Comment 2 for bug 1378355

Also while I don't know yet how MTR handles this, at least mysqld_multi doesn't use the --default-group or --default-group-suffix option at all, it reads the suffix-specific group sections itself and then creates the mysqld command line with explicit option settings from that

e.g. with a minimalistic my.cnf like this:

  [mysqld1]
  datadir=/home/ubuntu/mariadb/data1
  server-id=1

  [mysqld2]
  datadir=/home/ubuntu/mariadb/data2
  server-id=2
  port=3307

the mysqld processes started by "mysqld_multi start" get invoked like this:

  /usr/sbin/mysqld --datadir=/home/ubuntu/mariadb/data1 --server-id=1
  /usr/sbin/mysqld --datadir=/home/ubuntu/mariadb/data2 --server-id=2 --port=3307 --socket=/tmp/mysqld2.sock

So the active datadir should rather be supplied to xtrabackup explicitly directly, instead of relying on passing the correct option file and have it picked up from there?