SET @start_global_value = @@global.XENGINE_WAL_DIR;
SELECT @start_global_value;
@start_global_value

"Trying to set variable @@global.XENGINE_WAL_DIR to 444. It should fail because it is readonly."
SET @@global.XENGINE_WAL_DIR   = 444;
ERROR HY000: Variable 'xengine_wal_dir' is a read only variable