Hello,

I have a problem when i tried to hotplug a disk to a VM. Here is the situation.

We use a VM (let's call this backupVM) which is responsible of doing our VM backup by :
- Making a snapshot of the VM we want to backup
- Attach the snapshot disk
- Make the copy using DD
- Unplug the snapshot disk and then delete snapshot.

For some reason that i don't know, for certain VM, the backupVM cannot hot plug the snapshot disk.

I'm using ovirt 4.2.4
Here is what is see in engine.log

2018-08-04 19:52:30,496-10 ERROR [org.ovirt.engine.core.bll.storage.disk.AttachDiskToVmCommand] (default task-61) [361eca85-ca85-4e23-b292-cd8303a1a86d] Command 'org.ovirt.engine.core.bll.storage.disk.AttachDiskToVmCommand' failed: EngineException: org.ovirt.engine.core.vdsbroker.vdsbroker.VDSErrorException: VDSGenericException: VDSErrorException: Failed to HotPlugDiskVDS, error = internal error: unable to execute QEMU command 'device_add': Failed to get shared "write" lock, code = 45 (Failed with error FailedToPlugDisk and code 45)

Here is the vdsm.log

2018-08-04 19:51:38,512-1000 INFO  (jsonrpc/5) [virt.vm] (vmId='78ef239f-3cb3-4eef-921f-f989724009ef') Hotplug disk xml: <?xml version='1.0' encoding='utf-8'?>
<disk address="" device="disk" snapshot="no" type="file">
    <source file="/var/lib/vdsm/transient/225ec856-d661-4374-bb7d-8ea7168fe5f2-b641bb09-325a-4932-a9af-32c907e4a381.D52Nkj" />
    <target bus="virtio" dev="vdg" />
    <serial>c312a0c3-8c58-472f-99ad-cfbabb42337d</serial>
    <driver cache="writethrough" error_policy="stop" io="threads" name="qemu" type="qcow2" />
</disk>
 (vm:3859)
2018-08-04 19:51:38,672-1000 ERROR (jsonrpc/5) [virt.vm] (vmId='78ef239f-3cb3-4eef-921f-f989724009ef') Hotplug failed (vm:3867)
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vm.py", line 3865, in hotplugDisk
    self._dom.attachDevice(driveXml)
  File "/usr/lib/python2.7/site-packages/vdsm/virt/virdomain.py", line 98, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/common/libvirtconnection.py", line 130, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/common/function.py", line 92, in wrapper
    return func(inst, *args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 570, in attachDevice
    if ret == -1: raise libvirtError ('virDomainAttachDevice() failed', dom=self)
libvirtError: internal error: unable to execute QEMU command 'device_add': Failed to get shared "write" lock

here is libvirtd log in /var/log/messages

libvirtd: 2018-08-05 05:51:38.637+0000: 4967: error : qemuMonitorJSONCheckError:389 : internal error: unable to execute QEMU command 'device_add': Failed to get shared "write" lock

Do you guys have any idea of what going on ? How can i check this lock ?
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