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Oracle tnsnames.ora file

Oracle Tips by Burleson Consulting
Don Burleson

Inside the tnsnames.ora file

Oracle databases are often shared across geographical areas, so it's imperative that the Oracle professional understand how database performance is affected by network communications. The Transparent Network Substrate (TNS), provided by Oracle, allows distributed communications between databases.  It's the tnsnames.ora file that governs the TNS.

The SDU and TDU parameters in the tnsnames.ora file

The session data unit (SDU) and transport date unit (TDU) parameters are located in the tnsnames.ora and listener.ora files. In the tnsnames.ora, SDU specifies the size of the packets to send over the network. Ideally, SDU should not surpass the size of the maximum transmission unit (MTU). MTU is a fixed value that depends on the actual network implementation used. Oracle recommends that SDU be set equal to MTU.

The TDU is the default packet size used in the tnsnames.ora file 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 for the tnsnames.ora:

  • The SDU should never be set greater than TDU 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 in tnsnames.ora to smaller values 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 in tnsnames.ora. On standard Ethernet networks, the default MTU size is set to 1,514 bytes. On standard token ring networks, the default MTU size is 4,202.
  • If the Multi-Threaded Server (Shared Servers, or MTS) must be used, you must also set the mts_dispatchers with the proper MTU TDU configuration in tnsnames.ora.
  • For Streams replication and for using Oracle in a WAN environment, Oracle recommends that you increase the SDU to 32k.  Also, the Oracle docs note:

    To take advantage of an increased SDU for Streams propagation, the receiving side sqlnet.ora file must include the DEFAULT_SDU_SIZE parameter. The receiving side listener.ora file must indicate the SDU change for the system identifier (SID). The sending side tnsnames.ora file connect string must also include the SDU modification for the particular service. 

    For Streams, the SEND_BUF_SIZE and RECV_BUF_SIZE parameters in the listener.ora file increase the performance of propagation on your system.  Oracle internal testing of Oracle Data Guard on a WAN has demonstrated that the maximum setting of 32K (32768) performs best on a WAN. The primary gain in performance when setting the SDU is a result of the reduced number of calls to packet the data.

    In addition to setting the SDU parameter, network throughput can often be substantially improved by using the SQLNET.SEND_BUF_SIZE and SQLNET.RECV_BUF_SIZE Oracle Net parameters to increase the size of the network TCP send and receive I/O buffers.

The SDU and TDU settings are a direct function of the connection speed between the hosts. For fast T1 lines, set SDU=TDU=MTU. For slower modem lines, experiment with smaller values of SDU and TDU.

Scripts for measuring changes to the tnsnames.ora can be found in the Oracle script collection.



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