Currently, FLUSH TABLES ... FOR EXPORT takes a read lock on a table, and this prevents any INSERT, UPDATE, or DELETE operations from affecting the table until the lock has been released with UNLOCK TABLES.
Some users would like to allow INSERT, UPDATE, or DELETE operations to affect a table during the export process. Would it be possible to implement this? Perhaps it could be implemented using the same kind of log used to track changes during a table rebuild with ALTER TABLE ... ALGORITHM=INPLACE? i.e. the log referred to by the innodb_online_alter_log_max_size system variable: