The --only-known-file-types option of the mysqlbackup command limits the backup to only those files that represent known data files from MySQL or its built-in storage engines, such as .frm, .ibd, .myd, and so on. (See the full list of extensions.) By default, the mysqlbackup command backs up all file extensions within the data directory, which could include files produced by many different storage engines. Use this option to omit the additional data files from other storage engines from the backup, for performance or space reasons.