Why prepareMerge task is stuck in the vdsm after cold merge

Issue

• There are prepareMerge tasks stuck in vdsm for a long time.

vdsm-client Host getAllTasksInfo
{
    "a15324c1-8f3d-4f10-a360-4f108ca8dd39": {
        "verb": "prepareMerge", 
        "id": "a15324c1-8f3d-4f10-a360-4f108ca8dd39"
    }
}