pt-online-schema-change
pt-online-schema-change --nocheck-replication-filters --recursion-method=none --charset=utf8 --max-lag=10 --critical-load="Threads_running:100" --alter " modify column emp_no varchar(100)" h=192.168.1.1,P=3306,u=root,p=password,D=employees,t=emp --print --execute
pt-online-schema-change --nocheck-replication-filters --recursion-method=none --charset=utf8 --max-lag=10 --critical-load="Threads_running:100" --alter " modify column emp_no varchar(100)" h=192.168.1.1,P=3306,u=root,p=123,D=employees,t=emp --dry-run