Code review comment for lp:~akopytov/percona-xtrabackup/replication-support-in-test-suite-1.6

Actually, on a closer look, MTR picks unique IDs only for multiple threads within the same MTR run, i.e. for --parallel. It doesn't warrant any uniqueness with multiple concurrent MTR runs. Which means it is absolutely identical to what XB test suite does: just check if a port is open and hope nothing grabs it before we start the server on it.