fsync.warningthresholdms in zoo.cfg not working

Config fsync.warningthresholdms in zoo.cfg does not work.
I find QuorumPeerConfig.parseProperties give fsync.warningthresholdms a prefix like "zookeeper.fsync.warningthresholdms". But in class FileTxnLog where fsync.warningthresholdms is used, code is :Long.getLong("fsync.warningthresholdms", 1000),without prefix "zookeeper.", therefore can not get fsync.warningthresholdms's value.

I wonder the speed of fsync, need this config to see whether the speed is good enough.