1、open_links含义
OPEN_LINKS_PER_INSTANCE specifies the maximum number of migratable open connections globally for each database instance. XA transactions use migratable open connections so that the connections are cached after a transaction is committed. Another transaction can use the connection, provided the user who created the connection is the same as the user who owns the transaction.
OPEN_LINKS_PER_INSTANCE is different from OPEN_LINKS, which indicates the number of connections from a session. The OPEN_LINKS parameter is not applicable to XA applications.
可能出现的错误：
ORA-02020: too many database links in use
Cause: The current session has exceeded the INIT.ORA open_links maximum.
Action: Increase the open_links limit, or free up some open links by committing or rolling back the transaction and canceling open cursors that reference remote databases.
2、查看open_links
Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.1.0
Connected as SYS
SQL> show parameters open_links;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_links integer 4
open_links_per_instance integer 4
3、修改open_links
alter system set open_links=255 scope=spfile;
alter system set open_links_per_instance=255 scope=spfile;
4、使修改后的open_links生效
修改后查看，发现值没有变化。
SQL> show parameters open_links;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_links integer 4
open_links_per_instance integer 4
SQL> alter system set open_links=255 scope=spfile;
System altered
SQL> alter system set open_links_per_instance=255 scope=spfile;
System altered
SQL>
SQL> show parameters open_links;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_links integer 4
open_links_per_instance integer 4
SQL>
重启一下数据库查看，值修改过来了：
[oracle@eds2db ~]$ dbshut
ORACLE_HOME_LISTNER is not SET, unable to auto-stop Oracle Net Listener
Usage: /u01/app/oracle/db/bin/dbshut ORACLE_HOME
Processing Database instance "wg97": log file /u01/app/oracle/db/shutdown.log
[oracle@eds2db ~]$ dbstart
ORACLE_HOME_LISTNER is not SET, unable to auto-start Oracle Net Listener
Usage: /u01/app/oracle/db/bin/dbstart ORACLE_HOME
Processing Database instance "wg97": log file /u01/app/oracle/db/startup.log
[oracle@eds2db ~]$
SQL> show parameters open_links;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_links integer 255
open_links_per_instance integer 255