There is another problem with the value of innodb_flush_method.

In 80, srv_unix_file_flush_method are only set to the value of innodb_flush_method in xtrabackup_backup_func, which means that srv_unix_file_flush_method is never set during --prepare.

This results in the flush option of all files in the --prepare phase being SRV_UNIX_FSYNC, regardless of whether user sets other value using params.