1。su - oracle
1.1. modify the /opt/oracle/admin/dbora817/pfile/initdbora817.ora,make sure that
    log_archive_start = true
    log_archive_dest_1 = "location=/opt/oracle/admin/dbora817/arch"
    log_archive_format = arch_%t_%s.arc
   are not commented.

2. sqlplus '/ as sysdba'
3. SQL>shutdown
   SQL>startup mount
   SQL>alter database archivelog;
   SQL>alter database open;
4. verify:
   SQL>archive log list

Database log mode              Archive Mode
Automatic archival             ENABLED
Archive destination            /opt/oracle/admin/dbora817/arch
Oldest online log sequence     19
Next log sequence to archive   21
Current log sequence           21