Oracle10g如何释放flash_recovery_area，解决ORA-19815错误

ORA-19815: WARNING: db_recovery_file_dest_size of 2147483648 bytes is 100.00% used, and has 0 remaining bytes available.
*************************************************************
You have the following choices to free up space from
flash recovery area:
1. Consider changing your RMAN retention policy.
   If you are using dataguard, then consider changing your
   RMAN archivelog deletion policy.
2. Backup files to tertiary device such as tape using the
   RMAN command BACKUP RECOVERY AREA.
3. Add disk space and increase the db_recovery_file_dest_size
   parameter to reflect the new space.
4. Delete unncessary files using the RMAN DELETE command.
   If an OS command was used to delete files, then use
   RMAN CROSSCHECK and DELETE EXPIRED commands.
*************************************************************