1. problem description:

   as the title show, i miss the first problem using pt-online-schema-change tool to modify the table online.

   no more message print in the console .

2. sulution:

   see the line 8489 of the file ‘pt-online-schema-change‘, you will see code

printf("%s -> %s:%s\n", $cxn->name(), $cxn->{dsn}->{h}, $cxn->{dsn}->{P});

  copy and annotation,

one by one , test name(), {dsn}, {dsn}->{h}, {dsn}->{P},

you will find that  $cxn->{dsn}->{P} is null.

take it away and try again, you will success!

Found 1 slaves: Use of uninitialized value in printf at /usr/local/percona-toolkit/bin/pt-online-schema-change line 8489