Public bug reported:
On Bionic, Qemu complains that it cannot acquire write lock when
commiting a snapshot if a read-only backing-store is opened by another
qemu process. This behavior does not happen with version 2.12 in Cosmic.
Reproducer
==========
Create two QCOW2 containers sharing the same base file as a backing store :
base.qcow2
|
+---------+---------+
| |
middle-vm02.img middle-vm02.img
| |
top-vm01.img top-vm02.img
# cat mkimage
#!/bin/bash
qemu-img create -f qcow2 base.qcow2 10G
qemu-img create -f qcow2 -b base.qcow2 middle-vm01.img 10G
qemu-img create -f qcow2 -b base.qcow2 middle-vm02.img 10G
qemu-img create -f qcow2 -b middle-vm01.img top-vm01.img 10G
qemu-img create -f qcow2 -b middle-vm01.img top-vm02.img 10G
Start two VM each using its own top-vm{id}.img
# cat runvm
#!/bin/bash
qemu-system-x86_64 -nographic -qmp unix:./qmp-1.sock,server,nowait -enable-kvm
-device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m
256M -drive file=./top-vm01.img,if=virtio,id=disk0 -smp 1 -smbios
type=1,manufacturer=test&
qemu-system-x86_64 -nographic -qmp unix:./qmp-2.sock,server,nowait -enable-kvm
-device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m
256M -drive file=./top-vm02.img,if=virtio,id=disk0 -smp 1 -smbios
type=1,manufacturer=test&
Create a snapshot
./scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1
(QEMU) blockdev-snapshot-sync device=disk0 snapshot-file=tmp.qcow2 format=qcow2
Formatting 'tmp.qcow2', fmt=qcow2 size=10737418240 backing_file=./top-vm01.img
backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
{"return": {}}
Commit the snapshot
(QEMU) block-commit device=disk0 base=top-vm01.img
{"error": {"class": "GenericError", "desc": "Failed to get \"write\" lock"}}
Expected Behavior
=================
The commit should complete succesfully as the base.img backing store is opened
read-only so no write lock is required
Current Behavior
================
The commit fails with "Failed to get "write" lock
** Affects: qemu (Ubuntu)
Importance: Undecided
Status: New
** Affects: qemu (Ubuntu Bionic)
Importance: Undecided
Status: Confirmed
** Affects: qemu (Ubuntu Disco)
Importance: Undecided
Status: Fix Released
** Also affects: qemu (Ubuntu Disco)
Importance: Undecided
Status: New
** Also affects: qemu (Ubuntu Bionic)
Importance: Undecided
Status: New
** Changed in: qemu (Ubuntu Bionic)
Status: New => Confirmed
** Changed in: qemu (Ubuntu Disco)
Status: New => Fix Released
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https://bugs.launchpad.net/bugs/1837869
Title:
Cannot complete snapshot if read-only backing store is opened by
another VM
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