oracle6中已经使用了4个db link
使用到第五个的时候就报了ORA-02020 过多的数据库连接在使用中

查看系统参数
SQL> show parameter open_link

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
open_links                           integer     4
open_links_per_instance              integer     4

修改系统参数open_links,open_links_per_instance为100

SQL> alter system set open_links=100;
alter system set open_links=100
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
不能在线修改，需要加上scope=spfile参数,且需要重启DB生效

SQL> alter system set open_links=100 scope=spfile;

System altered.

SQL> alter system set open_links_per_instance=100;
alter system set open_links_per_instance=100
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
不能在线修改，需要加上scope=spfile参数,且需要重启DB生效

SQL> alter system set open_links_per_instance=100 scope=spfile;

System altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 1603411968 bytes
Fixed Size                  2213776 bytes
Variable Size            1275070576 bytes
Database Buffers          318767104 bytes
Redo Buffers                7360512 bytes
Database mounted.
Database opened.
SQL> show parameter open_links

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
open_links                           integer     100
open_links_per_instance              integer     100
之后可以正常使用.