Comment 5 for bug 879194

I've looked at v2 and it looks like this might still matter.

If pt-table-checksum is run on master M1 but later M2 is made the master, the 'dsn' table will need to be updated to reflect the change in topology. Otherwise when pt-table-checksum is run on M2, it will see itself in the 'dsn' table as a slave and not see anything mentioned about M1.

But the best option IMHO is to have all databases (including all masters) in the 'dsn' table and have pt-table-checksum filter out the current master from the resultset.