Advice for optimizing RMAN backup

Attempting to implement RMAN backups in an environment in which the database has never been backed up and is currently in NoArchiveLog mode.

Use Oracle 10GR2 on HP-UX version 11.23 using file systems and not ASM.

Checked different initialization parameters:

asm_diskgroups                       string

asm_diskstring                       string

disk_asynch_io                       boolean     TRUE

db_block_checking                 string      FALSE

db_writer_processes                  integer     1

dbwr_io_slaves                       integer     0

backup_tape_io_slaves                boolean     FALSE

large_pool_size                      big integer  0

filesystemio_options                 string      asynch

sga_max_size                         big integer 3008M

sga_target                           big integer 3008M

parallel_max_servers                 integer     40

session_max_open_files               integer     10

Although disk_asynch_io is set to 'True', according to the documentation I read, asynch I/O is not supported with Oracle 10GR2 if RAW devices are not used.