I recently encountered this problem on a guest VM running on OVM 3.1.1.
The Oracle VM Manager stated machine status as UNKNOWN so I could not start nor stop the VM giving me error while performing any operation on it.
Further investigation above task details returned this error:
Error: Acquire running lock failed: 256
I connected via SSH to one of Oracle VM servers and run the following command:
[root@orclvmsrv1 ~]# xm list
Name ID Mem VCPUs State Time(s)
0004fb00000600007fbdb52a1b566a74 15 4096 1 -b---- 229.6
Domain-0 0 726 12 r----- 62921.7
This command returns all VMs (referred by VM ID) and theyr corresponding state.
For further informations on xm command I suggest you to visit this page: xm man
To stop the guest VM run:
xm shutdown VIRTUAL_MACHINE_ID
In this case:
[root@orclvmsrv1 ~]# xm shutdown 0004fb00000600007fbdb52a1b566a74
The guest VM shutdowns and should be correctly recognized as "Stopped" by VM Manager.
This solved my issue and I was able to start again using VM Manager.