Attaching RBD device after qemu upgrade fails with "Unknown protocol 'rbd'"

[Louis Bouchard]

After upgrading the following QEMU packages to latest Bionic version (1:2.11+dfsg-1ubuntu7.20) :

- qemu-system-x86
- qemu-system-common
- qemu-utils
- qemu-block-extra:amd64

Attaching an RBD device to a VM that was running prior to the upgrade fails with

"Unknown protocol 'rbd'

This only happens if this is the first RBD device to be attached after the upgrade. Attaching a device, then upgrading to 1:2.11+dfsg-1ubuntu7.20 will not generate the error message and the device will attach correctly.

Attaching a device to a new VM created _AFTER_ the upgrade does not trigger the problem either.

Downgrading qemu-block-extra:amd64 after the upgrade and prior to first attaching an RBD device will also not trigger the error.

Here are two examples :
Using :
ii qemu-block-extra:amd64 1:2.11+dfsg-1ubuntu7
ii qemu-system-common 1:2.11+dfsg-1ubuntu7
ii qemu-system-x86 1:2.11+dfsg-1ubuntu7
ii qemu-utils 1:2.11+dfsg-1ubuntu7
./qmp-shell -H /opt/vms/1/qmp.sock
(QEMU) drive_add auto format=raw,cache=writeback,file=rbd:rbd/disk-086264fd-9ca4-42a9-9deb-4a8be80f2bab:rbd_cache=true:id=compute-hv:conf=/etc/ceph/ceph.conf:keyring=/etc/ceph/ceph.client.compute-hv.keyring,id=volume-8b84f816-6e11-4a17-a2ce-0cdb6f486bfe,throttling.iops-total=10000,if=none
OK

With :
ii qemu-block-extra:amd64 1:2.11+dfsg-1ubuntu7
ii qemu-system-common 1:2.11+dfsg-1ubuntu7
ii qemu-system-x86 1:2.11+dfsg-1ubuntu7
ii qemu-utils 1:2.11+dfsg-1ubuntu7

and an existing QEMU process running

$ apt udpate && apt -y install qemu-utils qemu-system-x86 qemu-system-common qemu-block-extra:amd64
Using :
ii qemu-block-extra:amd64 1:2.11+dfsg-1ubuntu7.20
ii qemu-system-common 1:2.11+dfsg-1ubuntu7.20
ii qemu-system-x86 1:2.11+dfsg-1ubuntu7.20
ii qemu-utils 1:2.11+dfsg-1ubuntu7.20

./qmp-shell -H /opt/vms/1/qmp.sock
(QEMU) drive_add auto format=raw,cache=writeback,file=rbd:rbd/disk-086264fd-9ca4-42a9-9deb-4a8be80f2bab:rbd_cache=true:id=compute-hv:conf=/etc/ceph/ceph.conf:keyring=/etc/ceph/ceph.client.compute-hv.keyring,id=volume-8b84f816-6e11-4a17-a2ce-0cdb6f486bfe,throttling.iops-total=10000,if=none
Unknown protocol 'rbd'
(QEMU)

Restarting the QEMU process fixes the issue.

[Christian Ehrhardt  (paelzer)]

Hi Louis,
this is a known, but complex and probably very (very) long running issue.
I recently discussed it on the KVM Forum and we had a mail thread about it.

Read the full background on bug 1847361 - I'll mark this as a dup.