect/ok)
Author: Eric Blake <eblake>

    block: Add BDRV_BLOCK_EOF to bdrv_get_block_status()

No customer demand. This was reported on upstream mailing list by a qemu-kvm-ev 7.4 user, so I'm creating a bug just in case we need this.

(In reply to Fam Zheng from comment #4)
> No customer demand. This was reported on upstream mailing list by a
> qemu-kvm-ev 7.4 user, so I'm creating a bug just in case we need this.

OK, closing it as CURRENTRELEASE then, given it's fixed in RHEL-7.5 which is GA.