0 - Trace disabled.
  1 - Trace DBI method calls returning with results or errors.
  2 - Trace method entry with parameters and returning with results.
  3 - As above, adding some high-level information from the driver and some internal information from the DBI.
  4 - As above, adding more detailed information from the driver. Also includes DBI mutex information when using threaded Perl.
  5 - As above but with more and more obscure information.