TCP window controls in IBM Storage Protect

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The TCPWINDOWSIZE options for IBM Storage Protect server and clients override the operating system settings for the size of send and receive windows for TCP/IP sessions. The TCPWINDOWSIZE option is available as a server option and a client option. With each option, you specify one value, which is used as the size for both the send and receive windows.

During incremental backup operations for files, both client and server act as receivers of data:
  • The server sends metadata about the inventory of active backup versions to the client. The metadata consists of file names and attributes. For file systems that contain millions of files, this data can be a substantial amount, as much as hundreds of megabytes or even gigabytes.
  • The client sends backup copies of new and changed files to the server.

Typically the default value for the TCPWINDOWSIZE option works well. A larger window might improve communication performance, especially on fast networks with high latency, such as a long-distance wide area network (WAN) connection.

Specifying the TCPWINDOWSIZE option with a value of 0 causes IBM Storage Protect to use the operating system default for the TCP window size. If the operating system does not automatically tune the TCP window size, avoid using the operating system default. The operating system default might be optimized for other applications, which might not be the optimal setting for IBM Storage Protect.

If IBM Storage Protect clients and server are on the same subnet, a larger TCP window size is unlikely to improve throughput. Also, you might need more kernel memory if you set a large TCP receive window size. The risk of the increased memory requirements might be greater than the benefit of a larger TCP window size.

Modern operating systems provide TCP/IP stacks that commit the requested memory as it is needed. Therefore, these systems have less risk of increased kernel memory for the send and receive buffers. These operating systems also automatically tune the receive buffer size by observing the session transfer statistics, and either increasing or decreasing the receive window, as appropriate. For these operating systems only, you might set the IBM Storage Protect server TCPWINDOWSIZE option to 0 and use the automatic tuning feature. These settings are especially useful when clients that connect to the server are remote.

The TCPWINDOWSIZE option is not related to the TCPBUFSIZE server option or the tcpbuffsize client option. The TCPWINDOWSIZE option is also not related to the send and receive buffers that are allocated in client or server memory.