open_links_per_instance 和 open_links 参数说明
一. 先看官网的说明
| Property | Description |
| Parameter type | Integer |
| Default value | 4 |
| Modifiable | No --即修改需要重启实例 |
| Range of values | 0 to 255 |
OPEN_LINKS specifies the maximum number of concurrent open connections to remote databases in one session. These connections include database links, as well as external procedures and cartridges, each of which uses a separate process.
Oracle counts one open link for the following:
For each user that references a public or private database link
For each external procedure or cartridge connection when it is executed for the first time
Both types of connections close when the session ends. You can also close a database link connection explicitly by issuing an ALTER SESSION CLOSE DATABASE LINK statement.
You should set this parameter to allow for the external procedure and cartridge connections expected during the session plus the number of databases referred to in typical distributed transactions (that is, a single SQL statement that references multiple databases), so that all the databases can be open to execute the statement.
For example, if queries alternately access databases A, B, and C, and OPEN_LINKS is set to 2, time will be lost waiting while one connection is broken and another made. Increase the value if many different databases are accessed over time.
This parameter refers only to connections used for distributed transactions. Direct connections to a remote database specified as an application connects are not counted.
If you set OPEN_LINKS to 0, then no distributed transactions are allowed.
| Property | Description |
| Parameter type | Integer |
| Default value | 4 |
| Modifiable | No |
| Range of values | 0 to 4294967295 (4 GB -1) |
| Oracle RAC | Multiple instances can have different values. |
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.
二. 查看并修改
2.1 查看
SQL> show parameter open
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_cursors integer 300
open_links integer 4
open_links_per_instance integer 4
read_only_open_delayed boolean FALSE
session_max_open_files integer 10
2.2 与这个参数有关的错误是ORA-02020
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.
2.3 修改该参数
之前说过， 修改这个参数需要重启实例。可以只修改spfile里的值。 如：
SQL> alter system set open_links=10;
alter system set open_links=10
*
第 1 行出现错误:
ORA-02095: 无法修改指定的初始化参数
SQL> alter system set open_links=10 scope=spfile;
系统已更改。
在次查看：
SQL> show parameter open_links
NAME TYPE VALUE
------------------------------------ ----------- -------
open_links integer 4
open_links_per_instance integer 4
修改没有生效，还是需要重启实例的。
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