14.2 Configuring Session Data Unit

Under typical database configuration, Oracle Net encapsulates data into buffers the size of the Session Data Unit (SDU) before sending the data across the network.

Oracle Net sends each buffer when it is filled, flushed, or when an application tries to read data. Adjusting the size of the SDU buffers relative to the amount of data provided to Oracle Net to send at any one time can improve performance, network utilization, and memory consumption. When large amounts of data are being transmitted, increasing the SDU size can improve performance and network throughput. SDU size can be adjusted lower or higher to achieve higher throughput for a specific deployment.

The amount of data provided to Oracle Net to send at any one time is referred to as the message size.

The SDU size can range from 512 bytes to 2 MB. The wide range of sizes allows the network administrator to tune the SDU size for optimal network performance for a given deployment. A high SDU size value requires more memory.

The default SDU size for the client is 8192 bytes and the server is 65536 bytes.

The actual SDU size used is negotiated between the client and the server at connect time and is the smaller of the client and server values. Configuring an SDU size different from the default requires configuring the SDU on both the client and server computers, unless you are using shared servers.

You should consider changing the SDU size when the predominant message size is smaller or larger than 8192. The SDU size should be 70 bytes more than the predominant message size. If the predominant message size plus 70 bytes exceeds the maximum SDU, then the SDU should be set such that the message size is divided into the smallest number of equal parts where each part is 70 bytes less than the SDU size. To change the default, change the DEFAULT_SDU_SIZE parameter in the sqlnet.ora file.

For example, if the majority of the messages sent and received by the application are smaller than 8 KB, taking into account the 70 bytes for overhead, then setting the SDU to 8 KB will likely produce good results. If sufficient memory is available, then using the maximum value for the SDU minimizes the number of system calls and overhead for Oracle Net Services.

• Setting the SDU Size for the Database
  
• Setting the SDU Size for the Client
  

14.2.1 Setting the SDU Size for the Database

To set the SDU size for the database server, configure the following files:

• sqlnet.ora

  Configure the DEFAULT_SDU_SIZE parameter in the sqlnet.ora file, such as the following:

  DEFAULT_SDU_SIZE=32767
  

• Initialization parameter file

  If using shared server processes, then set the SDU size in the DISPATCHERS parameter in the initialization parameter file, as follows:

  DISPATCHERS="(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp))(SDU=8192))"
  

• listener.ora

  If the listener was configured with a list of targets in the listener.ora file, then the value for SDU in the SID_LIST parameter overrides the current setting in the sqlnet.ora file when using dedicated server processes.

14.2.2 Setting the SDU Size for the Client

To set the SDU size for the client, configure the following files:

• sqlnet.ora

  For global configuration on the client side, configure the DEFAULT_SDU_SIZE parameter in the sqlnet.ora file, such as the following:

  DEFAULT_SDU_SIZE=32767
  

• tnsnames.ora

  For a particular connect descriptor, you can specify the SDU parameter in the DESCRIPTION parameter.

  sales.us.example.com=
  (DESCRIPTION= 
      (SDU=11280) 
        (ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)(PORT=1521))
      (CONNECT_DATA=
       (SERVICE_NAME=sales.us.example.com))
  ) 

The SDU size applies to all Oracle Net protocols for the particular connect descriptor.