AllocateLVMVolume contains two policy: BINPACK/SPREAD.

DivideLVMPVCs divide pvcs into pvcsWithVG and pvcsWithoutVG.

DividePVCAccordingToMediaType divide pvcs into pvcsWithSSD and pvcsWithHDD.

GetCacheDeviceCount get Device count from ClusterNodeCache.

GetCacheMPCount get MP count from ClusterNodeCache.

GetFreeDevice divide nodeCache.Devices into freeDeviceSSD and freeDeviceHDD.

GetFreeMP divide nodeCache.MountPoints into freeMPSSD and freeMPHDD.

GetNodeVGMap make a copy map of NodeCache VGs.

If there is no readonly snapshot pvc, just return true.

不需要单独处理 readonly snapshot, 因为若filter阶段只选出一个node，不会进入score。.