Network tuning considerations

The best way to improve overall performance in a distributed database environment is to eliminate delays from the network.

It is common for network administrators to consider a network to be more efficient if it collects as much data as possible between transmissions. This approach doesn't work for applications such as distributed databases because it builds delays into the network. The end-user doesn't see the efficiency of the network, only the delays.

Most network devices have delay parameters, and most of them default to values that are very bad for distributed databases. To improve performance you should locate these parameters and if possible, set them to zero. In addition you should ensure that the buffer size on the device is large enough to prevent retransmits due to lost data. For instance, UNIX systems typically have a Transmit or Receive queue depth default of 32. For better results, set the queue depth to 150. A corresponding parameter on DLC settings is the Receive Depth, which should also be 150.

The IOBUF parameter is set too low at most sites. It is usually set at 500, but experience has shown that a value of 3992 works best if you are moving large amounts of data, especially for channel connections such as ESCON or 3172.

On a LAN system the DLC or LLC transmit and receive window sizes can have a dramatic effect on performance. The send value should be set to seven or more, and for most configurations a receive value of four or less works best.

If you are running Ethernet, you should set the TCP segment size to 1500 bytes. On a token ring or FDDI network this value should be 4400 bytes, and if you are using an ESCON adapter with TCP/IP, the segment size should always be 4096.

Finally, for TCP/IP networks, the TCP Send and Receive buffer sizes should be set higher than 32768. A value of 65536 is generally best.

Note: Establishing a connection from the gateway to the server (outbound connection) is much more expensive than establishing a connection from a client to the gateway (inbound connection). In an environment where thousands of clients frequently connect to and disconnect from the server through the gateway, a substantial amount of processing time is spent establishing outbound connections. Db2 Connect provides connection pooling over TCP/IP. When a client requests disconnection from the server, the gateway drops the inbound connection with the client, but keeps the outbound connection to the server in a pool. When a new client comes into the gateway to request a connection, the gateway provides an existing one from the pool thus reducing the overall connection time and saving the high CPU connect cost on the server.
A summary of network performance tuning methods is provided in Table 1.
Table 1. Network performance tuning methods
What to Look For Example Setting Notes
Deliberate Delays Delay parameters on network devices Set to 0. Defaults are usually higher.
Buffers IOBUF parameter Set up to 3992. Particularly useful for ESCON or other channel adapter.
Buffers RUSIZE Optimum size is 4096. Setting RUSIZE and RQRIOBLK to same size might give the best performance.
Buffers Pacing VPACING, PACING, and Mode Profiles should be set to 63. Use adaptive pacing where applicable.
Adapter Settings Transmit/Receive queue depth Recommended value is 150. Default is usually 32.
TCP Settings Segment Sizes 1500 on Ethernet, 4400 on token ring and FDDI. ESCON adapters used for TCP/IP should always be set to 4096.
TCP Settings Send/Receive Space Sizes Should be 64K for both. Default is only 8192 for Windows. Can be set in the Windows registry.