Parameters That Must Be Identical Across All Instances
Certain initialization parameters that are critical at database creation or that affect
certain database operations must have the same value for every instance in Real
Application Clusters. Specify these parameter values in the common parameter file,
or within each init_dbname.ora file on each instance. The following list shows
the parameters that must be identical on every instance.
DML_LOCKS (Only if set to zero)
GC_FILES_TO_LOCKS (Optional: You do not need to set this parameter
because Oracle automatically controls resource assignments.)
Parameters That Must Be Unique Across All Instances
If you use the THREAD or ROLLBACK_SEGMENTS parameters, Oracle Corporation
recommends setting unique values for them by using the sid identifier in the
server parameter file. However, you must set a unique value for INSTANCE_
NUMBER for each instance and you cannot use a default value.
--Oracle uses the INSTANCE_NUMBER parameter to distinguish among instances
at startup. Oracle also uses INSTANCE_NUMBER to assign free space to instances
using the INSTANCE option of the ALLOCATE EXTENT clause in the ALTER
TABLE or ALTER CLUSTER statements.
--Specify the THREAD parameter so instances avoid the overhead of acquiring
different thread numbers during startup and shutdown. Oracle uses the
THREAD number to assign redo log groups to specific instances. To simplify
administration, use the same number for both the THREAD and INSTANCE_
--Specify the ORACLE_SID environment variable which comprises the database
name and the number of the THREAD assigned to the instance.
--Specify INSTANCE_NAME to uniquely identify the instance. The default is the
instance’s sid and Oracle Corporation recommends that you use this for
--Oracle acquires private rollback segments upon instance startup based on the
rollback segment names you identify with the ROLLBACK_SEGMENTS
initialization parameter. If you do not declare rollback segment names with this
parameter for an instance, Oracle acquires public rollback segments for the
--If you specify UNDO_TABLESPACE with automatic undo management enabled,
set this parameter to a unique value for each instance.
Considerations for Parameters in Clusters
To enable a database to be started in Real Application Clusters mode, set this parameter to
Set this parameter to the number of instances in your Real Application Clusters
environment. A proper setting for this parameter can improve memory use.
See Also: Yourplatform-specific documentation for more information.
The CLUSTER_INTERCONNECTS parameter is supported on some platforms. Refer to your
platform-specific documentation for the specific use of this parameter, its syntax, and its
behavior. In general, however, you should not have to set CLUSTER_INTERCONNECTS.
You do not need to set this parameter if you have a single cluster interconnect. You also do
not need to set it if the default cluster interconnect meets the bandwidth requirements of
your Real Application Clusters database(s), which is typically the case.
Oracle uses information from CLUSTER_INTERCONNECTS to distribute interconnect traffic
among the various network interfaces if you specify more than interconnect with this
parameter. Note that the specified configuration inherits any limitations of the listed
interconnects and the associated operating system IPC services, such as availability.
In rare cases where a single cluster interconnect cannot meet your bandwidth
requirements, consider setting CLUSTER_INTERCONNECTS. For example, you might need
to set this parameter in some Data Warehouse environments with very high interconnect
bandwidth demands from one or more database(s).
For example, if you have two databases with high interconnect bandwidth requirements,
then you can override the default interconnect(s) provided by your operating system and
nominate a different interconnect for each database using the following syntax in each
server parameter file:
Database One: CLUSTER_INTERCONNECTS = ip1
Database Two: CLUSTER_INTERCONNECTS = ip2
Where ipn is an IP address in standard dotted-decimal format, for example,
However, if you have one database with very high bandwidth demands, then you can
nominate multiple interconnects, for example, using the following syntax:
CLUSTER_INTERCONNECTS = ip1:ip2:...:ipn
If you set multiple values for CLUSTER_INTERCONNECTS as in the previous example, then
Oracle uses all of the interconnects that you specify. This provides load balancing as long
as all of the listed interconnects remain operational.
If there is an operating system error writing to the interconnect that you specify with
CLUSTER_INTERCONNECTS, then Oracle returns an error even if some other interfaces are
available. This is because the communication protocols between Oracle and the
interconnect can vary greatly depending on your platform.
DB_NAME If you set a value for DB_NAME in instance-specific parameter files, it must be identical for
DISPATCHER To enable a shared server configuration, set the DISPATCHERS parameter. The
DISPATCHERS parameter may contain many attributes.
Oracle Corporation recommends that you configure at least the PROTOCOL and LISTENER
attributes. PROTOCOL specifies the network protocol for which the dispatcher generates a
listening end point. LISTENER specifies an alias name for the listeners with which the
PMON process registers dispatcher information. Set the alias to a name that is resolved
through a naming method such as a tnsnames.ora file.
DML_LOCKS Must be identical on all instances only if set to zero. The default value assumes an average
of four tables referenced per transaction. For some systems, this value may not be enough.
If you set the value of DML_LOCKS to 0, enqueues are disabled and performance is slightly
increased. However, you cannot use DROP TABLE, CREATE INDEX, or explicit lock
statements such as LOCK TABLE IN EXCLUSIVE MODE.
INSTANCE_NAME If specified, this parameter must have unique values on all instances. In Real Application
Clusters environments, all instances can be associated with a single database service.
Clients can override connection load balancing by specifying a particular instance by
which to connect to the database. INSTANCE_NAME specifies the unique name of this
instance. Oracle Corporation recommends that you set INSTANCE_NAME equivalent to the
This parameter is applicable only if you are using the redo log in ARCHIVELOG mode. Use
a text string and variables to specify the default filename format when archiving redo log
files. The string generated from this format is appended to the string specified in the LOG_
ARCHIVE_DEST_n parameter. You must include the thread number.
The following variables can be used in the format:
%s: log sequence number
%S: log sequence number, zero filled
%t: thread number
%T: thread number, zero filled
Using uppercase letters for the variables (for example, %S) causes the value to be fixed
length and padded to the left with zeros. An example of specifying the archive redo log
filename format is:
LOG_ARCHIVE_FORMAT = "LOG%s_%t.ARC"
This is a Real Application Clusters-specific parameter. However, you should not change it
except under a limited set of circumstances.
This parameter specifies the maximum amount of time allowed before the system change
number (SCN) held in the SGA of an instance is refreshed by the log writer process
(LGWR). It determines whether the local SCN should be refreshed from the lock value
when getting the snapshot SCN for a query. Units are in hundredths of seconds. Under
unusual circumstances involving rapid updates and queries of the same data from
different instances, the SCN might not be refreshed in a timely manner. Setting the
parameter to zero causes the SCN to be refreshed immediately after a commit. The default
value (700 hundredths of a second, or seven seconds) is an upper bound that enables the
preferred existing high performance mechanism to remain in place.
If you want commits to be seen immediately on remote instances, you may need to change
the value of this parameter.
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