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Don Burleson Blog 







tnsnames.ora tips

Oracle tnsnames.ora tips by Donald BurlesonAugust 30,  2015

The Transparent Network Substrate (TNS), provided by Oracle, allows distributed communications between databases and it's the tnsnames.ora file that governs the TNS.

 For example, a entry in the tnsnames.ora file for a database might look like this:

berlin =
         (ADDRESS =
           (COMMUNITY = TCP)
           (PROTOCOL = TCP)
          (HOST = hun)
           (PORT = 1521)
     (CONNECT_DATA = (SID = kraus))

Let's see how the tnsnames.ora is used to resolve a connection.  In the tnsnames.ora example above we see a TNS service name of berlin, which defines a connection to a server named hun that contains an $ORACLE_SID named kraus. When a remote connection request is made from the UNIX/Linux server, the /etc/host file is accessed to get the IP address for the hun server.  From the listing below, we see that the hum server is located at As we see, the /etc/host file is used to isolate the IP address from the tnsnames.ora file.

Tuning with the tnsnames.ora file

The session data unit (SDU) and transport date unit (TDU) parameters are located in both the tnsnames.ora and listener.ora files. SDU specifies the size of the packets to send over the network and ideally, SDU should not surpass the size of the maximum transmission unit (MTU).

MTU is a fixed value parameter in the tnsnames.ora that depends on the actual network implementation used. Oracle recommends that SDU be set equal to MTU in the tnsnames.ora file.

The TDU is the default packet size used within Oracle Net to group data together. The TDU parameter should ideally be a multiple of the SDU parameter. The default value for both SDU and TDU is 2,048, and the maximum value is 32,767 bytes.  The following guidelines apply to SDU and TDU:

  • The SDU should never be set greater than TDU in the tnsnames.ora file because you'll waste network resources by shipping wasted space in each packet.
  • If your users are connecting via modem lines, you may want to set SDU and TDU to smaller values in your tnsnames.ora because of the frequent resends that occur over modem lines.
  • On fast network connections (T1 or T3 lines), you should set SDU and TDU equal to the MTU for your network. On standard Ethernet networks, the default MTU size for the tnsnames.ora is set to 1,514 bytes. On standard token ring networks, the default MTU size is 4,202.
  • If the Multi-Threaded Server (MTS) must is used (not recommeneded), you must also set the mts_dispatchers with the proper MTU TDU configuration in the tnsnames.ora.
  • For Streams replication and for using Oracle in a WAN environment, Oracle recommends that you increase the SDU to 32k. 


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