nova spawn instance的时候，会先create_p_w_picpath，下面是获取的base p_w_picpath的函数

defget_cache_fname(p_w_picpaths,key):"""ReturnafilenamebasedontheSHA1hashofagivenp_w_picpathID.Imagefilesstoredinthe_basedirectorythatmatchthispatternareconsideredforcleanupbythep_w_picpathcachemanager.Thecachemanagerconsidersthefiletobeinuseifitmatchesaninstance'sp_w_picpath_ref,kernel_idorramdisk_idproperty.However,ingrizzly-3andbefore,onlythep_w_picpath_refpropertywasconsidered.Thismeansthatit'sunsafetostorekernelandramdiskp_w_picpathsusingthispatternuntilwe'resurethatallcomputenodesarerunningacachemanagernewerthangrizzly-3.Fornow,werequireadminstoconfirmthatbysettingtheremove_unused_kernelsbooleanbut,atsomepointinthefuture,we'llbesafelyabletoassumethis."""p_w_picpath_id=str(p_w_picpaths[key])if((notCONF.libvirt.remove_unused_kernelsand#remove_unused_kernels默认值是Truekeyin['kernel_id','ramdisk_id'])):returnp_w_picpath_idelse:returnhashlib.sha1(p_w_picpath_id).hexdigest()#basep_w_picpathid生成