Virtualization New Added 29 September 2026

Proxmox: can't lock file '/var/lock/qemu-server/lock-100.conf' - got timeout

Another task still holds the lock on that VM's configuration, so the new start, stop or backup waited and gave up. The holder is usually a backup, snapshot or migration that is still running or hung, and killing the wrong process can leave the VM in a half-finished state. Check what holds the lock before clearing anything.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Is a task still running against VM 100?
qm config 100 | grep '^lock'
ps aux | grep -E 'vzdump|qm (start|stop|migrate|snapshot)' | grep 100

# Recent tasks on this node
tail -n 20 /var/log/pve/tasks/index

# Wait for a backup to finish, or stop it from the GUI task log.
# If nothing is running, the lock is stale: clear it
qm unlock 100

# A hung VM that will not stop
qm stop 100 --skiplock

# Last resort, only when no qm or vzdump process exists
# rm /var/lock/qemu-server/lock-100.conf

How to diagnose Virtualization errors

Virtualisation errors are usually about hardware virtualisation access: either it is disabled in firmware, or another hypervisor already holds it. On Windows, Hyper-V, WSL 2, Docker Desktop, Credential Guard and memory integrity all claim the virtualisation extensions, and VirtualBox or VMware then fails with a message that mentions VT-x but not the actual conflict.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Confirm virtualisation is enabled at the CPU level: grep -E 'vmx|svm' /proc/cpuinfo on Linux, or the Performance tab of Task Manager on Windows.
On Windows, check what else is using the hypervisor: bcdedit /enum | findstr hypervisorlaunchtype and whether Hyper-V, WSL 2 or Core Isolation are enabled.
On Linux, check /dev/kvm exists and that your user is in the kvm group; nested virtualisation must be enabled explicitly on cloud VMs.
Match guest additions and tools versions to the hypervisor version: mismatches cause shared folders, clipboard and display problems that look like guest OS bugs.
For WSL, run wsl --status and wsl --update; most WSL 2 errors are a stale kernel or a disabled Virtual Machine Platform feature.

Tools worth reaching for

grep -E 'vmx|svm' /proc/cpuinfo
wsl --status
bcdedit /enum
ls -l /dev/kvm
systemctl status libvirtd

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

pve.proxmox.com

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