the engine is not acid compliant. unlike oltp engines, it uses a "stream" format to disk with no block boundaries. the head of the archive file generated is a byte array representing the data format and contents of that file. in mysql 5.1, a copy of the mysql frm file is stored in the header of each archive file. the frm file, which represents the definition of a table, allows an archive file to be restored to a mysql server if the archive file is copied to the server.